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[over.match.best]
# 12 Overloading [[over]](./#over)
## 12.2 Overload resolution [[over.match]](over.match#best)
### 12.2.4 Best viable function [over.match.best]
#### [12.2.4.1](#general) General [[over.match.best.general]](over.match.best.general)
[1](#general-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1808)
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[.](#general-1.sentence-1)
[[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences") defines the implicit conversion sequences and [[over.ics.rank]](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences") defines what it means for one implicit conversion sequence to be
a better conversion sequence or worse conversion sequence than
another[.](#general-1.sentence-2)
[2](#general-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L1819)
Given these definitions,
a viable function F1 is defined to be a[*better*](#def:overloading,resolution,better_viable_function "12.2.4.1 General [over.match.best.general]") 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)](#general-2.1)
for some argument j,ICSj(F1) is a better conversion sequence thanICSj(F2), or, if not that,
- [(2.2)](#general-2.2)
the context is an initialization by user-defined conversion
(see [[dcl.init]](dcl.init "9.5 Initializers"),[[over.match.conv]](over.match.conv "12.2.2.6 Initialization by conversion function"), and [[over.match.ref]](over.match.ref "12.2.2.7 Initialization by conversion function for direct reference binding"))
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](#general-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)](#general-2.3)
the context is an initialization by conversion function for [direct
reference binding](over.match.ref "12.2.2.7 Initialization by conversion function for direct reference binding [over.match.ref]") 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](#general-example-2)*: template <class T> struct A {operator T&(); // #1operator T&&(); // #2};typedef int Fn();
A<Fn> a;
Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 — *end example*]
or, if not that,
- [(2.4)](#general-2.4)
F1 is not a function template specialization andF2 is a
function template
specialization, or, if not that,
- [(2.5)](#general-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]](temp.func.order "13.7.7.3 Partial ordering of function templates"),
or, if not that,
- [(2.6)](#general-2.6)
F1 and F2 are non-template functions andF1 is more partial-ordering-constrained thanF2 ([[temp.constr.order]](temp.constr.order "13.5.5 Partial ordering by constraints"))
[*Example [3](#general-example-3)*: template <typename T = int>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)](#general-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](#general-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)](#general-2.8)
F2 is a rewritten candidate ([[over.match.oper]](over.match.oper "12.2.2.3 Operators in expressions")) andF1 is not
[*Example [5](#general-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)](#general-2.9)
F1 and F2 are rewritten candidates, andF2 is a synthesized candidate
with reversed order of parameters
and F1 is not
[*Example [6](#general-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)](#general-2.10)
F1 and F2 are generated
from class template argument deduction ([[over.match.class.deduct]](over.match.class.deduct "12.2.2.9 Class template argument deduction"))
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)](#general-2.11)
F1 is generated from a[*deduction-guide*](temp.deduct.guide#nt:deduction-guide "13.7.2.3 Deduction guides [temp.deduct.guide]") ([[over.match.class.deduct]](over.match.class.deduct "12.2.2.9 Class template argument deduction"))
and F2 is not, or, if not that,
- [(2.12)](#general-2.12)
F1 is the [copy deduction candidate](over.match.class.deduct#def:copy_deduction_candidate "12.2.2.9 Class template argument deduction [over.match.class.deduct]") and F2 is not, or, if not that,
- [(2.13)](#general-2.13)
F1 is generated from a non-template constructor
and F2 is generated from a constructor template[.](#general-2.sentence-1)
[*Example [7](#general-example-7)*: template <class T> struct A {using value_type = T;
A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3template<class U> 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](#general-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2022)
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[.](#general-3.sentence-1)[105](#footnote-105 "The algorithm for selecting the best viable function is linear in the number of viable functions. Run a simple tournament to find a function W that is not worse than any opponent it faced. Although it is possible that another function F that W did not face is at least as good as W, F cannot be the best function because at some point in the tournament F encountered another function G such that F was not better than G. Hence, either W is the best function or there is no best function. So, make a second pass over the viable functions to verify that W is better than all other functions.")
