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

Explanation

explicit on a constructor with multiple arguments has no effect, since such constructors cannot take part in implicit conversions. However, for the purpose of implicit conversion, explicit will have an effect if a constructor has multiple arguments and all but one of the arguments has a default value.

A constructor with a single-parameter constructor that is declared without the function specifier explicit is called a conversion constructor.

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; // OK: implicit conversion from int to A
    A a2(2); // OK: explicit constructor call
    A a3 { 3 }; // OK: explicit constructor call
    int na1 = a1; // OK: implicit conversion from A to int
    int na2 = static_cast<int>(a1); // OK: explicit cast
    A a4 = (A)1; // OK: explicit conversion from int to A
 
    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
 
    B b4 = static_cast<B>(1); // OK: explicit conversion from int to B
    B b5 = (B)1; // OK: explicit conversion from int to B
}