cppdraft_translate/cppdraft/over/match.md

[over.match]

12 Overloading [over]

12.2 Overload resolution [over.match]

12.2.1 General [over.match.general]

1

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Overload resolution is a mechanism for selecting the best function to call given a list of expressions that are to be the arguments of the call and a set ofcandidate functions that can be called based on the context of the call.

The selection criteria for the best function are the number of arguments, how well the arguments match the parameter-type-list of the candidate function, how well (for non-static member functions) the object matches the object parameter, and certain other properties of the candidate function.

[Note 1:

The function selected by overload resolution is not guaranteed to be appropriate for the context.

Other restrictions, such as the accessibility of the function, can make its use in the calling context ill-formed.

— end note]

2

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Overload resolution selects the function to call in seven distinct contexts within the language:

• (2.1)

invocation of a function named in the function call syntax;

• (2.2)

invocation of a function call operator, a pointer-to-function conversion function, a reference-to-pointer-to-function conversion function, or a reference-to-function conversion function on a class object named in the function call syntax ([over.call.object]);

• (2.3)

invocation of the operator referenced in an expression ([over.match.oper]);

• (2.4)

invocation of a constructor for default- or direct-initialization ([dcl.init]) of a class object ([over.match.ctor]);

• (2.5)

invocation of a user-defined conversion forcopy-initialization of a class object ([over.match.copy]);

• (2.6)

invocation of a conversion function for initialization of an object of a non-class type from an expression of class type ([over.match.conv]); and

• (2.7)

invocation of a conversion function for conversion in which a reference ([dcl.init.ref]) will be directly bound.

Each of these contexts defines the set of candidate functions and the list of arguments in its own unique way.

But, once the candidate functions and argument lists have been identified, the selection of the best function is the same in all cases:

• (2.8)

  First, a subset of the candidate functions (those that have the proper number of arguments and meet certain other conditions) is selected to form a set ofviable functions ([over.match.viable]).

• (2.9)

  Then the best viable function is selected based on theimplicit conversion sequences needed to match each argument to the corresponding parameter of each viable function.

3

#

If a best viable function exists and is unique, overload resolution succeeds and produces it as the result.

Otherwise overload resolution fails and the invocation is ill-formed.

When overload resolution succeeds, and the best viable function is not accessible in the context in which it is used, the program is ill-formed.

4

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Overload resolution results in a usable candidate if overload resolution succeeds and the selected candidate is either not a function ([over.built]), or is a function that is not deleted and is accessible from the context in which overload resolution was performed.

12.2.2 Candidate functions and argument lists [over.match.funcs]

12.2.2.1 General [over.match.funcs.general]

1

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The subclauses of [over.match.funcs] describe the set of candidate functions and the argument list submitted to overload resolution in each context in which overload resolution is used.

The source transformations and constructions defined in these subclauses are only for the purpose of describing the overload resolution process.

An implementation is not required to use such transformations and constructions.

2

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The set of candidate functions can contain both member and non-member functions to be resolved against the same argument list.

If a member function is

• (2.1)

an implicit object member function that is not a constructor, or

• (2.2)

a static member function and the argument list includes an implied object argument,

it is considered to have an extra first parameter, called the implicit object parameter, which represents the object for which the member function has been called.

3

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Similarly, when appropriate, the context can construct an argument list that contains animplied object argument as the first argument in the list to denote the object to be operated on.

4

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For implicit object member functions, the type of the implicit object parameter is

• (4.1)

“lvalue reference to cv X” for functions declared without a ref-qualifier or with the& ref-qualifier

• (4.2)

“rvalue reference to cv X” for functions declared with the&& ref-qualifier

whereX is the class of which the function is a direct member andcv is the cv-qualification on the member function declaration.

[Example 1:

For aconst member function of classX, the extra parameter is assumed to have type “lvalue reference toconst X”.

— end example]

For conversion functions that are implicit object member functions, the function is considered to be a member of the class of the implied object argument for the purpose of defining the type of the implicit object parameter.

For non-conversion functions that are implicit object member functions nominated by a using-declaration in a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit object parameter.

For static member functions, the implicit object parameter is considered to match any object (since if the function is selected, the object is discarded).

[Note 1:

No actual type is established for the implicit object parameter of a static member function, and no attempt will be made to determine a conversion sequence for that parameter ([over.match.best]).

— end note]

5

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During overload resolution, the implied object argument is indistinguishable from other arguments.

The implicit object parameter, however, retains its identity since no user-defined conversions can be applied to achieve a type match with it.

For implicit object member functions declared without a ref-qualifier, even if the implicit object parameter is not const-qualified, an rvalue can be bound to the parameter as long as in all other respects the argument can be converted to the type of the implicit object parameter.

[Note 2:

The fact that such an argument is an rvalue does not affect the ranking of implicit conversion sequences.

— end note]

6

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Because other than in list-initialization only one user-defined conversion is allowed in an implicit conversion sequence, special rules apply when selecting the best user-defined conversion ([over.match.best], [over.best.ics]).

[Example 2: class T {public: T();};

class C : T {public: C(int);}; T a = 1; // error: no viable conversion (T(C(1)) not considered) — end example]

7

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In each case where conversion functions of a class S are considered for initializing an object or reference of type T, the candidate functions include the result of a search for the conversion-function-id operator T in S.

[Note 3:

This search can find a specialization of a conversion function template ([basic.lookup]).

— end note]

Each such case also defines sets of permissible types for explicit and non-explicit conversion functions; each (non-template) conversion function that

• (7.1)

is a non-hidden member of S,

• (7.2)

yields a permissible type, and,

• (7.3)

for the former set, is non-explicit

is also a candidate function.

If initializing an object, for any permissible type cv U, anycv2 U, cv2 U&, or cv2 U&& is also a permissible type.

If the set of permissible types for explicit conversion functions is empty, any candidates that are explicit are discarded.

8

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In each case where a candidate is a function template, candidate function template specializations are generated using template argument deduction ([temp.over], [temp.deduct]).

If a constructor template or conversion function template has an explicit-specifier whose constant-expression is value-dependent ([temp.dep]), template argument deduction is performed first and then, if the context admits only candidates that are not explicit and the generated specialization is explicit ([dcl.fct.spec]), it will be removed from the candidate set.

Those candidates are then handled as candidate functions in the usual way.99

A given name can refer to, or a conversion can consider, one or more function templates as well as a set of non-template functions.

In such a case, the candidate functions generated from each function template are combined with the set of non-template candidate functions.

9

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A defaulted move special member function ([class.copy.ctor], [class.copy.assign]) that is defined as deleted is excluded from the set of candidate functions in all contexts.

A constructor inherited from class type C ([class.inhctor.init]) that has a first parameter of type “reference to cv1 P” (including such a constructor instantiated from a template) is excluded from the set of candidate functions when constructing an object of type cv2 D if the argument list has exactly one argument andC is reference-related to P andP is reference-related to D.

[Example 3: struct A { A(); // #1 A(A &&); // #2template A(T &&); // #3};struct B : A {using A::A; B(const B &); // #4 B(B &&) = default; // #5, implicitly deletedstruct X { X(X &&) = delete; } x;};extern B b1; B b2 = static_cast<B&&>(b1); // calls #4: #1 is not viable, #2, #3, and #5 are not candidatesstruct C { operator B&&(); }; B b3 = C(); // calls #4 — end example]

99)99)

The process of argument deduction fully determines the parameter types of the function template specializations, i.e., the parameters of function template specializations contain no template parameter types.

Therefore, except where specified otherwise, function template specializations and non-template functions ([dcl.fct]) are treated equivalently for the remainder of overload resolution.

12.2.2.2 Function call syntax [over.match.call]

12.2.2.2.1 General [over.match.call.general]

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In a function call

postfix-expression ( expression-listopt )

if the postfix-expression names at least one function or function template, overload resolution is applied as specified in [over.call.func].

If the postfix-expression denotes an object of class type, overload resolution is applied as specified in [over.call.object].

2

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If the postfix-expression is the address of an overload set, overload resolution is applied using that set as described above.

[Note 1:

No implied object argument is added in this case.

— end note]

If the function selected by overload resolution is an implicit object member function, the program is ill-formed.

[Note 2:

The resolution of the address of an overload set in other contexts is described in [over.over].

— end note]

12.2.2.2.2 Call to designated function [over.call.func]

1

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Of interest in [over.call.func] are only those function calls in which the postfix-expression ultimately contains an id-expression or splice-expression that designates one or more functions.

Such apostfix-expression, perhaps nested arbitrarily deep in parentheses, has one of the following forms:

postfix-expression:
postfix-expression . id-expression
postfix-expression . splice-expression
postfix-expression -> id-expression
postfix-expression -> splice-expression
id-expression
splice-expression

These represent two syntactic subcategories of function calls: qualified function calls and unqualified function calls.