[*Example [8](#general-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](#general-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2083)
[*Note [1](#general-note-1)*:
If the best viable function was made viable by one or more default arguments,
additional requirements apply ([[over.match.viable]](over.match.viable "12.2.3 Viable functions"))[.](#general-4.sentence-1)
— *end note*]
[105)](#footnote-105)[105)](#footnoteref-105)
The algorithm
for selecting the best viable function is linear in the number
of viable
functions[.](#footnote-105.sentence-1)
Run a simple tournament to find a functionW that is not
worse than any
opponent it faced[.](#footnote-105.sentence-2)
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[.](#footnote-105.sentence-3)
Hence,
either W is
the best function or there is no best function[.](#footnote-105.sentence-4)
So, make a second pass over
the viable
functions to verify thatW is better than all other functions[.](#footnote-105.sentence-5)
#### [12.2.4.2](#over.best.ics) Implicit conversion sequences [[over.best.ics]](over.best.ics)
#### [12.2.4.2.1](#over.best.ics.general) General [[over.best.ics.general]](over.best.ics.general)
[1](#over.best.ics.general-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2095)
An [*implicit conversion sequence*](#def:conversion_sequence,implicit "12.2.4.2.1 General [over.best.ics.general]") 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[.](#over.best.ics.general-1.sentence-1)
The
sequence of conversions is an implicit conversion as defined in[[conv]](conv "7.3 Standard conversions"), which means it is governed by the rules for
initialization of an object or reference by a single
expression ([[dcl.init]](dcl.init "9.5 Initializers"), [[dcl.init.ref]](dcl.init.ref "9.5.4 References"))[.](#over.best.ics.general-1.sentence-2)
[2](#over.best.ics.general-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2106)
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[.](#over.best.ics.general-2.sentence-1)
[*Note [1](#over.best.ics.general-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](dcl.fct.def.delete "9.6.3 Deleted definitions [dcl.fct.def.delete]"), are ignored[.](#over.best.ics.general-2.sentence-2)
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[.](#over.best.ics.general-2.sentence-3)
— *end note*]
[3](#over.best.ics.general-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2121)
A
well-formed implicit conversion
sequence is one of the following forms:
- [(3.1)](#over.best.ics.general-3.1)
a [standard conversion sequence](#over.ics.scs "12.2.4.2.2 Standard conversion sequences [over.ics.scs]"),
- [(3.2)](#over.best.ics.general-3.2)
a [user-defined conversion sequence](#over.ics.user "12.2.4.2.3 User-defined conversion sequences [over.ics.user]"), or
- [(3.3)](#over.best.ics.general-3.3)
an [ellipsis conversion sequence](#over.ics.ellipsis "12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]")[.](#over.best.ics.general-3.sentence-1)
[4](#over.best.ics.general-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2134)
However, if the target is
- [(4.1)](#over.best.ics.general-4.1)
the first parameter of a constructor or
- [(4.2)](#over.best.ics.general-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.best.ics.general-4.3)
[[over.match.ctor]](over.match.ctor "12.2.2.4 Initialization by constructor"), when the argument is the temporary in the second
step of a class copy-initialization,
- [(4.4)](#over.best.ics.general-4.4)
[[over.match.copy]](over.match.copy "12.2.2.5 Copy-initialization of class by user-defined conversion"), [[over.match.conv]](over.match.conv "12.2.2.6 Initialization by conversion function"), or [[over.match.ref]](over.match.ref "12.2.2.7 Initialization by conversion function for direct reference binding") (in all cases), or
- [(4.5)](#over.best.ics.general-4.5)
the second phase of [[over.match.list]](over.match.list "12.2.2.8 Initialization by list-initialization") 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[.](#over.best.ics.general-4.sentence-1)
[*Note [2](#over.best.ics.general-note-2)*:
These rules prevent more than one user-defined conversion from being
applied during overload resolution, thereby avoiding infinite recursion[.](#over.best.ics.general-4.sentence-2)
— *end note*]
[*Example [1](#over.best.ics.general-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](#over.best.ics.general-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2170)
