Converting constructor
A constructor that is not declared with the specifier explicitand 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.
[edit]Example
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() [](...){}(a1, a4, a4, a5, b5);// may suppress "unused variable" warnings}