2

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In qualified function calls, the function is designated by an id-expression or splice-expression E preceded by an -> or . operator.

Since the constructA->B is generally equivalent to(*A).B, the rest of[over] assumes, without loss of generality, that all member function calls have been normalized to the form that uses an object and the. operator.

Furthermore, [over] assumes that thepostfix-expression that is the left operand of the. operator has type “cv T” whereT denotes a class.100

The set of candidate functions either is the set found by name lookup ([class.member.lookup]) if E is an id-expression or is the set determined as specified in [expr.prim.splice] if E is a splice-expression.

The argument list is theexpression-list in the call augmented by the addition of the left operand of the. operator in the normalized member function call as the implied object argument ([over.match.funcs]).

3

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In unqualified function calls, the function is designated by an id-expression or a splice-expression E.

The set of candidate functions either is the set found by name lookup ([basic.lookup]) if E is an id-expression or is the set determined as specified in [expr.prim.splice] if E is a splice-expression.

The set of candidate functions consists either entirely of non-member functions or entirely of member functions of some classT.

In the former case or if E is either a splice-expression or the address of an overload set, the argument list is the same as theexpression-list in the call.

Otherwise, the argument list is theexpression-list in the call augmented by the addition of an implied object argument as in a qualified function call.

If the current class is, or is derived from, T, and the keywordthis ([expr.prim.this]) refers to it,

• (3.1)

if the unqualified function call appears in a precondition assertion of a constructor or a postcondition assertion of a destructor and overload resolution selects a non-static member function, the call is ill-formed;

• (3.2)

otherwise, the implied object argument is(*this).

Otherwise,

• (3.3)

if overload resolution selects a non-static member function, the call is ill-formed;

• (3.4)

otherwise, a contrived object of typeT becomes the implied object argument.101

[Example 1: struct C {bool a(); void b() { a(); // OK, (*this).a()}void c(this const C&); // #1void c() &; // #2static void c(int = 0); // #3void d() { c(); // error: ambiguous between #2 and #3(C::c)(); // error: as above(&(C::c))(); // error: cannot resolve address of overloaded this->C​::​c ([over.over])(&C::c)(C{}); // selects #1(&C::c)(*this); // error: selects #2, and is ill-formed ([over.match.call.general])(&C::c)(); // selects #3}void f(this const C&); void g() const { f(); // OK, (*this).f() f(*this); // error: no viable candidate for (*this).f(*this)this->f(); // OK}static void h() { f(); // error: contrived object argument, but overload resolution// picked a non-static member function f(C{}); // error: no viable candidate C{}.f(); // OK}void k(this int); operator int() const; void m(this const C& c) { c.k(); // OK} C() pre(a()) // error: implied this in constructor precondition pre(this->a()) // OK post(a()); // OK~C() pre(a()) // OK post(a()) // error: implied this in destructor postcondition post(this->a()); // OK}; — end example]

100)100)

Note that cv-qualifiers on the type of objects are significant in overload resolution for both glvalue and class prvalue objects.

101)101)

An implied object argument is contrived to correspond to the implicit object parameter attributed to member functions during overload resolution.

It is not used in the call to the selected function.

Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.

12.2.2.2.3 Call to object of class type [over.call.object]

1

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If the postfix-expression E in the function call syntax evaluates to a class object of type “cv T”, then the set of candidate functions includes at least the function call operators of T.

The function call operators of T are the results of a search for the name operator() in the scope of T.

2

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In addition, for each non-explicit conversion function declared in T of the form

operator conversion-type-id ( ) cv-qualifier-seqopt ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt ;

where the optionalcv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than,cv, and whereconversion-type-id denotes the type “pointer to function of (P1,…,Pn) returning R”, or the type “reference to pointer to function of (P1,…,Pn) returning R”, or the type “reference to function of (P1,…,Pn) returning R”, a surrogate call function with the unique namecall-function and having the form

R call-function ( conversion-type-id F, P1 a1, …, Pn an) { return F (a1, …, an); }

is also considered as a candidate function.

Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class ofT provided the function is not hidden withinT by another intervening declaration.102

3

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The argument list submitted to overload resolution consists of the argument expressions present in the function call syntax preceded by the implied object argument(E).

[Note 1:

When comparing the call against the function call operators, the implied object argument is compared against the object parameter of the function call operator.

When comparing the call against a surrogate call function, the implied object argument is compared against the first parameter of the surrogate call function.

— end note]

[Example 1: int f1(int);int f2(float);typedef int (*fp1)(int);typedef int (*fp2)(float);struct A {operator fp1() { return f1; }operator fp2() { return f2; }} a;int i = a(1); // calls f1 via pointer returned from conversion function — end example]

102)102)

Note that this construction can yield candidate call functions that cannot be differentiated one from the other by overload resolution because they have identical declarations or differ only in their return type.

The call will be ambiguous if overload resolution cannot select a match to the call that is uniquely better than such undifferentiable functions.

12.2.2.3 Operators in expressions [over.match.oper]

1

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If no operand of an operator in an expression has a type that is a class or an enumeration, the operator is assumed to be a built-in operator and interpreted according to [expr.compound].

[Note 1:

Because.,.*, and​::​ cannot be overloaded, these operators are always built-in operators interpreted according to[expr.compound].

?: cannot be overloaded, but the rules in this subclause are used to determine the conversions to be applied to the second and third operands when they have class or enumeration type ([expr.cond]).

— end note]

[Example 1: struct String { String (const String&); String (const char*); operator const char* ();}; String operator + (const String&, const String&);

void f() {const char* p= "one" + "two"; // error: cannot add two pointers; overloaded operator+ not considered// because neither operand has class or enumeration typeint I = 1 + 1; // always evaluates to 2 even if class or enumeration types exist// that would perform the operation.} — end example]

2

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If either operand has a type that is a class or an enumeration, a user-defined operator function can be declared that implements this operator or a user-defined conversion can be necessary to convert the operand to a type that is appropriate for a built-in operator.

In this case, overload resolution is used to determine which operator function or built-in operator is to be invoked to implement the operator.

Therefore, the operator notation is first transformed to the equivalent function-call notation as summarized in Table 18 (where @ denotes one of the operators covered in the specified subclause).

However, the operands are sequenced in the order prescribed for the built-in operator ([expr.compound]).

Table 18 — Relationship between operator and function call notation [tab:over.match.oper]

🔗 Subclause	Expression	As member function	As non-member function
🔗 [over.unary]	@a	(a).operator@ ( )	operator@(a)
🔗 [over.binary]	a@b	(a).operator@ (b)	operator@(a, b)
🔗 [over.assign]	a=b	(a).operator= (b)
🔗 [over.sub]	a[b]	(a).operator
🔗 [over.ref]	a->	(a).operator->( )
🔗 [over.inc]	a@	(a).operator@ (0)	operator@(a, 0)

3

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For a unary operator @ with an operand of type cv1 T1, and for a binary operator @ with a left operand of type cv1 T1 and a right operand of type cv2 T2, four sets of candidate functions, designatedmember candidates,non-member candidates,built-in candidates, andrewritten candidates, are constructed as follows:

• (3.1)

  If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of a search foroperator@ in the scope of T1; otherwise, the set of member candidates is empty.

• (3.2)

  For the operators =, [], or ->, the set of non-member candidates is empty; otherwise, it includes the result of unqualified lookup foroperator@ in the rewritten function call ([basic.lookup.unqual], [basic.lookup.argdep]), ignoring all member functions. However, if no operand has a class type, only those non-member functions in the lookup set that have a first parameter of typeT1 or “reference to cv T1”, whenT1 is an enumeration type, or (if there is a right operand) a second parameter of typeT2 or “reference to cv T2”, whenT2 is an enumeration type, are candidate functions.

• (3.3)

  For the operator,, the unary operator&, or the operator->, the built-in candidates set is empty. For all other operators, the built-in candidates include all of the candidate operator functions defined in [over.built] that, compared to the given operator,

  • (3.3.1)

have the same operator name, and

• (3.3.2)

accept the same number of operands, and

• (3.3.3)

accept operand types to which the given operand or operands can be converted according to [over.best.ics], and

• (3.3.4)

do not have the same parameter-type-list as any non-member candidate or rewritten non-member candidate that is not a function template specialization.

• (3.4)

  The rewritten candidate set is determined as follows:

  • (3.4.1)

    For the relational ([expr.rel]) operators, the rewritten candidates include all non-rewritten candidates for the expression x <=> y.

  • (3.4.2)

    For the relational ([expr.rel]) and three-way comparison ([expr.spaceship]) operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expressiony <=> x.

  • (3.4.3)

    For the != operator ([expr.eq]), the rewritten candidates include all non-rewritten candidates for the expression x == y that are rewrite targets with first operand x (see below).

  • (3.4.4)

    For the equality operators, the rewritten candidates also include a synthesized candidate, with the order of the two parameters reversed, for each non-rewritten candidate for the expression y == x that is a rewrite target with first operand y.