For the case where the parameter type is a reference, see [[over.ics.ref]](#over.ics.ref "12.2.4.2.5 Reference binding")[.](#over.best.ics.general-5.sentence-1)
[6](#over.best.ics.general-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2173)
When the parameter type is not a reference, the implicit conversion
sequence models a copy-initialization of the parameter from the argument
expression[.](#over.best.ics.general-6.sentence-1)
The implicit conversion sequence is the one required to convert the
argument expression to a prvalue of the type of
the parameter[.](#over.best.ics.general-6.sentence-2)
[*Note [3](#over.best.ics.general-note-3)*:
When the parameter has a class type, this is a conceptual conversion
defined for the purposes of [[over]](over "12 Overloading"); the actual initialization is
defined in terms of constructors and is not a conversion[.](#over.best.ics.general-6.sentence-3)
— *end note*]
[7](#over.best.ics.general-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2186)
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[.](#over.best.ics.general-7.sentence-1)
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[.](#over.best.ics.general-7.sentence-2)
A derived-to-base conversion has Conversion rank ([[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"))[.](#over.best.ics.general-7.sentence-3)
[*Note [4](#over.best.ics.general-note-4)*:
There is no such standard conversion; this derived-to-base conversion exists
only in the description of implicit conversion sequences[.](#over.best.ics.general-7.sentence-4)
— *end note*]
[*Example [2](#over.best.ics.general-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.ctor "12.2.2.4 Initialization by constructor"), [[over.match.viable]](over.match.viable "12.2.3 Viable functions"))[.](#over.best.ics.general-7.sentence-5)
The implicit conversion sequence for that case is the identity sequence; it
contains no “conversion” from const A to A[.](#over.best.ics.general-7.sentence-6)
— *end example*]
[8](#over.best.ics.general-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2210)
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[.](#over.best.ics.general-8.sentence-1)
[9](#over.best.ics.general-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2215)
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[.](#over.best.ics.general-9.sentence-1)
[*Note [5](#over.best.ics.general-note-5)*:
When a conversion to the explicit object parameter occurs,
it can include user-defined conversion sequences[.](#over.best.ics.general-9.sentence-2)
— *end note*]
[10](#over.best.ics.general-10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2224)
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]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"))[.](#over.best.ics.general-10.sentence-1)
[11](#over.best.ics.general-11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2229)
If no sequence of conversions can be found to convert an argument
to a parameter type, an implicit conversion sequence cannot be formed[.](#over.best.ics.general-11.sentence-1)
[12](#over.best.ics.general-12)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2233)
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 the[*ambiguous conversion sequence*](#def:conversion_sequence,ambiguous "12.2.4.2.1 General [over.best.ics.general]")[.](#over.best.ics.general-12.sentence-1)
For the purpose of ranking implicit conversion sequences as described
in [[over.ics.rank]](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences"), the ambiguous conversion sequence is treated
as a user-defined conversion sequence that is indistinguishable from any
other user-defined conversion sequence[.](#over.best.ics.general-12.sentence-2)
[*Note [6](#over.best.ics.general-note-6)*:
This rule prevents a function from becoming non-viable because of an ambiguous
conversion sequence for one of its parameters[.](#over.best.ics.general-12.sentence-3)
[*Example [3](#over.best.ics.general-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[.](#over.best.ics.general-12.sentence-4)
[13](#over.best.ics.general-13)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2266)
The three forms of implicit conversion sequences mentioned above
are defined in the following subclauses[.](#over.best.ics.general-13.sentence-1)
#### [12.2.4.2.2](#over.ics.scs) Standard conversion sequences [[over.ics.scs]](over.ics.scs)
[1](#over.ics.scs-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2272)
