Namespaces
Variants
Views
Actions

Difference between revisions of "cpp/language/explicit"

From cppreference.com
< cpp‎ | language
(Added item)
(c -> c/core.)
 
(11 intermediate revisions by 5 users not shown)
Line 1: Line 1:
−
{{title|explicit specifier}}
+
{{title|{{tt|explicit}} specifier}}
 
{{cpp/language/classes/navbar}}
 
{{cpp/language/classes/navbar}}
 +
 +
===Syntax===
 
{{sdsc begin}}
 
{{sdsc begin}}
−
{{sdsc|num=1|1=
+
{{sdsc|num=1|
−
{{ttb|explicit}}
+
{{ttb|explicit}}
 
}}
 
}}
−
{{sdsc|num=2|notes={{mark since c++20}}|1=
+
{{sdsc|num=2|notes={{mark since c++20}}|
−
{{ttb|explicit (}} {{spar|expression}} {{ttb|) }}
+
{{ttb|explicit (}} {{spar|expression}} {{ttb|)}}
 
}}
 
}}
 
{{sdsc end}}
 
{{sdsc end}}
 
{{par begin}}
 
{{par begin}}
−
{{par | {{spar|expression}} | {{rlp|constant expression#Converted constant expression|contextually converted constant expression of type {{c|bool}}}} }}
+
{{par|{{spar|expression}}|{{rlp|constant expression#Converted constant expression|contextually converted constant expression of type {{c/core|bool}}}}}}
 
{{par end}}
 
{{par end}}
  
−
@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]].
 
  
−
{{rrev | since=c++20|
+
@1@ Specifies that a constructor {{rev inl|since=c++11|or conversion function}}{{rev inl|since=c++17|or {{rlp|ctad|deduction guide}}}} is explicit, that is, it cannot be used for {{rlp|implicit conversion}}s and {{rlp|copy initialization|copy-initialization}}.
−
@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}}.
+
 
 +
{{rrev|since=c++20|
 +
@2@ The {{c/core|explicit}} specifier may be used with a constant expression. The function is explicit if and only if that constant expression evaluates to {{c|true}}.
 
}}
 
}}
  
−
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.
+
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===
−
A constructor {{rev inl|until=c++11|with a single non-default parameter}} that is declared without the function specifier {{c|explicit}} is called a {{rlp|converting constructor}}.
+
A constructor {{rev inl|until=c++11|with a single non-default parameter}} that is declared without the function specifier {{c/core|explicit}} is called a {{rlp|converting constructor}}.
  
−
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 {{c/core|explicit}} does not change.
  
 
{{rrev|since=c++20|1=
 
{{rrev|since=c++20|1=
−
A {{tt|(}} token that follows {{tt|explicit}} is parsed as part of the explicit specifier:
+
A {{ttb|(}} token that follows {{c/core|explicit}} is always 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
Line 35: Line 39:
 
}}
 
}}
 
}}
 
}}
 +
{{feature test macro|value=201806L|std=C++20|__cpp_conditional_explicit|conditional {{c/core|explicit}}}}
 +
 +
===Keywords===
 +
{{ltt|cpp/keyword/explicit}}
 +
 
===Example===
 
===Example===
 
{{example|code=
 
{{example|code=
 
struct A
 
struct A
 
{
 
{
−
     A(int) { }      // converting constructor
+
     A(int) {}      // converting constructor
−
     A(int, int) { } // converting constructor (C++11)
+
     A(int, int) {} // converting constructor (C++11)
 
     operator bool() const { return true; }
 
     operator bool() const { return true; }
 
};
 
};
Line 46: Line 55:
 
struct B
 
struct B
 
{
 
{
−
     explicit B(int) { }
+
     explicit B(int) {}
−
     explicit B(int, int) { }
+
     explicit B(int, int) {}
 
     explicit operator bool() const { return true; }
 
     explicit operator bool() const { return true; }
 
};
 
};
Line 58: Line 67:
 
     A a4 = {4, 5}; // OK: copy-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
 
     A a5 = (A)1;  // OK: explicit cast performs static_cast
−
     if (a1) ;      // OK: A::operator bool()
+
     if (a1) { }    // OK: A::operator bool()
 
     bool na1 = a1; // OK: copy-initialization selects 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
 
     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 b1 = 1;      // error: copy-initialization does not consider B::B(int)
 
     B b2(2);      // OK: direct-initialization selects 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 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 b4 = {4, 5}; // error: copy-list-initialization does not consider B::B(int, int)
 
     B b5 = (B)1;  // OK: explicit cast performs static_cast
 
     B b5 = (B)1;  // OK: explicit cast performs static_cast
−
     if (b2) ;      // OK: B::operator bool()
+
     if (b2) { }    // OK: B::operator bool()
 
//  bool nb1 = b2; // error: copy-initialization does not consider 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
 
     bool nb2 = static_cast<bool>(b2); // OK: static_cast performs direct-initialization
 +
   
 +
    [](...){}(a4, a5, na1, na2, b5, nb2); // suppresses “unused variable” warnings
 
}
 
}
 
}}
 
}}
  
 
===See also===
 
===See also===
−
* {{rlp | converting constructor}}
+
* {{rlp|converting constructor}}
−
* {{rlp | copy assignment}}
+
* {{rlp|initialization}}
−
* {{rlp | copy constructor}}
+
* {{rlp|copy initialization}}
−
* {{rlp | default constructor}}
+
* {{rlp|direct initialization}}
−
* {{rlp | destructor}}
+
−
* {{rlp | initialization}}
+
−
** {{rlp | aggregate initialization}}
+
−
** {{rlp | copy initialization}}
+
−
** {{rlp | default initialization}}
+
−
** {{rlp | direct initialization}}
+
−
** {{rlp | initializer list}}
+
−
** {{rlp | list initialization}}
+
−
** {{rlp | value initialization}}
+
−
* {{rlp | move assignment}}
+
−
* {{rlp | move constructor}}
+
−
* {{rlp | new}}
+
  
−
[[de:cpp/language/explicit]]
+
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
−
[[es:cpp/language/explicit]]
+
−
[[fr:cpp/language/explicit]]
+
−
[[it:cpp/language/explicit]]
+
−
[[ja:cpp/language/explicit]]
+
−
[[pt:cpp/language/explicit]]
+
−
[[ru:cpp/language/explicit]]
+
−
[[zh:cpp/language/explicit]]
+

Latest revision as of 23:29, 11 August 2024

 
C++
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
const/volatile
decltype (C++11)
auto (C++11)
constexpr (C++11)
consteval (C++20)
constinit (C++20)
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
explicit (C++11)
static

Special member functions
Templates
Miscellaneous
 
Classes
 

Contents

[edit] Syntax

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)or deduction guide(since C++17) 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.

[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)
Feature-test macro Value Std Feature
__cpp_conditional_explicit 201806L (C++20) conditional explicit

[edit] Keywords

explicit

[edit] 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); // suppresses “unused variable” warnings
}

[edit] See also

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/explicit&oldid=174581"