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# user-defined conversion

From cppreference.com
< cpp‎ | language

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 explicit return type and with the name of the form:

`operator` conversion-type-id | (1) |
`explicit` `operator` conversion-type-id | (2) | (since C++11)

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.

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, 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 on 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::operator&
    }

When making an explicit call to the conversion function, the type-id is greedy: it is the longest possible sequence of tokens that is a valid type id (including attributes, if any):

    & x.operator int * a; // parsed as & (x.operator int*) a
                          // not as & (x.operator int) * a

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
    };
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.

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