Table [19](#tab:over.ics.scs "Table 19: Conversions") summarizes the conversions defined in [[conv]](conv "7.3 Standard conversions") and
partitions them into four disjoint categories: Lvalue Transformation,
Qualification Adjustment, Promotion, and Conversion[.](#over.ics.scs-1.sentence-1)
[*Note [1](#over.ics.scs-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[.](#over.ics.scs-1.sentence-2)
— *end note*]
[2](#over.ics.scs-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2287)
[*Note [2](#over.ics.scs-note-2)*:
As described in [[conv]](conv "7.3 Standard conversions"),
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[.](#over.ics.scs-2.sentence-1)
If there are two or more conversions in the sequence, the
conversions are applied in the canonical order:**Lvalue Transformation**,**Promotion** or**Conversion**,**Qualification Adjustment**[.](#over.ics.scs-2.sentence-2)
— *end note*]
[3](#over.ics.scs-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2303)
Each conversion in Table [19](#tab:over.ics.scs "Table 19: Conversions") also has an associated rank (Exact
Match, Promotion, or Conversion)[.](#over.ics.scs-3.sentence-1)
These are used
to [rank standard conversion sequences](#over.ics.rank "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.scs-3.sentence-2)
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](#over.ics.ref "12.2.4.2.5 Reference binding [over.ics.ref]")[.](#over.ics.scs-3.sentence-3)
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[.](#over.ics.scs-3.sentence-4)
Table [19](#tab:over.ics.scs) — Conversions [[tab:over.ics.scs]](./tab:over.ics.scs)
| [🔗](#tab:over.ics.scs-row-1)<br>**Conversion** | **Category** | **Rank** | **Subclause** |
| --- | --- | --- | --- |
| [🔗](#tab:over.ics.scs-row-2)<br>No conversions required | Identity | | |
| [🔗](#tab:over.ics.scs-row-3)<br> Lvalue-to-rvalue conversion | | | [[conv.lval]](conv.lval "7.3.2 Lvalue-to-rvalue conversion") |
| [🔗](#tab:over.ics.scs-row-4)<br> Array-to-pointer conversion | Lvalue Transformation | | [[conv.array]](conv.array "7.3.3 Array-to-pointer conversion") |
| [🔗](#tab:over.ics.scs-row-5)<br> Function-to-pointer conversion | | Exact Match | [[conv.func]](conv.func "7.3.4 Function-to-pointer conversion") |
| [🔗](#tab:over.ics.scs-row-6)<br> Qualification conversions | | | [[conv.qual]](conv.qual "7.3.6 Qualification conversions") |
| [🔗](#tab:over.ics.scs-row-7)<br> Function pointer conversion | Qualification Adjustment | | [[conv.fctptr]](conv.fctptr "7.3.14 Function pointer conversions") |
| [🔗](#tab:over.ics.scs-row-8)<br>Integral promotions | | | [[conv.prom]](conv.prom "7.3.7 Integral promotions") |
| [🔗](#tab:over.ics.scs-row-9)<br> Floating-point promotion | Promotion | Promotion | [[conv.fpprom]](conv.fpprom "7.3.8 Floating-point promotion") |
| [🔗](#tab:over.ics.scs-row-10)<br>Integral conversions | | | [[conv.integral]](conv.integral "7.3.9 Integral conversions") |
| [🔗](#tab:over.ics.scs-row-11)<br> Floating-point conversions | | | [[conv.double]](conv.double "7.3.10 Floating-point conversions") |
| [🔗](#tab:over.ics.scs-row-12)<br> Floating-integral conversions | | | [[conv.fpint]](conv.fpint "7.3.11 Floating-integral conversions") |
| [🔗](#tab:over.ics.scs-row-13)<br> Pointer conversions | Conversion | Conversion | [[conv.ptr]](conv.ptr "7.3.12 Pointer conversions") |
| [🔗](#tab:over.ics.scs-row-14)<br> Pointer-to-member conversions | | | [[conv.mem]](conv.mem "7.3.13 Pointer-to-member conversions") |
| [🔗](#tab:over.ics.scs-row-15)<br> Boolean conversions | | | [[conv.bool]](conv.bool "7.3.15 Boolean conversions") |
#### [12.2.4.2.3](#over.ics.user) User-defined conversion sequences [[over.ics.user]](over.ics.user)
[1](#over.ics.user-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2339)
A [*user-defined conversion sequence*](#def:conversion_sequence,user-defined "12.2.4.2.3 User-defined conversion sequences [over.ics.user]") consists of an initial
standard conversion sequence followed by a user-defined
conversion ([[class.conv]](class.conv "11.4.8 Conversions")) followed by a second standard
conversion sequence[.](#over.ics.user-1.sentence-1)
If the user-defined conversion is specified
by a constructor ([[class.conv.ctor]](class.conv.ctor "11.4.8.2 Conversion by constructor")), the initial standard
conversion sequence converts the source type to the type of the
first parameter of that constructor[.](#over.ics.user-1.sentence-2)
