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# explicit specifier

From cppreference.com
< cpp‎ | language

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

### [edit] 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)

### [edit] 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.

### [edit] Example

Run this code

    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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