  • (3.4.5)

    For all other operators, the rewritten candidate set is empty.

  [Note 2: A candidate synthesized from a member candidate has its object parameter as the second parameter, thus implicit conversions are considered for the first, but not for the second, parameter. — end note]

4

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A non-template function or function template F named operator== is a rewrite target with first operand o unless a search for the name operator!= in the scope S from the instantiation context of the operator expression finds a function or function template that would correspond ([basic.scope.scope]) to F if its name were operator==, where S is the scope of the class type of o if F is a class member, and the namespace scope of which F is a member otherwise.

A function template specialization named operator== is a rewrite target if its function template is a rewrite target.

[Example 2: struct A {};template bool operator==(A, T); // #1bool a1 = 0 == A(); // OK, calls reversed #1template bool operator!=(A, T);bool a2 = 0 == A(); // error, #1 is not a rewrite targetstruct B {bool operator==(const B&); // #2};struct C : B { C(); C(B); bool operator!=(const B&); // #3};bool c1 = B() == C(); // OK, calls #2; reversed #2 is not a candidate// because search for operator!= in C finds #3bool c2 = C() == B(); // error: ambiguous between #2 found when searching C and// reversed #2 found when searching Bstruct D {};template bool operator==(D, T); // #4inline namespace N {template bool operator!=(D, T); // #5}bool d1 = 0 == D(); // OK, calls reversed #4; #5 does not forbid #4 as a rewrite target — end example]

5

#

For the first parameter of the built-in assignment operators, only standard conversion sequences ([over.ics.scs]) are considered.

6

#

For all other operators, no such restrictions apply.

7

#

The set of candidate functions for overload resolution for some operator @ is the union of the member candidates, the non-member candidates, the built-in candidates, and the rewritten candidates for that operator @.

8

#

The argument list contains all of the operands of the operator.

The best function from the set of candidate functions is selected according to [over.match.viable] and [over.match.best].103

[Example 3: struct A {operator int();}; A operator+(const A&, const A&);void m() { A a, b; a + b; // operator+(a, b) chosen over int(a) + int(b)} — end example]

9

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If a rewritten operator<=> candidate is selected by overload resolution for an operator @,x @ y is interpreted as0 @ (y <=> x) if the selected candidate is a synthesized candidate with reversed order of parameters, or (x <=> y) @ 0 otherwise, using the selected rewritten operator<=> candidate.

Rewritten candidates for the operator @ are not considered in the context of the resulting expression.

10

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If a rewritten operator== candidate is selected by overload resolution for an operator @, its return type shall be cv bool, andx @ y is interpreted as:

• (10.1)

if @ is != and the selected candidate is a synthesized candidate with reversed order of parameters,!(y == x),

• (10.2)

otherwise, if @ is !=,!(x == y),

• (10.3)

otherwise (when @ is ==),y == x,

in each case using the selected rewritten operator== candidate.

11

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If a built-in candidate is selected by overload resolution, the operands of class type are converted to the types of the corresponding parameters of the selected operation function, except that the second standard conversion sequence of a user-defined conversion sequence is not applied.

Then the operator is treated as the corresponding built-in operator and interpreted according to [expr.compound].

[Example 4: struct X {operator double();};

struct Y {operator int*();};

int *a = Y() + 100.0; // error: pointer arithmetic requires integral operandint *b = Y() + X(); // error: pointer arithmetic requires integral operand — end example]

12

#

The second operand of operator-> is ignored in selecting anoperator-> function, and is not an argument when theoperator-> function is called.

Whenoperator-> returns, the operator-> is applied to the value returned, with the original second operand.104

13

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If the operator is the operator,, the unary operator&, or the operator->, and there are no viable functions, then the operator is assumed to be the built-in operator and interpreted according to[expr.compound].

14

#

[Note 3:

The lookup rules for operators in expressions are different than the lookup rules for operator function names in a function call, as shown in the following example:struct A { };void operator + (A, A);

struct B {void operator + (B); void f ();};

A a;

void B::f() {operator+ (a,a); // error: global operator hidden by member a + a; // OK, calls global operator+}

— end note]

103)103)

If the set of candidate functions is empty, overload resolution is unsuccessful.

104)104)

If the value returned by theoperator-> function has class type, this can result in selecting and calling anotheroperator-> function.

The process repeats until anoperator-> function returns a value of non-class type.

12.2.2.4 Initialization by constructor [over.match.ctor]

1

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When objects of class type are direct-initialized, copy-initialized from an expression of the same or a derived class type ([dcl.init]), or default-initialized, overload resolution selects the constructor.

For direct-initialization or default-initialization (including default-initialization in the context of copy-list-initialization), the candidate functions are all the constructors of the class of the object being initialized.

Otherwise, the candidate functions are all the non-explicit constructors ([class.conv.ctor]) of that class.

The argument list is theexpression-list or assignment-expression of the initializer.

For default-initialization in the context of copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.

12.2.2.5 Copy-initialization of class by user-defined conversion [over.match.copy]

1

#

Under the conditions specified in [dcl.init], as part of a copy-initialization of an object of class type, a user-defined conversion can be invoked to convert an initializer expression to the type of the object being initialized.

Overload resolution is used to select the user-defined conversion to be invoked.

[Note 1:

The conversion performed for indirect binding to a reference to a possibly cv-qualified class type is determined in terms of a corresponding non-reference copy-initialization.

— end note]

Assuming that “cv1 T” is the type of the object being initialized, withT a class type, the candidate functions are selected as follows:

• (1.1)

  The non-explicit constructors ([class.conv.ctor]) ofT are candidate functions.

• (1.2)

  When the type of the initializer expression is a class type “cv S”, conversion functions are considered. The permissible types for non-explicit conversion functions areT and any class derived from T. When initializing a temporary object ([class.mem]) to be bound to the first parameter of a constructor where the parameter is of type “reference to cv2 T” and the constructor is called with a single argument in the context of direct-initialization of an object of type “cv3 T”, the permissible types for explicit conversion functions are the same; otherwise there are none.

2

#

In both cases, the argument list has one argument, which is the initializer expression.

[Note 2:

This argument will be compared against the first parameter of the constructors and against the object parameter of the conversion functions.

— end note]

12.2.2.6 Initialization by conversion function [over.match.conv]

1

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Under the conditions specified in [dcl.init], as part of an initialization of an object of non-class type, a conversion function can be invoked to convert an initializer expression of class type to the type of the object being initialized.

Overload resolution is used to select the conversion function to be invoked.

Assuming that “cv T” is the type of the object being initialized, the candidate functions are selected as follows:

• (1.1)

The permissible types for non-explicit conversion functions are those that can be converted to type T via a standard conversion sequence ([over.ics.scs]). For direct-initialization, the permissible types for explicit conversion functions are those that can be converted to type T with a (possibly trivial) qualification conversion ([conv.qual]); otherwise there are none.

2

#

The argument list has one argument, which is the initializer expression.

[Note 1:

This argument will be compared against the object parameter of the conversion functions.

— end note]

12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]

1

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Under the conditions specified in [dcl.init.ref], a reference can be bound directly to the result of applying a conversion function to an initializer expression.

Overload resolution is used to select the conversion function to be invoked.

Assuming that “reference to cv1 T” is the type of the reference being initialized, the candidate functions are selected as follows:

• (1.1)

Let R be a set of types including

• (1.1.1)

“lvalue reference to cv2 T2” (when converting to an lvalue) and

• (1.1.2)

“cv2 T2” and “rvalue reference to cv2 T2” (when converting to an rvalue or an lvalue of function type)

for any T2. The permissible types for non-explicit conversion functions are the members of R where “cv1 T” is reference-compatible ([dcl.init.ref]) with “cv2 T2”. For direct-initialization, the permissible types for explicit conversion functions are the members of R where T2 can be converted to type T with a (possibly trivial) qualification conversion ([conv.qual]); otherwise there are none.

2

#

The argument list has one argument, which is the initializer expression.

[Note 1:

This argument will be compared against the object parameter of the conversion functions.

— end note]

12.2.2.8 Initialization by list-initialization [over.match.list]

1

#

When objects of non-aggregate class type T are list-initialized such that [dcl.init.list] specifies that overload resolution is performed according to the rules in this subclause or when forming a list-initialization sequence according to [over.ics.list], overload resolution selects the constructor in two phases:

• (1.1)

  If the initializer list is not empty or T has no default constructor, overload resolution is first performed where the candidate functions are the initializer-list constructors ([dcl.init.list]) of the class T and the argument list consists of the initializer list as a single argument.

• (1.2)

  Otherwise, or if no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list.

In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.

[Note 1:

This differs from other situations ([over.match.ctor], [over.match.copy]), where only non-explicit constructors are considered for copy-initialization.

This restriction only applies if this initialization is part of the final result of overload resolution.

