# `explicit` specifier

From cppreference.com
< cpp‎ | language

### Syntax

`explicit` | (1) |
`explicit (` expression `)` | (2) | (since C++20)

expression | - | contextually converted constant expression of type bool

1) Specifies that a constructor or conversion function (since C++11)or deduction guide (since C++17) is explicit, that is, it cannot be used for implicit conversions and copy-initialization.

2) The explicit specifier may be used with a constant expression. The function is explicit if and only if that constant expression evaluates to true. | (since C++20)

The explicit specifier may only appear within the decl-specifier-seq of the declaration of a constructor or conversion function (since C++11) within its class definition.

### 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.

Both constructors (other than copy/move) and user-defined conversion functions may be function templates; the meaning of `explicit` does not change.

A `(` token that follows `explicit` is parsed as part of the explicit specifier:
    struct S
    {
        explicit (S)(const S&);    // error in C++20, OK in C++17
        explicit (operator int)(); // error in C++20, OK in C++17
    };
| (since C++20)

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_conditional_explicit` | 201806L | (C++20) | explicit(bool)

### Example

Run this code

    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

        [](...){}(a4, a5, na1, na2, b5, nb2); // may suppress "unused variable" warnings
    }

### See also

- converting constructor
- initialization
- copy initialization
- direct initialization

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/explicit&oldid=148488"