Converting constructor

 

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A constructor that is not declared with the specifier explicit and which has one non-default parameter (until C++11) is called a converting constructor

Unlike explicit constructors, which are only considered during direct initialization (which includes explicit conversions such as static_cast), converting constructors are also considered during copy initialization, as part of user-defined conversion sequence.

It is said that a converting constructor specifies an implicit conversion from the types of its arguments to the type of its class. Note that non-explicit user-defined conversion function also specifies an implicit conversion.

Implicitly-declared and user-defined non-explicit copy constructors and move constructors are converting constructors.

Example

Run this code

struct A
{
    A(int) {} // converting constructor
    A(int, int) {} // converting constructor (C++11)
};
 
struct B
{
    explicit B(int) {}
    explicit B(int, int) {}
};
 
int main()
{
    A a1 = 1; // OK: copy-initialization selects A::A(int)
    A a2(2); // OK: direct-initialization selects A::A(int)
    A a3 {4,5}; // OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4,5}; // OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1; // OK: explicit cast performs static_cast, direct-initialization
 
//  B b1 = 1; // Error: copy-initialization does not consider B::B(int)
    B b2(2); // OK: direct-initialization selects B::B(int)
    B b3 {4,5}; // OK: direct-list-initialization selects B::B(int, int)
//  B b4 = {4,5}; // Error: copy-list-initialization does not consider B::B(int,int)
    B b5 = (B)1; // OK: explicit cast performs static_cast, direct-initialization
}