— end note]

12.2.2.9 Class template argument deduction [over.match.class.deduct]

1

#

When resolving a placeholder for a deduced class type ([dcl.type.class.deduct]) where the template-name or splice-type-specifier designates a primary class template C, a set of functions and function templates, called the guides of C, is formed comprising:

• (1.1)

  If C is defined, for each constructor of C, a function template with the following properties:

  • (1.1.1)

    The template parameters are the template parameters of C followed by the template parameters (including default template arguments) of the constructor, if any.

  • (1.1.2)

    The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of C and the associated constraints of the constructor, if any. [Note 1: A constraint-expression in the template-head of C is checked for satisfaction before any constraints from the template-head or trailing requires-clause of the constructor. — end note]

  • (1.1.3)

    The parameter-declaration-clause is that of the constructor.

  • (1.1.4)

    The return type is the class template specialization designated by C and template arguments corresponding to the template parameters of C.

• (1.2)

  If C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C().

• (1.3)

  An additional function template derived as above from a hypothetical constructor C(C), called the copy deduction candidate.

• (1.4)

  For each deduction-guide, a function or function template with the following properties:

  • (1.4.1)

    The template-head, if any, and parameter-declaration-clause are those of the deduction-guide.

  • (1.4.2)

    The return type is the simple-template-id of the deduction-guide.

In addition, if C is defined and its definition satisfies the conditions for an aggregate class ([dcl.init.aggr]) with the assumption that any dependent base class has no virtual functions and no virtual base classes, and the initializer is a non-empty braced-init-list or parenthesized expression-list, and there are no deduction-guides for C, the set contains an additional function template, called the aggregate deduction candidate, defined as follows.

Let x1,…,xn be the elements of the initializer-list ordesignated-initializer-list of the braced-init-list, or of the expression-list.

For each xi, let ei be the corresponding aggregate element of C or of one of its (possibly recursive) subaggregates that would be initialized by xi ([dcl.init.aggr]) if

• (1.5)

brace elision is not considered for any aggregate element that has

• (1.5.1)

a dependent non-array type,

• (1.5.2)

an array type with a value-dependent bound, or

• (1.5.3)

an array type with a dependent array element type and xi is a string literal; and

• (1.6)

each non-trailing aggregate element that is a pack expansion is assumed to correspond to no elements of the initializer list, and

• (1.7)

a trailing aggregate element that is a pack expansion is assumed to correspond to all remaining elements of the initializer list (if any).

If there is no such aggregate element ei for any xi, the aggregate deduction candidate is not added to the set.

The aggregate deduction candidate is derived as above from a hypothetical constructor C(T1,…,Tn), where

• (1.8)

if ei is of array type andxi is a braced-init-list,Ti is an rvalue reference to the declared type of ei, and

• (1.9)

if ei is of array type andxi is a string-literal,Ti is an lvalue reference to the const-qualified declared type of ei, and

• (1.10)

otherwise, Ti is the declared type of ei,

except that additional parameter packs of the form Pj... are inserted into the parameter list in their original aggregate element position corresponding to each non-trailing aggregate element of type Pj that was skipped because it was a parameter pack, and the trailing sequence of parameters corresponding to a trailing aggregate element that is a pack expansion (if any) is replaced by a single parameter of the form Tn....

In addition, if C is defined and inherits constructors ([namespace.udecl]) from a direct base class denoted in the base-specifier-list by a class-or-decltype B, let A be an alias template whose template parameter list is that of C and whose defining-type-id is B.

If A is a deducible template ([dcl.type.simple]), the set contains the guides of A with the return type R of each guide replaced with typename CC​::​type given a class templatetemplate class CC; whose primary template is not defined and with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of A with the template argument list of A ([temp.dep.type]) having a member typedef type designating a template specialization with the template argument list of A but with C as the template.

[Note 2:

Equivalently, the template parameter list of the specialization is that of C, the template argument list of the specialization is B, and the member typedef names C with the template argument list of C.

— end note]

2

#

[Example 1: template struct B { B(T);};template struct C : public B {using B::B;};template struct D : public B {};

C c(42); // OK, deduces C D d(42); // error: deduction failed, no inherited deduction guides B(int) -> B; C c2(42); // OK, deduces Ctemplate struct E : public B {using B::B;};

E e(42); // error: deduction failed, arguments of E cannot be deduced from introduced guidestemplate <typename T, typename U, typename V> struct F { F(T, U, V);};template <typename T, typename U> struct G : F<U, T, int> {using G::F::F;} G g(true, 'a', 1); // OK, deduces G<char, bool>template<class T, std::size_t N>struct H { T array[N];};template<class T, std::size_t N>struct I {volatile T array[N];};template<std::size_t N>struct J {unsigned char array[N];};

H h = { "abc" }; // OK, deduces H<char, 4> (not T = const char) I i = { "def" }; // OK, deduces I<char, 4> J j = { "ghi" }; // error: cannot bind reference to array of unsigned char to array of char in deduction — end example]

3

#

When resolving a placeholder for a deduced class type ([dcl.type.simple]) where the template-name or splice-type-specifier designates an alias template A, the defining-type-id of A must be of the form

typenameopt nested-name-specifieropt templateopt simple-template-id

as specified in [dcl.type.simple].

The guides of A are the set of functions or function templates formed as follows.

For each function or function template f in the guides of the template named by the simple-template-id of the defining-type-id, the template arguments of the return type of f are deduced from the defining-type-id of A according to the process in [temp.deduct.type] with the exception that deduction does not fail if not all template arguments are deduced.

If deduction fails for another reason, proceed with an empty set of deduced template arguments.

Let g denote the result of substituting these deductions into f.

If substitution succeeds, form a function or function template f' with the following properties and add it to the set of guides of A:

• (3.1)

  The function type of f' is the function type of g.

• (3.2)

  If f is a function template,f' is a function template whose template parameter list consists of all the template parameters of A (including their default template arguments) that appear in the above deductions or (recursively) in their default template arguments, followed by the template parameters of f that were not deduced (including their default template arguments), otherwise f' is not a function template.

• (3.3)

  The associated constraints ([temp.constr.decl]) are the conjunction of the associated constraints of g and a constraint that is satisfied if and only if the arguments of A are deducible (see below) from the return type.

• (3.4)

  If f is a copy deduction candidate, then f' is considered to be so as well.

• (3.5)

  If f was generated from a deduction-guide ([temp.deduct.guide]), then f' is considered to be so as well.

• (3.6)

  The explicit-specifier of f' is the explicit-specifier of g (if any).

4

#

The arguments of a template A are said to be deducible from a type T if, given a class templatetemplate class AA; with a single partial specialization whose template parameter list is that of A and whose template argument list is a specialization of A with the template argument list of A ([temp.dep.type]),AA matches the partial specialization.

5

#

Initialization and overload resolution are performed as described in [dcl.init] and [over.match.ctor], [over.match.copy], or [over.match.list] (as appropriate for the type of initialization performed) for an object of a hypothetical class type, where the guides of the template named by the placeholder are considered to be the constructors of that class type for the purpose of forming an overload set, and the initializer is provided by the context in which class template argument deduction was performed.

The following exceptions apply:

• (5.1)

  The first phase in [over.match.list] (considering initializer-list constructors) is omitted if the initializer list consists of a single expression of type cv U, where U is, or is derived from, a specialization of the class template directly or indirectly named by the placeholder.

• (5.2)

  During template argument deduction for the aggregate deduction candidate, the number of elements in a trailing parameter pack is only deduced from the number of remaining function arguments if it is not otherwise deduced.

If the function or function template was generated from a constructor or deduction-guide that had an explicit-specifier, each such notional constructor is considered to have that same explicit-specifier.

All such notional constructors are considered to be public members of the hypothetical class type.

