explicit specifier

 

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Specifies constructors and (since C++11) conversion operators that don't allow implicit conversions or copy-initialization.

Syntax

explicit class_name ( params )
explicit operator type ( ) (since C++11)

1) specifies that this constructor is only considered for direct initialization

2) specifies that this user-defined conversion function is only considered for direct initialization

Example

Run this code

struct A
{
    A(int) {}
    operator int() const { return 0; }
};
 
struct B
{
    explicit B(int) {}
    explicit operator int() const { return 0; }
};
 
int main()
{
    // A has no explicit ctor / conversion, everything is fine
    A a1 = 1;
    A a2 ( 2 );
    A a3 { 3 };
    int na1 = a1;
    int na2 = static_cast<int>( a1 );
 
    B b1 = 1; // Error: implicit conversion from int to B
    B b2 ( 2 ); // OK: explicit constructor call
    B b3 { 3 }; // OK: explicit constructor call
    int nb1 = b2; // Error: implicit conversion from B to int
    int nb2 = static_cast<int>( b2 ); // OK: explicit cast
}