Difference between revisions of "cpp/language/explicit"
From cppreference.com
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{{title|{{tt|explicit}} specifier}} | {{title|{{tt|explicit}} specifier}} | ||
{{cpp/language/classes/navbar}} | {{cpp/language/classes/navbar}} | ||
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| + | ===Syntax=== | ||
{{sdsc begin}} | {{sdsc begin}} | ||
{{sdsc|num=1|1= | {{sdsc|num=1|1= | ||
| − | + | {{ttb|explicit}} | |
}} | }} | ||
{{sdsc|num=2|notes={{mark since c++20}}|1= | {{sdsc|num=2|notes={{mark since c++20}}|1= | ||
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A {{ttb|(}} token that follows {{tt|explicit}} is parsed as part of the explicit specifier: | A {{ttb|(}} token that follows {{tt|explicit}} is parsed as part of the explicit specifier: | ||
{{source|1= | {{source|1= | ||
| − | struct S { | + | struct S |
| + | { | ||
explicit (S)(const S&); // error in C++20, OK in C++17 | explicit (S)(const S&); // error in C++20, OK in C++17 | ||
explicit (operator int)(); // error in C++20, OK in C++17 | explicit (operator int)(); // error in C++20, OK in C++17 | ||
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bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization | bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization | ||
| − | [a4, a5, na1, na2, b5, nb2 | + | [](...){}(a4, a5, na1, na2, b5, nb2); // may suppress "unused variable" warnings |
} | } | ||
}} | }} | ||
Revision as of 04:03, 23 February 2023
Contents |
Syntax
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)or deduction guide(since C++17) 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 does not 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) |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__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 }