6

#

[Example 2: template struct A {explicit A(const T&, ...) noexcept; // #1 A(T&&, ...); // #2};

int i; A a1 = { i, i }; // error: explicit constructor #1 selected in copy-list-initialization during deduction,// cannot deduce from non-forwarding rvalue reference in #2 A a2{i, i}; // OK, #1 deduces to A and also initializes A a3{0, i}; // OK, #2 deduces to A and also initializes A a4 = {0, i}; // OK, #2 deduces to A and also initializestemplate A(const T&, const T&) -> A<T&>; // #3template explicit A(T&&, T&&) -> A; // #4 A a5 = {0, 1}; // error: explicit deduction guide #4 selected in copy-list-initialization during deduction A a6{0,1}; // OK, #4 deduces to A and #2 initializes A a7 = {0, i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructor A a8{0,i}; // error: #3 deduces to A<int&>, #1 and #2 declare same constructortemplate struct B {template using TA = T; template B(U, TA);};

B b{(int*)0, (char*)0}; // OK, deduces B<char*>template struct S { T x; T y;};

template struct C { S s; T t;};

template struct D { S s; T t;};

C c1 = {1, 2}; // error: deduction failed C c2 = {1, 2, 3}; // error: deduction failed C c3 = {{1u, 2u}, 3}; // OK, deduces C D d1 = {1, 2}; // error: deduction failed D d2 = {1, 2, 3}; // OK, braces elided, deduces Dtemplate struct E { T t; decltype(t) t2;};

E e1 = {1, 2}; // OK, deduces Etemplate <typename... T>struct Types {};

template <typename... T>struct F : Types<T...>, T... {};

struct X {};struct Y {};struct Z {};struct W { operator Y(); };

F f1 = {Types<X, Y, Z>{}, {}, {}}; // OK, F<X, Y, Z> deduced F f2 = {Types<X, Y, Z>{}, X{}, Y{}}; // OK, F<X, Y, Z> deduced F f3 = {Types<X, Y, Z>{}, X{}, W{}}; // error: conflicting types deduced; operator Y not considered — end example]

7

#

[Example 3: template <class T, class U> struct C { C(T, U); // #1};template<class T, class U> C(T, U) -> C<T, std::type_identity_t>; // #2template using A = C<V *, V *>;template<std::integral W> using B = A;

int i{};double d{}; A a1(&i, &i); // deduces A A a2(i, i); // error: cannot deduce V * from i A a3(&i, &d); // error: #1: cannot deduce (V*, V*) from (int *, double *)// #2: cannot deduce A from C<int *, double *> B b1(&i, &i); // deduces B B b2(&d, &d); // error: cannot deduce B from C<double *, double *>

Possible exposition-only implementation of the above procedure:// The following concept ensures a specialization of A is deduced.template class AA;template class AA<A> { };template concept deduces_A = requires { sizeof(AA); };

// f1 is formed from the constructor #1 of C, generating the following function templatetemplate<class T, class U>auto f1(T, U) -> C<T, U>;

// Deducing arguments for C<T, U> from C<V , V> deduces T as V * and U as V *;// f1' is obtained by transforming f1 as described by the above procedure.template requires deduces_A<C<V *, V *>>auto f1_prime(V , V) -> C<V *, V *>;

// f2 is formed from the deduction-guide #2 of Ctemplate<class T, class U> auto f2(T, U) -> C<T, std::type_identity_t>;

// Deducing arguments for C<T, std​::​type_identity_t> from C<V , V> deduces T as V *;// f2' is obtained by transforming f2 as described by the above procedure.template<class V, class U>requires deduces_A<C<V *, std::type_identity_t>>auto f2_prime(V *, U) -> C<V *, std::type_identity_t>;

// The following concept ensures a specialization of B is deduced.template class BB;template class BB<B> { };template concept deduces_B = requires { sizeof(BB); };

// The guides for B derived from the above f1' and f2' for A are as follows:template<std::integral W>requires deduces_A<C<W *, W *>> && deduces_B<C<W *, W *>>auto f1_prime_for_B(W *, W *) -> C<W *, W *>;

template<std::integral W, class U>requires deduces_A<C<W *, std::type_identity_t>> && deduces_B<C<W *, std::type_identity_t>>auto f2_prime_for_B(W *, U) -> C<W *, std::type_identity_t>;

— end example]

12.2.3 Viable functions [over.match.viable]

1

#

From the set of candidate functions constructed for a given context ([over.match.funcs]), a set of viable functions is chosen, from which the best function will be selected by comparing argument conversion sequences and associated constraints ([temp.constr.decl]) for the best fit ([over.match.best]).

The selection of viable functions considers associated constraints, if any, and relationships between arguments and function parameters other than the ranking of conversion sequences.

2

#

First, to be a viable function, a candidate function shall have enough parameters to agree in number with the arguments in the list.

• (2.1)

  If there are m arguments in the list, all candidate functions having exactly m parameters are viable.

• (2.2)

  A candidate function having fewer than m parameters is viable only if it has an ellipsis in its parameter list ([dcl.fct]). For the purposes of overload resolution, any argument for which there is no corresponding parameter is considered to “match the ellipsis” ([over.ics.ellipsis]).

• (2.3)

  A candidate function C having more than m parameters is viable only if the set of scopes G, as defined below, is not empty. G consists of every scope X that satisfies all of the following:

  • (2.3.1)

    There is a declaration of C, whose host scope is X, considered by the overload resolution.

  • (2.3.2)

    For every kth parameter P where k > m, there is a reachable declaration, whose host scope is X, that specifies a default argument ([dcl.fct.default]) for P.

  If C is selected as the best viable function ([over.match.best]):

  • (2.3.3)

    G shall contain exactly one scope (call it S).

  • (2.3.4)

    If the candidates are denoted by a splice-expression, then S shall not be a block scope.

  • (2.3.5)

    The default arguments used in the call to C are the default arguments specified by the reachable declarations whose host scope is S.

  For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.

[Example 1: namespace A {extern "C" void f(int, int = 5); extern "C" void f(int = 6, int);}namespace B {extern "C" void f(int, int = 7);}void use() {[:^^A::f:](3, 4); // OK, default argument was not used for viability:^^A::f:; // error: default argument provided by declarations from two scopes:^^A::f:; // OK, default arguments provided by declarations in the scope of Ausing A::f; using B::f; f(3, 4); // OK, default argument was not used for viability f(3); // error: default argument provided by declaration from two scopes f(); // OK, default arguments provided by declarations in the scope of Avoid g(int = 8); g(); // OK:^^g:; // error: host scope is block scope}void h(int = 7);constexpr std::meta::info r = ^^h;void poison() {void h(int = 8); h(); // OK, calls h(8):^^h:; // error: default argument provided by declarations from two scopes}void call_h() {:^^h:; // error: default argument provided by declarations from two scopes:r:; // error: default argument provided by declarations from two scopes}template<typename... Ts>int k(int = 3, Ts...);int i = k(); // error: no default argument for the second parameterint j = k<>(); // OK — end example]

3

#

Second, for a function to be viable, if it has associated constraints ([temp.constr.decl]), those constraints shall be satisfied ([temp.constr.constr]).

4

#

Third, forF to be a viable function, there shall exist for each argument animplicit conversion sequence that converts that argument to the corresponding parameter ofF.

If the parameter has reference type, the implicit conversion sequence includes the operation of binding the reference, and the fact that an lvalue reference to non-const cannot bind to an rvalue and that an rvalue reference cannot bind to an lvalue can affect the viability of the function (see [over.ics.ref]).

12.2.4 Best viable function [over.match.best]

12.2.4.1 General [over.match.best.general]

1

#

Define ICSi(F) as the implicit conversion sequence that converts the ith argument in the list to the type of the ith parameter of viable function F.

[over.best.ics] defines the implicit conversion sequences and [over.ics.rank] defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.

2

#

Given these definitions, a viable function F1 is defined to be abetter function than another viable function F2 if for all arguments i,ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then

• (2.1)

for some argument j,ICSj(F1) is a better conversion sequence thanICSj(F2), or, if not that,

• (2.2)

the context is an initialization by user-defined conversion (see [dcl.init],[over.match.conv], and [over.match.ref]) and the standard conversion sequence from the result of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the result of F2 to the destination type [Example 1: struct A { A(); operator int(); operator double();} a;int i = a; // a.operator int() followed by no conversion is better than// a.operator double() followed by a conversion to intfloat x = a; // ambiguous: both possibilities require conversions,// and neither is better than the other — end example] or, if not that,

• (2.3)

the context is an initialization by conversion function for direct reference binding of a reference to function type, the return type of F1 is the same kind of reference (lvalue or rvalue) as the reference being initialized, and the return type of F2 is not [Example 2: template struct A {operator T&(); // #1operator T&&(); // #2};typedef int Fn(); A a; Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 — end example] or, if not that,

• (2.4)

F1 is not a function template specialization andF2 is a function template specialization, or, if not that,

• (2.5)

F1 andF2 are function template specializations, and the function template forF1 is more specialized than the template forF2 according to the partial ordering rules described in [temp.func.order], or, if not that,

• (2.6)

F1 and F2 are non-template functions andF1 is more partial-ordering-constrained thanF2 ([temp.constr.order]) [Example 3: template struct S {constexpr void f(); // #1constexpr void f(this S&) requires true; // #2};

void test() { S<> s;

s.f(); // calls #2} — end example] or, if not that,

• (2.7)

F1 is a constructor for a class D,F2 is a constructor for a base class B of D, and for all arguments the corresponding parameters of F1 and F2 have the same type [Example 4: struct A { A(int = 0);};

struct B: A {using A::A;

B();};

int main() { B b; // OK, B​::​B()} — *end example*]

or, if not that,

• (2.8)

F2 is a rewritten candidate ([over.match.oper]) andF1 is not [Example 5: struct S {friend auto operator<=>(const S&, const S&) = default; // #1friend bool operator<(const S&, const S&); // #2};bool b = S() < S(); // calls #2 — end example] or, if not that,

