explicit specifier
From cppreference.com
explicit
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(1) | ||||||||
explicit ( expression )
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(2) | (since C++20) | |||||||
| expression | - | contextually converted constant expression of type bool |
1) Specifies that a constructor or conversion function(since C++11) is explicit, that is, it cannot be used for implicit conversions and copy-initialization.
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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.
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(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 doesn't change.
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A 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) |
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 }
