explicit specifier

 

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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 always parsed as part of the explicit specifier: struct S {explicit(S)(const S&);// error in C++20, OK in C++17explicit(operator int)();// error in C++20, OK in C++17};

(since C++20)

Feature-test macro	Value	Std	Feature
__cpp_conditional_explicit	201806L	(C++20)	conditional explicit

Keywords

explicit

Example

Run this code

struct A { A(int){}// converting constructor A(int, int){}// converting constructor (C++11) operator bool()const{returntrue;}};   struct B {explicit B(int){}explicit B(int, int){}explicit operator bool()const{returntrue;}};   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_castif(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_castif(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);// suppresses “unused variable” warnings}

See also

• converting constructor
• initialization
• copy initialization
• direct initialization