• (2.9)

F1 and F2 are rewritten candidates, andF2 is a synthesized candidate with reversed order of parameters and F1 is not [Example 6: struct S {friend std::weak_ordering operator<=>(const S&, int); // #1friend std::weak_ordering operator<=>(int, const S&); // #2};bool b = 1 < S(); // calls #2 — end example] or, if not that,

• (2.10)

F1 and F2 are generated from class template argument deduction ([over.match.class.deduct]) for a class D, andF2 is generated from inheriting constructors from a base class of D while F1 is not, and for each explicit function argument, the corresponding parameters of F1 and F2 are either both ellipses or have the same type, or, if not that,

• (2.11)

F1 is generated from adeduction-guide ([over.match.class.deduct]) and F2 is not, or, if not that,

• (2.12)

F1 is the copy deduction candidate and F2 is not, or, if not that,

• (2.13)

F1 is generated from a non-template constructor and F2 is generated from a constructor template. [Example 7: template struct A {using value_type = T; A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3template A(int, T, U); // #4// #5 is the copy deduction candidate, A(A)};

A x(1, 2, 3); // uses #3, generated from a non-template constructortemplate <class T> A(T) -> A<T>; // #6, less specialized than #5 A a(42); // uses #6 to deduce A<int> and #1 to initialize A b = a; // uses #5 to deduce A<int> and #2 to initializetemplate <class T> A(A<T>) -> A<A<T>>; // #7, as specialized as #5 A b2 = a; // uses #7 to deduce A<A<int>> and #1 to initialize — *end example*]

3

#

If there is exactly one viable function that is a better function than all other viable functions, then it is the one selected by overload resolution; otherwise the call is ill-formed.105

[Example 8: void Fcn(const int*, short);void Fcn(int*, int);

int i;short s = 0;

void f() { Fcn(&i, s); // is ambiguous because &i → int* is better than &i → const int*// but s → short is also better than s → int Fcn(&i, 1L); // calls Fcn(int*, int), because &i → int* is better than &i → const int*// and 1L → short and 1L → int are indistinguishable Fcn(&i, 'c'); // calls Fcn(int*, int), because &i → int* is better than &i → const int*// and 'c' → int is better than 'c' → short} — end example]

4

#

[Note 1:

If the best viable function was made viable by one or more default arguments, additional requirements apply ([over.match.viable]).

— end note]

105)105)

The algorithm for selecting the best viable function is linear in the number of viable functions.

Run a simple tournament to find a functionW that is not worse than any opponent it faced.

Although it is possible that another functionF thatW did not face is at least as good asW,F cannot be the best function because at some point in the tournamentF encountered another functionG such thatF was not better thanG.

Hence, either W is the best function or there is no best function.

So, make a second pass over the viable functions to verify thatW is better than all other functions.

12.2.4.2 Implicit conversion sequences [over.best.ics]

12.2.4.2.1 General [over.best.ics.general]

1

#

An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called.

The sequence of conversions is an implicit conversion as defined in[conv], which means it is governed by the rules for initialization of an object or reference by a single expression ([dcl.init], [dcl.init.ref]).

2

#

Implicit conversion sequences are concerned only with the type, cv-qualification, and value category of the argument and how these are converted to match the corresponding properties of the parameter.

[Note 1:

Other properties, such as the lifetime, storage duration, linkage, alignment, accessibility of the argument, whether the argument is a bit-field, and whether a function is deleted, are ignored.

So, although an implicit conversion sequence can be defined for a given argument-parameter pair, the conversion from the argument to the parameter might still be ill-formed in the final analysis.

— end note]

3

#

A well-formed implicit conversion sequence is one of the following forms:

• (3.1)

a standard conversion sequence,

• (3.2)

a user-defined conversion sequence, or

• (3.3)

an ellipsis conversion sequence.

4

#

However, if the target is

• (4.1)

the first parameter of a constructor or

• (4.2)

the object parameter of a user-defined conversion function

and the constructor or user-defined conversion function is a candidate by

• (4.3)

[over.match.ctor], when the argument is the temporary in the second step of a class copy-initialization,

• (4.4)

[over.match.copy], [over.match.conv], or [over.match.ref] (in all cases), or

• (4.5)

the second phase of [over.match.list] when the initializer list has exactly one element that is itself an initializer list, and the target is the first parameter of a constructor of class X, and the conversion is to X or reference to cv X,

user-defined conversion sequences are not considered.

[Note 2:

These rules prevent more than one user-defined conversion from being applied during overload resolution, thereby avoiding infinite recursion.

— end note]

[Example 1: struct Y { Y(int); };struct A { operator int(); }; Y y1 = A(); // error: A​::​operator int() is not a candidatestruct X { X(); };struct B { operator X(); }; B b; X x{{b}}; // error: B​::​operator X() is not a candidate — end example]

5

#

For the case where the parameter type is a reference, see [over.ics.ref].

6

#

When the parameter type is not a reference, the implicit conversion sequence models a copy-initialization of the parameter from the argument expression.

The implicit conversion sequence is the one required to convert the argument expression to a prvalue of the type of the parameter.

[Note 3:

When the parameter has a class type, this is a conceptual conversion defined for the purposes of [over]; the actual initialization is defined in terms of constructors and is not a conversion.

— end note]

7

#

When the cv-unqualified version of the type of the argument expression is the same as the parameter type, the implicit conversion sequence is an identity conversion.

When the parameter has a class type and the argument expression has a (possibly cv-qualified) derived class type, the implicit conversion sequence is a derived-to-baseconversion from the derived class to the base class.

A derived-to-base conversion has Conversion rank ([over.ics.scs]).

[Note 4:

There is no such standard conversion; this derived-to-base conversion exists only in the description of implicit conversion sequences.

— end note]

[Example 2:

An implicit conversion sequence from an argument of type const A to a parameter of type A can be formed, even if overload resolution for copy-initialization of A from the argument would not find a viable function ([over.match.ctor], [over.match.viable]).

The implicit conversion sequence for that case is the identity sequence; it contains no “conversion” from const A to A.

— end example]

8

#

When the parameter is the implicit object parameter of a static member function, the implicit conversion sequence is a standard conversion sequence that is neither better nor worse than any other standard conversion sequence.

9

#

In all contexts, when converting to the implicit object parameter or when converting to the left operand of an assignment operation only standard conversion sequences are allowed.

[Note 5:

When a conversion to the explicit object parameter occurs, it can include user-defined conversion sequences.

— end note]

10

#

If no conversions are required to match an argument to a parameter type, the implicit conversion sequence is the standard conversion sequence consisting of the identity conversion ([over.ics.scs]).

11

#

If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.

12

#

If there are multiple well-formed implicit conversion sequences converting the argument to the parameter type, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated theambiguous conversion sequence.

For the purpose of ranking implicit conversion sequences as described in [over.ics.rank], the ambiguous conversion sequence is treated as a user-defined conversion sequence that is indistinguishable from any other user-defined conversion sequence.

[Note 6:

This rule prevents a function from becoming non-viable because of an ambiguous conversion sequence for one of its parameters.

[Example 3: class B;class A { A (B&);};class B { operator A (); };class C { C (B&); };void f(A) { }void f(C) { } B b; f(b); // error: ambiguous because there is a conversion b → C (via constructor)// and an (ambiguous) conversion b → A (via constructor or conversion function)void f(B) { } f(b); // OK, unambiguous — end example]

— end note]

If a function that uses the ambiguous conversion sequence is selected as the best viable function, the call will be ill-formed because the conversion of one of the arguments in the call is ambiguous.

13

#

The three forms of implicit conversion sequences mentioned above are defined in the following subclauses.

12.2.4.2.2 Standard conversion sequences [over.ics.scs]

1

#

Table 19 summarizes the conversions defined in [conv] and partitions them into four disjoint categories: Lvalue Transformation, Qualification Adjustment, Promotion, and Conversion.

[Note 1:

These categories are orthogonal with respect to value category, cv-qualification, and data representation: the Lvalue Transformations do not change the cv-qualification or data representation of the type; the Qualification Adjustments do not change the value category or data representation of the type; and the Promotions and Conversions do not change the value category or cv-qualification of the type.

— end note]

2

#

[Note 2:

As described in [conv], a standard conversion sequence either is the Identity conversion by itself (that is, no conversion) or consists of one to three conversions from the other four categories.

If there are two or more conversions in the sequence, the conversions are applied in the canonical order:Lvalue Transformation,Promotion orConversion,Qualification Adjustment.

— end note]

3

#

Each conversion in Table 19 also has an associated rank (Exact Match, Promotion, or Conversion).

These are used to rank standard conversion sequences.

The rank of a conversion sequence is determined by considering the rank of each conversion in the sequence and the rank of any reference binding.

