A constructor that is not declared with the specifier explicit and which can be called with a single 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 (if any) 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.
struct A
{
A() { } // converting constructor (since C++11)
A(int) { } // converting constructor
A(int, int) { } // converting constructor (since C++11)
};
struct B
{
explicit 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 selected an explicit constructor
// B::B(int, int)
B b5 = (B)1; // OK: explicit cast performs static_cast, direct-initialization
B b6; // OK, default-initialization
B b7{}; // OK, direct-list-initialization
// B b8 = {}; // error: copy-list-initialization selected an explicit constructor
// B::B()
}
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