If the user-defined
conversion is specified by a [conversion function](class.conv.fct "11.4.8.3 Conversion functions [class.conv.fct]"), the
initial standard conversion sequence
converts the source type to the type of the
object parameter of that conversion function[.](#over.ics.user-1.sentence-3)
[2](#over.ics.user-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2354)
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[.](#over.ics.user-2.sentence-1)
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]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))[.](#over.ics.user-2.sentence-2)
[3](#over.ics.user-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2364)
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[.](#over.ics.user-3.sentence-1)
[4](#over.ics.user-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2369)
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[.](#over.ics.user-4.sentence-1)
#### [12.2.4.2.4](#over.ics.ellipsis) Ellipsis conversion sequences [[over.ics.ellipsis]](over.ics.ellipsis)
[1](#over.ics.ellipsis-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2380)
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]](expr.call "7.6.1.3 Function call"))[.](#over.ics.ellipsis-1.sentence-1)
#### [12.2.4.2.5](#over.ics.ref) Reference binding [[over.ics.ref]](over.ics.ref)
[1](#over.ics.ref-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2388)
When a parameter of type “reference to cv T”
binds directly ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) to an argument expression:
- [(1.1)](#over.ics.ref-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]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))[.](#over.ics.ref-1.1.sentence-1)
- [(1.2)](#over.ics.ref-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[.](#over.ics.ref-1.2.sentence-1)
- [(1.3)](#over.ics.ref-1.3)
Otherwise,
if T is a function type,
the implicit conversion sequence is a function pointer conversion[.](#over.ics.ref-1.3.sentence-1)
- [(1.4)](#over.ics.ref-1.4)
Otherwise, the implicit conversion sequence is a qualification conversion[.](#over.ics.ref-1.4.sentence-1)
[*Example [1](#over.ics.ref-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]](#over.ics.user "12.2.4.2.3 User-defined conversion sequences"))
whose second standard conversion sequence is
determined by the above rules[.](#over.ics.ref-1.sentence-2)
[2](#over.ics.ref-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2435)
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]](#over.best.ics "12.2.4.2 Implicit conversion sequences")[.](#over.ics.ref-2.sentence-1)
Conceptually, this conversion sequence corresponds to copy-initializing a
temporary of the referenced type with the argument expression[.](#over.ics.ref-2.sentence-2)
Any difference
in top-level cv-qualification is subsumed by the initialization itself and
does not constitute a conversion[.](#over.ics.ref-2.sentence-3)
[3](#over.ics.ref-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2445)
Except for an implicit object parameter, for which see [[over.match.funcs]](over.match.funcs "12.2.2 Candidate functions and argument lists"),
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[.](#over.ics.ref-3.sentence-1)
[*Note [1](#over.ics.ref-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]](dcl.init.ref "9.5.4 References"))[.](#over.ics.ref-3.sentence-2)
— *end note*]
[4](#over.ics.ref-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2460)
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[.](#over.ics.ref-4.sentence-1)
[*Example [2](#over.ics.ref-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[.](#over.ics.ref-4.sentence-2)
The formation of implicit conversion sequences
treats theint bit-field as anint lvalue and finds an exact
match with the parameter[.](#over.ics.ref-4.sentence-3)
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]](dcl.init.ref "9.5.4 References"))[.](#over.ics.ref-4.sentence-4)
— *end example*]
#### [12.2.4.2.6](#over.ics.list) List-initialization sequence [[over.ics.list]](over.ics.list)
[1](#over.ics.list-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2484)
When an argument is an initializer list ([[dcl.init.list]](dcl.init.list "9.5.5 List-initialization")), it is not an expression and special rules apply for converting it to a parameter type[.](#over.ics.list-1.sentence-1)
[2](#over.ics.list-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2488)