If any of those has Conversion rank, the sequence has Conversion rank; otherwise, if any of those has Promotion rank, the sequence has Promotion rank; otherwise, the sequence has Exact Match rank.

Table 19 — Conversions [tab:over.ics.scs]

🔗 Conversion	Category	Rank	Subclause
🔗 No conversions required	Identity
🔗 Lvalue-to-rvalue conversion			[conv.lval]
🔗 Array-to-pointer conversion	Lvalue Transformation		[conv.array]
🔗 Function-to-pointer conversion		Exact Match	[conv.func]
🔗 Qualification conversions			[conv.qual]
🔗 Function pointer conversion	Qualification Adjustment		[conv.fctptr]
🔗 Integral promotions			[conv.prom]
🔗 Floating-point promotion	Promotion	Promotion	[conv.fpprom]
🔗 Integral conversions			[conv.integral]
🔗 Floating-point conversions			[conv.double]
🔗 Floating-integral conversions			[conv.fpint]
🔗 Pointer conversions	Conversion	Conversion	[conv.ptr]
🔗 Pointer-to-member conversions			[conv.mem]
🔗 Boolean conversions			[conv.bool]

12.2.4.2.3 User-defined conversion sequences [over.ics.user]

1

#

A user-defined conversion sequence consists of an initial standard conversion sequence followed by a user-defined conversion ([class.conv]) followed by a second standard conversion sequence.

If the user-defined conversion is specified by a constructor ([class.conv.ctor]), the initial standard conversion sequence converts the source type to the type of the first parameter of that constructor.

If the user-defined conversion is specified by a conversion function, the initial standard conversion sequence converts the source type to the type of the object parameter of that conversion function.

2

#

The second standard conversion sequence converts the result of the user-defined conversion to the target type for the sequence; any reference binding is included in the second standard conversion sequence.

Since an implicit conversion sequence is an initialization, the special rules for initialization by user-defined conversion apply when selecting the best user-defined conversion for a user-defined conversion sequence (see [over.match.best] and [over.best.ics]).

3

#

If the user-defined conversion is specified by a specialization of a conversion function template, the second standard conversion sequence shall have Exact Match rank.

4

#

A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a constructor (i.e., a user-defined conversion function) is called for those cases.

12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]

1

#

An ellipsis conversion sequence occurs when an argument in a function call is matched with the ellipsis parameter specification of the function called (see [expr.call]).

12.2.4.2.5 Reference binding [over.ics.ref]

1

#

When a parameter of type “reference to cv T” binds directly ([dcl.init.ref]) to an argument expression:

• (1.1)

  If the argument expression has a type that is a derived class of the parameter type, the implicit conversion sequence is a derived-to-base conversion ([over.best.ics]).

• (1.2)

  Otherwise, if the type of the argument is possibly cv-qualified T, or if T is an array type of unknown bound with element type U and the argument has an array type of known bound whose element type is possibly cv-qualified U, the implicit conversion sequence is the identity conversion.

• (1.3)

  Otherwise, if T is a function type, the implicit conversion sequence is a function pointer conversion.

• (1.4)

  Otherwise, the implicit conversion sequence is a qualification conversion.

[Example 1: struct A {};struct B : public A {} b;int f(A&);int f(B&);int i = f(b); // calls f(B&), an exact match, rather than f(A&), a conversionvoid g() noexcept;int h(void (&)() noexcept); // #1int h(void (&)()); // #2int j = h(g); // calls #1, an exact match, rather than #2, a function pointer conversion — end example]

If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence ([over.ics.user]) whose second standard conversion sequence is determined by the above rules.

2

#

When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the referenced type according to [over.best.ics].

Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the referenced type with the argument expression.

Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.

3

#

Except for an implicit object parameter, for which see [over.match.funcs], an implicit conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue of object type.

[Note 1:

This means, for example, that a candidate function cannot be a viable function if it has a non-const lvalue reference parameter (other than the implicit object parameter) and the corresponding argument would require a temporary to be created to initialize the lvalue reference (see [dcl.init.ref]).

— end note]

4

#

Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of an implicit conversion sequence, however.

[Example 2:

A function with an “lvalue reference to int” parameter can be a viable candidate even if the corresponding argument is anint bit-field.

The formation of implicit conversion sequences treats theint bit-field as anint lvalue and finds an exact match with the parameter.

If the function is selected by overload resolution, the call will nonetheless be ill-formed because of the prohibition on binding a non-const lvalue reference to a bit-field ([dcl.init.ref]).

— end example]

12.2.4.2.6 List-initialization sequence [over.ics.list]

1

#

When an argument is an initializer list ([dcl.init.list]), it is not an expression and special rules apply for converting it to a parameter type.

2

#

If the initializer list is a designated-initializer-list and the parameter is not a reference, a conversion is only possible if the parameter has an aggregate type that can be initialized from the initializer list according to the rules for aggregate initialization ([dcl.init.aggr]), in which case the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.

[Note 1:

Aggregate initialization does not require that the members are declared in designation order.

If, after overload resolution, the order does not match for the selected overload, the initialization of the parameter will be ill-formed ([dcl.init.list]).

[Example 1: struct A { int x, y; };struct B { int y, x; };void f(A a, int); // #1void f(B b, ...); // #2void g(A a); // #3void g(B b); // #4void h() { f({.x = 1, .y = 2}, 0); // OK; calls #1 f({.y = 2, .x = 1}, 0); // error: selects #1, initialization of a fails// due to non-matching member order ([dcl.init.list]) g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4} — end example]

— end note]

3

#

Otherwise, if the parameter type is an aggregate class X and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence is the one required to convert the element to the parameter type.

4

#

Otherwise, if the parameter type is a character array106 and the initializer list has a single element that is an appropriately-typedstring-literal ([dcl.init.string]), the implicit conversion sequence is the identity conversion.

5

#

Otherwise, if the parameter type is std​::​initializer_list and all the elements of the initializer list can be implicitly converted to X, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X, or if the initializer list has no elements, the identity conversion.

This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor.

[Example 2: void f(std::initializer_list); f( {} ); // OK, f(initializer_list) identity conversion f( {1,2,3} ); // OK, f(initializer_list) identity conversion f( {'a','b'} ); // OK, f(initializer_list) integral promotion f( {1.0} ); // error: narrowingstruct A { A(std::initializer_list); // #1 A(std::initializer_list<std::complex>); // #2 A(std::initializer_liststd::string); // #3}; A a{ 1.0,2.0 }; // OK, uses #1void g(A); g({ "foo", "bar" }); // OK, uses #3typedef int IA[3];void h(const IA&); h({ 1, 2, 3 }); // OK, identity conversion — end example]

6

#

Otherwise, if the parameter type is “array of N X” or “array of unknown bound of X”, if there exists an implicit conversion sequence from each element of the initializer list (and from {} in the former case if N exceeds the number of elements in the initializer list) to X, the implicit conversion sequence is the worst such implicit conversion sequence.

7

#

Otherwise, if the parameter is a non-aggregate class X and overload resolution per [over.match.list] chooses a single best constructor C ofX to perform the initialization of an object of type X from the argument initializer list:

• (7.1)

  If C is not an initializer-list constructor and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence has Exact Match rank if U is X, or Conversion rank if U is derived from X.

• (7.2)

  Otherwise, the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.

If multiple constructors are viable but none is better than the others, the implicit conversion sequence is the ambiguous conversion sequence.

User-defined conversions are allowed for conversion of the initializer list elements to the constructor parameter types except as noted in [over.best.ics].

[Example 3: struct A { A(std::initializer_list);};void f(A); f( {'a', 'b'} ); // OK, f(A(std​::​initializer_list)) user-defined conversionstruct B { B(int, double);};void g(B); g( {'a', 'b'} ); // OK, g(B(int, double)) user-defined conversion g( {1.0, 1.0} ); // error: narrowingvoid f(B); f( {'a', 'b'} ); // error: ambiguous f(A) or f(B)struct C { C(std::string);};void h(C); h({"foo"}); // OK, h(C(std​::​string("foo")))struct D { D(A, C);};void i(D); i({ {1,2}, {"bar"} }); // OK, i(D(A(std​::​initializer_list{1,2}), C(std​::​string("bar")))) — end example]

8

#

Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization ([dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence whose second standard conversion sequence is an identity conversion.

[Example 4: struct A {int m1; double m2;};

void f(A); f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowing — end example]

9

#

Otherwise, if the parameter is a reference, see [over.ics.ref].

[Note 2:

The rules in this subclause will apply for initializing the underlying temporary for the reference.

