Specifies constructors and conversion operators (since C++11) that don't allow implicit conversions or copy-initialization.
Syntax
explicit class_name ( params ) | (1) | |
explicit operator type ( ) (since C++11) | (2) |
1) specifies that this constructor is only considered for direct initialization (including explicit conversions)
2) specifies that this user-defined conversion function is only considered for direct initialization (including explicit conversions)
Notes
A constructor with a single non-default parameter (until C++11) that is declared without the function specifier explicit is called a converting constructor.
An explicit default constructor can be used to perform both default initialization and value initialization: the use of explicit with a default constructor has no effect unless it has default arguments that would allow it to be used in copy-initialization context.
Both constructors (other than copy/move) and user-defined conversion functions may be function templates; the meaning of explicit doesn't change.
Example
struct A
{
A(int) {} // converting constructor
A(int, int) {} // converting constructor (C++11)
operator int() const { return 0; }
};
struct B
{
explicit B(int) {}
explicit B(int, int) {}
explicit operator int() const { return 0; }
};
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)
int na1 = a1; // OK: copy-initialization selects A::operator int()
int na2 = static_cast<int>(a1); // OK: static_cast performs direct-initialization
A a5 = (A)1; // OK: explicit cast performs static_cast
// 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)
// int nb1 = b2; // Error: copy-initialization does not consider B::operator int()
int nb2 = static_cast<int>(b2); // OK: static_cast performs direct-initialization
B b5 = (B)1; // OK: explicit cast performs static_cast
}
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