If the initializer list is a [*designated-initializer-list*](dcl.init.general#nt:designated-initializer-list "9.5.1 General [dcl.init.general]") 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]](dcl.init.aggr "9.5.2 Aggregates")),
in which case the implicit conversion sequence is
a user-defined conversion sequence
whose second standard conversion sequence
is an identity conversion[.](#over.ics.list-2.sentence-1)
[*Note [1](#over.ics.list-note-1)*:
Aggregate initialization does not require that
the members are declared in designation order[.](#over.ics.list-2.sentence-2)
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]](dcl.init.list "9.5.5 List-initialization"))[.](#over.ics.list-2.sentence-3)
[*Example [1](#over.ics.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]](dcl.init.list "9.5.5 List-initialization")) g({.x = 1, .y = 2}); // error: ambiguous between #3 and #4} — *end example*]
— *end note*]
[3](#over.ics.list-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2523)
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[.](#over.ics.list-3.sentence-1)
[4](#over.ics.list-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2530)
Otherwise, if the parameter type is a character array[106](#footnote-106 "Since there are no parameters of array type, this will only occur as the referenced type of a reference parameter.") and the initializer list has a single element that is an appropriately-typed[*string-literal*](lex.string#nt:string-literal "5.13.5 String literals [lex.string]") ([[dcl.init.string]](dcl.init.string "9.5.3 Character arrays")), the implicit conversion
sequence is the identity conversion[.](#over.ics.list-4.sentence-1)
[5](#over.ics.list-5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2540)
Otherwise, if the parameter type is std​::​initializer_list<X> 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[.](#over.ics.list-5.sentence-1)
This conversion can be a user-defined conversion even in
the context of a call to an initializer-list constructor[.](#over.ics.list-5.sentence-2)
[*Example [2](#over.ics.list-example-2)*: void f(std::initializer_list<int>);
f( {} ); // OK, f(initializer_list<int>) identity conversion f( {1,2,3} ); // OK, f(initializer_list<int>) identity conversion f( {'a','b'} ); // OK, f(initializer_list<int>) integral promotion f( {1.0} ); // error: narrowingstruct A { A(std::initializer_list<double>); // #1 A(std::initializer_list<std::complex<double>>); // #2 A(std::initializer_list<std::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](#over.ics.list-6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2572)
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[.](#over.ics.list-6.sentence-1)
[7](#over.ics.list-7)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2582)
Otherwise, if the parameter is a non-aggregate class X and overload
resolution per [[over.match.list]](over.match.list "12.2.2.8 Initialization by list-initialization") chooses a single best constructor C ofX to perform the initialization of an object of type X from the
argument initializer list:
- [(7.1)](#over.ics.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[.](#over.ics.list-7.1.sentence-1)
- [(7.2)](#over.ics.list-7.2)
Otherwise, the implicit conversion sequence is a user-defined
conversion sequence whose second standard conversion sequence is an
identity conversion[.](#over.ics.list-7.2.sentence-1)
If multiple constructors are viable but none is better than
the others, the implicit conversion sequence is the ambiguous conversion
sequence[.](#over.ics.list-7.sentence-2)
User-defined conversions are allowed for conversion of the initializer
list elements to the constructor parameter types except as noted
in [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences")[.](#over.ics.list-7.sentence-3)
[*Example [3](#over.ics.list-example-3)*: struct A { A(std::initializer_list<int>);};void f(A);
f( {'a', 'b'} ); // OK, f(A(std​::​initializer_list<int>)) 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<int>{1,2}), C(std​::​string("bar")))) — *end example*]
[8](#over.ics.list-8)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2636)
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]](dcl.init.aggr "9.5.2 Aggregates")), the implicit conversion sequence is a
user-defined conversion sequence whose second standard conversion
sequence is an identity conversion[.](#over.ics.list-8.sentence-1)