— end note]

[Example 5: struct A {int m1; double m2;};

void f(const A&); f( {'a', 'b'} ); // OK, f(A(int,double)) user-defined conversion f( {1.0} ); // error: narrowingvoid g(const double &); g({1}); // same conversion as int to double — end example]

10

#

Otherwise, if the parameter type is not a class:

• (10.1)

if the initializer list has one element that is not itself an initializer list, the implicit conversion sequence is the one required to convert the element to the parameter type; [Example 6: void f(int); f( {'a'} ); // OK, same conversion as char to int f( {1.0} ); // error: narrowing — end example]

• (10.2)

if the initializer list has no elements, the implicit conversion sequence is the identity conversion. [Example 7: void f(int); f( { } ); // OK, identity conversion — end example]

11

#

In all cases other than those enumerated above, no conversion is possible.

106)106)

Since there are no parameters of array type, this will only occur as the referenced type of a reference parameter.

12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]

1

#

This subclause defines a partial ordering of implicit conversion sequences based on the relationshipsbetter conversion sequence andbetter conversion.

If an implicit conversion sequence S1 is defined by these rules to be a better conversion sequence than S2, then it is also the case that S2 is aworse conversion sequence than S1.

If conversion sequence S1 is neither better than nor worse than conversion sequence S2, S1 and S2 are said to beindistinguishable conversion sequences.

2

#

When comparing the basic forms of implicit conversion sequences (as defined in [over.best.ics])

• (2.1)

a standard conversion sequence is a better conversion sequence than a user-defined conversion sequence or an ellipsis conversion sequence, and

• (2.2)

a user-defined conversion sequence is a better conversion sequence than an ellipsis conversion sequence.

3

#

Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

• (3.1)

  List-initialization sequence L1 is a better conversion sequence than list-initialization sequence L2 if

  • (3.1.1)

L1 converts to std​::​initializer_list for some X andL2 does not, or, if not that,

• (3.1.2)

L1 and L2 convert to arrays of the same element type, and either the number of elements n1 initialized by L1 is less than the number of elements n2 initialized by L2, orn1=n2 andL2 converts to an array of unknown bound and L1 does not,

even if one of the other rules in this paragraph would otherwise apply. [Example 1: void f1(int); // #1void f1(std::initializer_list); // #2void g1() { f1({42}); } // chooses #2void f2(std::pair<const char*, const char*>); // #3void f2(std::initializer_liststd::string); // #4void g2() { f2({"foo","bar"}); } // chooses #4 — end example] [Example 2: void f(int (&&)[] ); // #1void f(double (&&)[] ); // #2void f(int (&&)[2]); // #3 f( {1} ); // Calls #1: Better than #2 due to conversion, better than #3 due to bounds f( {1.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1.0, 2.0} ); // Calls #2: Identity conversion is better than floating-integral conversion f( {1, 2} ); // Calls #3: Converting to array of known bound is better than to unknown bound,// and an identity conversion is better than floating-integral conversion — end example]

• (3.2)

  Standard conversion sequenceS1 is a better conversion sequence than standard conversion sequenceS2 if

  • (3.2.1)

S1 is a proper subsequence ofS2 (comparing the conversion sequences in the canonical form defined by [over.ics.scs], excluding any Lvalue Transformation; the identity conversion sequence is considered to be a subsequence of any non-identity conversion sequence) or, if not that,

• (3.2.2)

the rank ofS1 is better than the rank ofS2, orS1 andS2 have the same rank and are distinguishable by the rules in the paragraph below, or, if not that,

• (3.2.3)

S1 and S2 include reference bindings ([dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and S1 binds an rvalue reference to an rvalue and S2 binds an lvalue reference [Example 3: int i;int f1();int&& f2();int g(const int&);int g(const int&&);int j = g(i); // calls g(const int&)int k = g(f1()); // calls g(const int&&)int l = g(f2()); // calls g(const int&&)struct A { A& operator<<(int); void p() &; void p() &&;}; A& operator<<(A&&, char); A() << 1; // calls A​::​operator<<(int) A() << 'c'; // calls operator<<(A&&, char) A a; a << 1; // calls A​::​operator<<(int) a << 'c'; // calls A​::​operator<<(int) A().p(); // calls A​::​p()&& a.p(); // calls A​::​p()& — end example] or, if not that,

• (3.2.4)

S1 and S2 include reference bindings ([dcl.init.ref]) andS1 binds an lvalue reference to an lvalue of function type andS2 binds an rvalue reference to an lvalue of function type [Example 4: int f(void(&)()); // #1int f(void(&&)()); // #2void g();int i1 = f(g); // calls #1 — end example] or, if not that,

• (3.2.5)

S1 and S2 differ only in their qualification conversion ([conv.qual]) and yield similar types T1 and T2, respectively (where a standard conversion sequence that is a reference binding is considered to yield the cv-unqualified referenced type), where T1 and T2 are not the same type, andconst T2 is reference-compatible with T1 ([dcl.init.ref]) [Example 5: int f(const volatile int );int f(const int );int i;int j = f(&i); // calls f(const int)int g(const int);int g(const volatile int* const&);int* p;int k = g(p); // calls g(const int*) — end example] or, if not that,

• (3.2.6)

S1 andS2 bind “reference to T1” and “reference to T2”, respectively ([dcl.init.ref]), where T1 and T2 are not the same type, andT2 is reference-compatible with T1 [Example 6: int f(const int &);int f(int &);int g(const int &);int g(int);

    int i;int j = f(i); // calls f(int &)int k = g(i); // ambiguousstruct X {void f() const; void f();};void g(const X& a, X b) { a.f(); // calls X​::​f() const b.f(); // calls X​::​f()}int h(int (&)[]);int h(int (&)[1]);void g2() {int a[1];

h(a); // calls h(int (&)[1])} — end example] or, if not that,

• (3.2.7)

S1 and S2 bind the same reference type “reference to T” and have source types V1 and V2, respectively, where the standard conversion sequence from V1* to T* is better than the standard conversion sequence from V2* to T*. [Example 7: struct Z {};

    struct A {operator Z&(); operator const Z&(); // #1};

    struct B {operator Z(); operator const Z&&(); // #2};

    const Z& r1 = A(); // OK, uses #1const Z&& r2 = B(); // OK, uses #2 — *end example*]

• (3.3)

  User-defined conversion sequenceU1 is a better conversion sequence than another user-defined conversion sequenceU2 if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence ofU1 is better than the second standard conversion sequence ofU2. [Example 8: struct A {operator short();} a;int f(int);int f(float);int i = f(a); // calls f(int), because short → int is// better than short → float. — end example]

4

#

Standard conversion sequences are ordered by their ranks: an Exact Match is a better conversion than a Promotion, which is a better conversion than a Conversion.

Two conversion sequences with the same rank are indistinguishable unless one of the following rules applies:

• (4.1)

  A conversion that does not convert a pointer or a pointer to member tobool is better than one that does.

• (4.2)

  A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.

• (4.3)

  A conversion in either direction between floating-point type FP1 and floating-point type FP2 is better than a conversion in the same direction between FP1 and arithmetic type T3 if

  • (4.3.1)

the floating-point conversion rank ([conv.rank]) of FP1 is equal to the rank of FP2, and

• (4.3.2)

T3 is not a floating-point type, orT3 is a floating-point type whose rank is not equal to the rank of FP1, or the floating-point conversion subrank ([conv.rank]) of FP2 is greater than the subrank of T3. [Example 9: int f(std::float32_t);int f(std::float64_t);int f(long long);float x; std::float16_t y;int i = f(x); // calls f(std​::​float32_t) on implementations where// float and std​::​float32_t have equal conversion ranksint j = f(y); // error: ambiguous, no equal conversion rank — end example]

• (4.4)

  If classB is derived directly or indirectly from classA, conversion ofB* toA* is better than conversion ofB* tovoid*, and conversion ofA* tovoid* is better than conversion ofB* tovoid*.

• (4.5)

  If classB is derived directly or indirectly from classA and classC is derived directly or indirectly fromB,

  • (4.5.1)

conversion ofC* toB* is better than conversion ofC* toA*, [Example 10: struct A {};struct B : public A {};struct C : public B {}; C* pc;int f(A*);int f(B*);int i = f(pc); // calls f(B*) — end example]

• (4.5.2)

binding of an expression of typeC to a reference to typeB is better than binding an expression of typeC to a reference to typeA,

• (4.5.3)

conversion ofA​::​* toB​::​* is better than conversion ofA​::​* toC​::​*,

• (4.5.4)

conversion ofC toB is better than conversion ofC toA,

• (4.5.5)

conversion ofB* toA* is better than conversion ofC* toA*,

• (4.5.6)

binding of an expression of typeB to a reference to typeA is better than binding an expression of typeC to a reference to typeA,

• (4.5.7)

conversion ofB​::​* toC​::​* is better than conversion ofA​::​* toC​::​*, and

• (4.5.8)

conversion ofB toA is better than conversion ofC toA.

[Note 1: Compared conversion sequences will have different source types only in the context of comparing the second standard conversion sequence of an initialization by user-defined conversion (see [over.match.best]); in all other contexts, the source types will be the same and the target types will be different. — end note]