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
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
}
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