[class.conv] # 11 Classes [[class]](./#class) ## 11.4 Class members [[class.mem]](class.mem#class.conv) ### 11.4.8 Conversions [class.conv] #### [11.4.8.1](#general) General [[class.conv.general]](class.conv.general) [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2441) Type conversions of class objects can be specified by constructors and by conversion functions[.](#general-1.sentence-1) These conversions are called[*user-defined conversions*](#def:conversion,user-defined "11.4.8.1 General [class.conv.general]") and are used for implicit type conversions ([[conv]](conv "7.3 Standard conversions")), for initialization ([[dcl.init]](dcl.init "9.5 Initializers")), and for explicit type conversions ([[expr.type.conv]](expr.type.conv "7.6.1.4 Explicit type conversion (functional notation)"), [[expr.cast]](expr.cast "7.6.3 Explicit type conversion (cast notation)"), [[expr.static.cast]](expr.static.cast "7.6.1.9 Static cast"))[.](#general-1.sentence-2) [2](#general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2455) User-defined conversions are applied only where they are unambiguous ([[class.member.lookup]](class.member.lookup "6.5.2 Member name lookup"), [[class.conv.fct]](#fct "11.4.8.3 Conversion functions"))[.](#general-2.sentence-1) Conversions obey the access control rules ([[class.access]](class.access "11.8 Member access control"))[.](#general-2.sentence-2) Access control is applied after ambiguity resolution ([[basic.lookup]](basic.lookup "6.5 Name lookup"))[.](#general-2.sentence-3) [3](#general-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2460) [*Note [1](#general-note-1)*: See [[over.match]](over.match "12.2 Overload resolution") for a discussion of the use of conversions in function calls[.](#general-3.sentence-1) — *end note*] [4](#general-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2465) At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value[.](#general-4.sentence-1) [*Example [1](#general-example-1)*: struct X {operator int();}; struct Y {operator X();}; Y a;int b = a; // error: no viable conversion (a.operator X().operator int() not considered)int c = X(a); // OK, a.operator X().operator int() — *end example*] #### [11.4.8.2](#ctor) Conversion by constructor [[class.conv.ctor]](class.conv.ctor) [1](#ctor-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2488) A constructor that is not explicit ([[dcl.fct.spec]](dcl.fct.spec "9.2.3 Function specifiers")) specifies a conversion from the types of its parameters (if any) to the type of its class[.](#ctor-1.sentence-1) [*Example [1](#ctor-example-1)*: [🔗](#:Jessie) struct X { X(int); X(const char*, int = 0); X(int, int);}; void f(X arg) { X a = 1; // a = X(1) X b = "Jessie"; // b = X("Jessie",0) a = 2; // a = X(2) f(3); // f(X(3)) f({1, 2}); // f(X(1,2))} — *end example*] [2](#ctor-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2512) [*Note [1](#ctor-note-1)*: An explicit constructor constructs objects just like non-explicit constructors, but does so only where the direct-initialization syntax ([[dcl.init]](dcl.init "9.5 Initializers")) or where casts ([[expr.static.cast]](expr.static.cast "7.6.1.9 Static cast"), [[expr.cast]](expr.cast "7.6.3 Explicit type conversion (cast notation)")) are explicitly used; see also [[over.match.copy]](over.match.copy "12.2.2.5 Copy-initialization of class by user-defined conversion")[.](#ctor-2.sentence-1) A default constructor can be an explicit constructor; such a constructor will be used to perform default-initialization or value-initialization ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#ctor-2.sentence-2) [*Example [2](#ctor-example-2)*: struct Z {explicit Z(); explicit Z(int); explicit Z(int, int);}; Z a; // OK, default-initialization performed Z b{}; // OK, direct initialization syntax used Z c = {}; // error: copy-list-initialization Z a1 = 1; // error: no implicit conversion Z a3 = Z(1); // OK, direct initialization syntax used Z a2(1); // OK, direct initialization syntax used Z* p = new Z(1); // OK, direct initialization syntax used Z a4 = (Z)1; // OK, explicit cast used Z a5 = static_cast(1); // OK, explicit cast used Z a6 = { 3, 4 }; // error: no implicit conversion — *end example*] — *end note*] #### [11.4.8.3](#fct) Conversion functions [[class.conv.fct]](class.conv.fct) [conversion-function-id:](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") operator [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") [conversion-type-id:](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") [*type-specifier-seq*](dcl.type.general#nt:type-specifier-seq "9.2.9.1 General [dcl.type.general]") [*conversion-declarator*](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]")opt [conversion-declarator:](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]") [*ptr-operator*](dcl.decl.general#nt:ptr-operator "9.3.1 General [dcl.decl.general]") [*conversion-declarator*](#nt:conversion-declarator "11.4.8.3 Conversion functions [class.conv.fct]")opt [1](#fct-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2563) A declaration whose [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") has an [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") that is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") declares a [*conversion function*](#def:function,conversion "11.4.8.3 Conversion functions [class.conv.fct]"); its [*declarator*](dcl.decl.general#nt:declarator "9.3.1 General [dcl.decl.general]") shall be a function declarator ([[dcl.fct]](dcl.fct "9.3.4.6 Functions")) of the form [*noptr-declarator*](dcl.decl.general#nt:noptr-declarator "9.3.1 General [dcl.decl.general]") [*parameters-and-qualifiers*](dcl.decl.general#nt:parameters-and-qualifiers "9.3.1 General [dcl.decl.general]") where the [*noptr-declarator*](dcl.decl.general#nt:noptr-declarator "9.3.1 General [dcl.decl.general]") consists solely of an [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]"), an optional [*attribute-specifier-seq*](dcl.attr.grammar#nt:attribute-specifier-seq "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]"), and