[*Example [4](#over.ics.list-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](#over.ics.list-9)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2655)
Otherwise, if the parameter is a reference, see [[over.ics.ref]](#over.ics.ref "12.2.4.2.5 Reference binding")[.](#over.ics.list-9.sentence-1)
[*Note [2](#over.ics.list-note-2)*:
The rules in this subclause will apply for initializing the underlying temporary
for the reference[.](#over.ics.list-9.sentence-2)
— *end note*]
[*Example [5](#over.ics.list-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](#over.ics.list-10)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2677)
Otherwise, if the parameter type is not a class:
- [(10.1)](#over.ics.list-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](#over.ics.list-example-6)*: void f(int);
f( {'a'} ); // OK, same conversion as char to int f( {1.0} ); // error: narrowing — *end example*]
- [(10.2)](#over.ics.list-10.2)
if the initializer list has no elements, the implicit conversion sequence
is the identity conversion[.](#over.ics.list-10.sentence-1)
[*Example [7](#over.ics.list-example-7)*: void f(int);
f( { } ); // OK, identity conversion — *end example*]
[11](#over.ics.list-11)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2701)
In all cases other than those enumerated above, no conversion is possible[.](#over.ics.list-11.sentence-1)
[106)](#footnote-106)[106)](#footnoteref-106)
Since there are no parameters of array type,
this will only occur as the referenced type of a reference parameter[.](#footnote-106.sentence-1)
#### [12.2.4.3](#over.ics.rank) Ranking implicit conversion sequences [[over.ics.rank]](over.ics.rank)
[1](#over.ics.rank-1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2706)
This subclause defines a partial ordering of implicit conversion
sequences based on the relationships[*better conversion sequence*](#def:conversion_sequence,better "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]") and[*better conversion*](#def:conversion,better "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.rank-1.sentence-1)
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 a[*worse conversion sequence*](#def:conversion_sequence,worse "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]") than S1[.](#over.ics.rank-1.sentence-2)
If conversion sequence S1 is neither better
than nor worse than conversion sequence S2, S1 and S2 are said to
be[*indistinguishable conversion sequences*](#def:conversion_sequence,indistinguishable "12.2.4.3 Ranking implicit conversion sequences [over.ics.rank]")[.](#over.ics.rank-1.sentence-3)
[2](#over.ics.rank-2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2722)
When comparing the basic forms of implicit conversion sequences
(as defined in [[over.best.ics]](#over.best.ics "12.2.4.2 Implicit conversion sequences"))
- [(2.1)](#over.ics.rank-2.1)
a [standard conversion sequence](#over.ics.scs "12.2.4.2.2 Standard conversion sequences [over.ics.scs]") is a better
conversion sequence than a user-defined conversion sequence
or an ellipsis conversion sequence, and
- [(2.2)](#over.ics.rank-2.2)
a [user-defined conversion sequence](#over.ics.user "12.2.4.2.3 User-defined conversion sequences [over.ics.user]") is a
better conversion sequence than an [ellipsis conversion
sequence](#over.ics.ellipsis "12.2.4.2.4 Ellipsis conversion sequences [over.ics.ellipsis]")[.](#over.ics.rank-2.sentence-1)
[3](#over.ics.rank-3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2736)
Two implicit conversion sequences of the same form are
indistinguishable conversion sequences unless one of the
following rules applies:
- [(3.1)](#over.ics.rank-3.1)
List-initialization sequence L1 is a better conversion sequence than
list-initialization sequence L2 if
* [(3.1.1)](#over.ics.rank-3.1.1)
L1 converts to std​::​initializer_list<X> for some X andL2 does not, or, if not that,
* [(3.1.2)](#over.ics.rank-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[.](#over.ics.rank-3.1.sentence-1)
[*Example [1](#over.ics.rank-example-1)*: void f1(int); // #1void f1(std::initializer_list<long>); // #2void g1() { f1({42}); } // chooses #2void f2(std::pair<const char*, const char*>); // #3void f2(std::initializer_list<std::string>); // #4void g2() { f2({"foo","bar"}); } // chooses #4 — *end example*]
[*Example [2](#over.ics.rank-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)](#over.ics.rank-3.2)
Standard conversion sequenceS1 is a better conversion
sequence than standard conversion sequenceS2 if
