The explicit specifier specifies that a constructor or conversion function (since C++11) doesn't allow implicit conversions or copy-initialization. It may only appear within the decl-specifier-seq of the declaration of such a function within its class definition.
A constructor with a single non-default parameter (until C++11) that is declared without the function specifier explicit is called a converting constructor.
Both constructors (other than copy/move) and user-defined conversion functions may be function templates; the meaning of explicit doesn't change.
struct A
{
A(int) { } // converting constructor
A(int, int) { } // converting constructor (C++11)
operator bool() const { return true; }
};
struct B
{
explicit B(int) { }
explicit B(int, int) { }
explicit operator bool() const { return true; }
};
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
if (a1) ; // OK: A::operator bool()
bool na1 = a1; // OK: copy-initialization selects A::operator bool()
bool na2 = static_cast<bool>(a1); // OK: static_cast performs 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
if (b2) ; // OK: B::operator bool()
// bool nb1 = b2; // error: copy-initialization does not consider B::operator bool()
bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization
}
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
http://en.cppreference.com/w/cpp/language/explicit