User-defined conversion function

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Enables implicit conversion or explicit conversion from a class type to another type.

[edit] Syntax

Conversion function is declared like a non-static member function or member function template with no parameters, no explicit return type, and with the name of the form:

operator conversion-type-id (1)
explicit operator conversion-type-id (2) (since C++11)
explicit ( expression ) operator conversion-type-id (3) (since C++20)
1) Declares a user-defined conversion function that participates in all implicit and explicit conversions.
2) Declares a user-defined conversion function that participates in direct-initialization and explicit conversions only.
3) Declares a user-defined conversion function that is conditionally explicit.

conversion-type-id is a type-id except that function and array operators [] or () are not allowed in its declarator (thus conversion to types such as pointer to array requires a type alias/typedef or an identity template: see below). Regardless of typedef, conversion-type-id cannot represent an array or a function type.

Although the return type is not allowed in the declaration of a user-defined conversion function, the decl-specifier-seq of the declaration grammar may be present and may include any specifier other than type-specifier or the keyword static, In particular, besides explicit, the specifiers inline, virtual, constexpr(since C++11), consteval(since C++20), and friend are also allowed (note that friend requires a qualified name: friend A::operator B();).

When such member function is declared in class X, it performs conversion from X to conversion-type-id:

struct X
    // implicit conversion
    operator int() const { return 7; }
    // explicit conversion
    explicit operator int*() const { return nullptr; }
    // Error: array operator not allowed in conversion-type-id
//  operator int(*)[3]() const { return nullptr; }
    using arr_t = int[3];
    operator arr_t*() const { return nullptr; } // OK if done through typedef
//  operator arr_t () const; // Error: conversion to array not allowed in any case
int main()
    X x;
    int n = static_cast<int>(x);   // OK: sets n to 7
    int m = x;                     // OK: sets m to 7
    int* p = static_cast<int*>(x); // OK: sets p to null
//  int* q = x; // Error: no implicit conversion
    int (*pa)[3] = x;  // OK

[edit] Explanation

User-defined conversion function is invoked in the second stage of the implicit conversion, which consists of zero or one converting constructor or zero or one user-defined conversion function.

If both conversion functions and converting constructors can be used to perform some user-defined conversion, the conversion functions and constructors are both considered by overload resolution in copy-initialization and reference-initialization contexts, but only the constructors are considered in direct-initialization contexts.

struct To
    To() = default;
    To(const struct From&) {} // converting constructor
struct From
    operator To() const {return To();} // conversion function
int main()
    From f;
    To t1(f);  // direct-initialization: calls the constructor
    // Note: if converting constructor is not available, implicit copy constructor
    // will be selected, and conversion function will be called to prepare its argument
//  To t2 = f; // copy-initialization: ambiguous
    // Note: if conversion function is from a non-const type, e.g.
    // From::operator To();, it will be selected instead of the ctor in this case
    To t3 = static_cast<To>(f); // direct-initialization: calls the constructor
    const To& r = f;            // reference-initialization: ambiguous

Conversion function to its own (possibly cv-qualified) class (or to a reference to it), to the base of its own class (or to a reference to it), and to the type void can be defined, but can not be executed as part of the conversion sequence, except, in some cases, through virtual dispatch:

struct D;
struct B
    virtual operator D() = 0;
struct D : B
    operator D() override { return D(); }
int main()
    D obj;
    D obj2 = obj; // does not call D::operator D()
    B& br = obj;
    D obj3 = br;  // calls D::operator D() through virtual dispatch

It can also be called using member function call syntax:

struct B {};
struct X : B
    operator B&() { return *this; };
int main()
    X x;
    B& b1 = x;                  // does not call X::operatorB&()
    B& b2 = static_cast<B&>(x); // does not call X::operatorB&
    B& b3 = x.operator B&();    // calls X::operatorB&

When making an explicit call to the conversion function, conversion-type-id is greedy: it is the longest sequence of tokens that could possibly form a conversion-type-id (including attributes, if any)(since C++11):

& x.operator int * a; // error: parsed as & (x.operator int*) a,
                      //           not as & (x.operator int) * a
operator int [[noreturn]] (); // error: noreturn attribute applied to a type

The placeholder auto can be used in conversion-type-id, indicating a deduced return type:

struct X
    operator int(); // OK
    operator auto() -> short; // error: trailing return type not part of syntax
    operator auto() const { return 10; } // OK: deduced return type
    operator decltype(auto)() const { return 10l; } // OK: deduced return type

Note: a conversion function template is not allowed to have a deduced return type.

(since C++14)

Conversion functions can be inherited and can be virtual, but cannot be static. A conversion function in the derived class does not hide a conversion function in the base class unless they are converting to the same type.

Conversion function can be a template member function, for example, std::auto_ptr<T>::operator auto_ptr<Y>. See member template and template argument deduction for applicable special rules.

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 296 C++98 conversion functions could be static they cannot be declared static
CWG 2016 C++98 conversion functions could not specify return types,
but the types are present in conversion-type-id
return types cannot be specified in the
declaration specifiers of conversion functions
CWG 2175 C++11 it was unclear whether the [[noreturn]] in
operator int [[noreturn]] (); is parsed as a part of
noptr-declarator (of function declarator) or conversion-type-id
it is parsed as a part of