explicit specifier - cppreference.com (original) (raw)
[edit] Syntax
| explicit | (1) | |
| explicit ( expression ) | (2) | (since C++20) |
| 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) |
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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.
[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.
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++17 explicit (operator int)(); // error in C++20, OK in C++17 }; | (since C++20) |
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| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
| __cpp_conditional_explicit | 201806L | (C++20) | conditional explicit |
[edit] Keywords
[edit] Example
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(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(b2); // OK: static_cast performs direct-initialization {}(a4, a5, na1, na2, b5, nb2); // suppresses “unused variable” warnings }