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Difference between revisions of "cpp/language/explicit"

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< cpp‎ | language
(example wouldn't compile)
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{{title|explicit specifier}}
 
{{title|explicit specifier}}
 
{{cpp/language/classes/navbar}}
 
{{cpp/language/classes/navbar}}
 +
{{sdsc begin}}
 +
{{sdsc|num=1|1=
 +
{{ttb|explicit}}
 +
}}
 +
{{sdsc|num=2|notes={{mark since c++20}}|1=
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{{ttb|explicit (}} {{spar|expression}} {{ttb|) }}
 +
}}
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{{sdsc end}}
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{{par begin}}
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{{par | {{spar|expression}} | {{rlp|constant expression#Converted constant expression|contextually converted constant expression of type {{c|bool}}}} }}
 +
{{par end}}
  
−
The {{c|explicit}} specifier specifies that a constructor {{rev inl|since=c++11| or conversion function}} doesn't allow [[cpp/language/implicit_cast|implicit conversions]] or [[cpp/language/copy_initialization|copy-initialization]]. It may only appear within the {{spar|decl-specifier-seq}} of the declaration of such a function within its class definition.
+
@1@ Specifies that a constructor {{rev inl|since=c++11| or conversion function}} is explicit, that is, it cannot be used for [[cpp/language/implicit_cast|implicit conversions]] and [[cpp/language/copy_initialization|copy-initialization]].
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 +
{{rrev | since=c++20|
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@2@ The {{c|explicit}} specifier may be used with a constant expression. The function is explicit if and only if that constant expression evaluates to {{c|true}}.
 +
}}
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 +
The explicit specifier may only appear within the {{spar|decl-specifier-seq}} of the declaration of a constructor {{rev inl|since=c++11| or conversion function}} within its class definition.
  
 
===Notes===
 
===Notes===
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Both constructors (other than {{rlp|copy_constructor|copy}}/{{rlp|move_constructor|move}}) and user-defined conversion functions may be function templates; the meaning of {{tt|explicit}} doesn't change.
 
Both constructors (other than {{rlp|copy_constructor|copy}}/{{rlp|move_constructor|move}}) and user-defined conversion functions may be function templates; the meaning of {{tt|explicit}} doesn't change.
  
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{{rrev|since=c++20|1=
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A {{tt|(}} token that follows {{tt|explicit}} is parsed as part of the explicit specifier:
 +
{{source|1=
 +
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
 +
};
 +
}}
 +
}}
 
===Example===
 
===Example===
 
{{example|code=
 
{{example|code=

Revision as of 10:13, 9 June 2018

 
C++
 
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Classes
 
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) 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 doesn't change.

A ( token that follows explicit is 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)

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
}
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