optional surrounding parentheses, and the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") has one of the following forms: - [(1.1)](#fct-1.1) in a [*member-declaration*](class.mem.general#nt:member-declaration "11.4.1 General [class.mem.general]") that belongs to the [*member-specification*](class.mem.general#nt:member-specification "11.4.1 General [class.mem.general]") of a class or class template but is not a friend declaration ([[class.friend]](class.friend "11.8.4 Friends")), the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]"); - [(1.2)](#fct-1.2) otherwise, the [*id-expression*](expr.prim.id.general#nt:id-expression "7.5.5.1 General [expr.prim.id.general]") is a [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") whose [*unqualified-id*](expr.prim.id.unqual#nt:unqualified-id "7.5.5.2 Unqualified names [expr.prim.id.unqual]") is a [*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]")[.](#fct-1.sentence-1) [2](#fct-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2589) A conversion function shall have no non-object parameters and shall be a non-static member function of a class or class template X; its declared return type is the [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") and it specifies a conversion from X to the type specified by the [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]"), interpreted as a [*type-id*](dcl.name#nt:type-id "9.3.2 Type names [dcl.name]") ([[dcl.name]](dcl.name "9.3.2 Type names"))[.](#fct-2.sentence-1) A [*decl-specifier*](dcl.spec.general#nt:decl-specifier "9.2.1 General [dcl.spec.general]") in the [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]") of a conversion function (if any) shall not be a [*defining-type-specifier*](dcl.type.general#nt:defining-type-specifier "9.2.9.1 General [dcl.type.general]")[.](#fct-2.sentence-2) [3](#fct-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2600) [*Note [1](#fct-note-1)*: A conversion function is never invoked for implicit or explicit conversions of an object to the same object type (or a reference to it), to a base class of that type (or a reference to it), or to cv void[.](#fct-3.sentence-1) Even though never directly called to perform a conversion, such conversion functions can be declared and can potentially be reached through a call to a virtual conversion function in a base class[.](#fct-3.sentence-2) — *end note*] [*Example [1](#fct-example-1)*: struct X {operator int(); operator auto() -> short; // error: trailing return type}; void f(X a) {int i = int(a); i = (int)a; i = a;} In all three cases the value assigned will be converted byX​::​operator int()[.](#fct-3.sentence-3) — *end example*] [4](#fct-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2628) A conversion function may be explicit ([[dcl.fct.spec]](dcl.fct.spec "9.2.3 Function specifiers")), in which case it is only considered as a user-defined conversion for direct-initialization ([[dcl.init]](dcl.init "9.5 Initializers"))[.](#fct-4.sentence-1) Otherwise, user-defined conversions are not restricted to use in assignments and initializations[.](#fct-4.sentence-2) [*Example [2](#fct-example-2)*: class Y { };struct Z {explicit operator Y() const;}; void h(Z z) { Y y1(z); // OK, direct-initialization Y y2 = z; // error: no conversion function candidate for copy-initialization Y y3 = (Y)z; // OK, cast notation}void g(X a, X b) {int i = (a) ? 1+a : 0; int j = (a&&b) ? a+b : i; if (a) {}} — *end example*] [5](#fct-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2652) The[*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") shall not represent a function type nor an array type[.](#fct-5.sentence-1) The[*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]") in a[*conversion-function-id*](#nt:conversion-function-id "11.4.8.3 Conversion functions [class.conv.fct]") is the longest sequence of tokens that could possibly form a [*conversion-type-id*](#nt:conversion-type-id "11.4.8.3 Conversion functions [class.conv.fct]")[.](#fct-5.sentence-2) [*Note [2](#fct-note-2)*: This prevents ambiguities between the declarator operator * and its expression counterparts[.](#fct-5.sentence-3) [*Example [3](#fct-example-3)*: &ac.operator int*i; // syntax error:// parsed as: &(ac.operator int *)i// not as: &(ac.operator int)*i The * is the pointer declarator and not the multiplication operator[.](#fct-5.sentence-4) — *end example*] This rule also prevents ambiguities for attributes[.](#fct-5.sentence-5) [*Example [4](#fct-example-4)*: operator int [[noreturn]] (); // error: noreturn attribute applied to a type — *end example*] — *end note*] [6](#fct-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2681) [*Note [3](#fct-note-3)*: A conversion function in a derived class hides only conversion functions in base classes that convert to the same type[.](#fct-6.sentence-1) A conversion function template with a dependent return type hides only templates in base classes that correspond to it ([[class.member.lookup]](class.member.lookup "6.5.2 Member name lookup")); otherwise, it hides and is hidden as a non-template function[.](#fct-6.sentence-2) Function overload resolution ([[over.match.best]](over.match.best "12.2.4 Best viable function")) selects the best conversion function to perform the conversion[.](#fct-6.sentence-3) [*Example [5](#fct-example-5)*: struct X {operator int();}; struct Y : X {operator char();}; void f(Y& a) {if (a) { // error: ambiguous between X​::​operator int() and Y​::​operator char()}} — *end example*] — *end note*] [7](#fct-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2709) Conversion functions can be virtual[.](#fct-7.sentence-1) [8](#fct-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/classes.tex#L2713) A conversion function template shall not have a deduced return type ([[dcl.spec.auto]](dcl.spec.auto "9.2.9.7 Placeholder type specifiers"))[.](#fct-8.sentence-1) [*Example [6](#fct-example-6)*: struct S {operator auto() const { return 10; } // OKtemplateoperator auto() const { return 1.2; } // error: conversion function template}; — *end example*]