* [(3.2.1)](#over.ics.rank-3.2.1)
S1 is a proper subsequence ofS2 (comparing the conversion sequences in the canonical form defined
by [[over.ics.scs]](#over.ics.scs "12.2.4.2.2 Standard conversion sequences"), 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)](#over.ics.rank-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)](#over.ics.rank-3.2.3)
S1 and S2 include reference bindings ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) and
neither refers to an implicit object parameter of a non-static member function
declared without a [*ref-qualifier*](dcl.decl.general#nt:ref-qualifier "9.3.1 General [dcl.decl.general]"),
and S1 binds an rvalue reference to an
rvalue and S2 binds an lvalue reference
[*Example [3](#over.ics.rank-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)](#over.ics.rank-3.2.4)
S1 and S2 include reference bindings ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")) andS1 binds an lvalue reference to an lvalue of function type andS2 binds an rvalue reference to an lvalue of function type
[*Example [4](#over.ics.rank-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)](#over.ics.rank-3.2.5)
S1 and S2 differ only
in their qualification conversion ([[conv.qual]](conv.qual "7.3.6 Qualification conversions")) 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]](dcl.init.ref "9.5.4 References"))
[*Example [5](#over.ics.rank-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)](#over.ics.rank-3.2.6)
S1 andS2 bind “reference to T1” and “reference to T2”,
respectively ([[dcl.init.ref]](dcl.init.ref "9.5.4 References")),
where T1 and T2 are not the same type, andT2 is reference-compatible with T1
[*Example [6](#over.ics.rank-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)](#over.ics.rank-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*[.](#over.ics.rank-3.2.sentence-1)
[*Example [7](#over.ics.rank-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)](#over.ics.rank-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[.](#over.ics.rank-3.3.sentence-1)
[*Example [8](#over.ics.rank-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](#over.ics.rank-4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L2973)
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[.](#over.ics.rank-4.sentence-1)
Two conversion sequences with the same rank are indistinguishable unless
one of the following rules applies:
- [(4.1)](#over.ics.rank-4.1)
A conversion that does not convert a pointer or a pointer to member
tobool is better than one that does[.](#over.ics.rank-4.1.sentence-1)
- [(4.2)](#over.ics.rank-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[.](#over.ics.rank-4.2.sentence-1)
- [(4.3)](#over.ics.rank-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)](#over.ics.rank-4.3.1)
the floating-point conversion rank ([[conv.rank]](conv.rank "6.9.6 Conversion ranks")) of FP1 is equal to the rank of FP2, and
* [(4.3.2)](#over.ics.rank-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]](conv.rank "6.9.6 Conversion ranks")) of FP2 is greater than the subrank of T3[.](#over.ics.rank-4.3.sentence-1)
[*Example [9](#over.ics.rank-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)](#over.ics.rank-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*[.](#over.ics.rank-4.4.sentence-1)
- [(4.5)](#over.ics.rank-4.5)
If classB is derived directly or indirectly from classA and classC is derived directly or indirectly fromB,
* [(4.5.1)](#over.ics.rank-4.5.1)
conversion ofC* toB* is better than conversion ofC* toA*,
[*Example [10](#over.ics.rank-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)](#over.ics.rank-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)](#over.ics.rank-4.5.3)
conversion ofA​::​* toB​::​* is better than conversion ofA​::​* toC​::​*,
* [(4.5.4)](#over.ics.rank-4.5.4)
conversion ofC toB is better than conversion ofC toA,
* [(4.5.5)](#over.ics.rank-4.5.5)
conversion ofB* toA* is better than conversion ofC* toA*,
* [(4.5.6)](#over.ics.rank-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)](#over.ics.rank-4.5.7)
conversion ofB​::​* toC​::​* is better than conversion
ofA​::​* toC​::​*,
and
* [(4.5.8)](#over.ics.rank-4.5.8)
conversion ofB toA is better than conversion ofC toA[.](#over.ics.rank-4.5.sentence-1)
[*Note [1](#over.ics.rank-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[.](#over.ics.rank-4.5.sentence-2)
— *end note*]
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