explicit specifier| General topics | ||||||||||||||||
| Flow control | ||||||||||||||||
| Conditional execution statements | ||||||||||||||||
| Iteration statements (loops) | ||||||||||||||||
| Jump statements | ||||||||||||||||
| Functions | ||||||||||||||||
| Function declaration | ||||||||||||||||
| Lambda function expression | ||||||||||||||||
inline specifier | ||||||||||||||||
| Dynamic exception specifications(until C++17*) | ||||||||||||||||
noexcept specifier(C++11) | ||||||||||||||||
| Exceptions | ||||||||||||||||
| Namespaces | ||||||||||||||||
| Types | ||||||||||||||||
| Specifiers | ||||||||||||||||
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| Storage duration specifiers | ||||||||||||||||
| Initialization | ||||||||||||||||
| Expressions | |||||||||||||||||||||||||||||||
| Alternative representations | |||||||||||||||||||||||||||||||
| Literals | |||||||||||||||||||||||||||||||
| Boolean -Integer -Floating-point | |||||||||||||||||||||||||||||||
| Character -String -nullptr(C++11) | |||||||||||||||||||||||||||||||
| User-defined(C++11) | |||||||||||||||||||||||||||||||
| Utilities | |||||||||||||||||||||||||||||||
| Attributes(C++11) | |||||||||||||||||||||||||||||||
| Types | |||||||||||||||||||||||||||||||
typedef declaration | |||||||||||||||||||||||||||||||
| Type alias declaration(C++11) | |||||||||||||||||||||||||||||||
| Casts | |||||||||||||||||||||||||||||||
| Memory allocation | |||||||||||||||||||||||||||||||
| Classes | |||||||||||||||||||||||||||||||
| Class-specific function properties | |||||||||||||||||||||||||||||||
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| Special member functions | |||||||||||||||||||||||||||||||
| Templates | |||||||||||||||||||||||||||||||
| Miscellaneous | |||||||||||||||||||||||||||||||
Contents |
explicit | (1) | ||||||||
explicit (expression) | (2) | (since C++20) | |||||||
| expression | - | contextually converted constant expression of typebool |
2) Theexplicit specifier may be used with a constant expression. The function is explicit if and only if that constant expression evaluates totrue. | (since C++20) |
The explicit specifier may only appear within thedecl-specifier-seq of the declaration of a constructoror conversion function(since C++11) within its class definition.
A constructorwith a single non-default parameter(until C++11) that is declared without the function specifierexplicit is called aconverting constructor.
Both constructors (other thancopy/move) and user-defined conversion functions may be function templates; the meaning ofexplicit does not change.
A struct S{explicit(S)(const S&);// error in C++20, OK in C++17explicit(operatorint)();// error in C++20, OK in C++17}; | (since C++20) |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_conditional_explicit | 201806L | (C++20) | conditionalexplicit |
struct A{ A(int){}// converting constructor A(int,int){}// converting constructor (C++11) operatorbool()const{returntrue;}}; struct B{explicit B(int){}explicit B(int,int){}explicit operatorbool()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}
