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Difference between revisions of "cpp/named req/CopyConstructible"

From cppreference.com
< cpp‎ | named req
m (Text replace - "{{concept" to "{{named req")
(Example: CopyConstructible is false, is_copy_constructible is true)
 
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{{cpp/named req/navbar}}
  
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Specifies that an instance of the type can be copy-constructed from an [[cpp/language/value_category|lvalue expression]].
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Specifies that an instance of the type can be copy-constructed from an [[cpp/language/value category|lvalue expression]].
  
 
===Requirements===
 
===Requirements===
−
The type {{tt|T}} satisfies {{tt|CopyConstructible}} if
+
The type {{tt|T}} satisfies {{named req/core|CopyConstructible}} if
  
 
* The type {{tt|T}} satisfies {{named req|MoveConstructible}}, and
 
* The type {{tt|T}} satisfies {{named req|MoveConstructible}}, and
  
 
Given
 
Given
−
* {{tt|v}}, an [[cpp/language/value_category|lvalue]] expression of type {{tt|T}} or {{tt|const T}} or an [[cpp/language/value_category|rvalue]] expression of type {{tt|const T}}
+
* {{c|v}}, an [[cpp/language/value_category#lvalue|lvalue]] expression of type {{tt|T}} or {{c/core|const T}} or an [[cpp/language/value category#rvalue|rvalue]] expression of type {{c/core|const T}},
−
* {{tt|u}}, an arbitrary identifier
+
* {{c|u}}, an arbitrary identifier.
−
 
+
−
The following expressions must be valid and have their specified effects
+
  
 +
The following expressions must be valid and have their specified effects:
 
{|table class=wikitable
 
{|table class=wikitable
 
|-
 
|-
 
!Expression||Post-conditions
 
!Expression||Post-conditions
 
|-
 
|-
−
| {{c|1=T u = v;}}
+
|{{c|1=T u = v;}}
−
| The value of {{tt|u}} is equivalent to the value of {{tt|v}}.
+
|The value of {{c|u}} is equivalent to the value of {{c|v}}.
−
The value of {{tt|v}} is unchanged
+
The value of {{c|v}} is unchanged.
 
|-
 
|-
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| {{c|T(v)}}
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|{{c|T(v)}}
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| The value of {{tt|T(v)}} is equivalent to the value of {{tt|v}}.
+
|The value of {{c|T(v)}} is equivalent to the value of {{c|v}}.
−
The value of {{tt|v}} is unchanged.
+
The value of {{c|v}} is unchanged.
 
|}
 
|}
  
 
{{rev begin}}
 
{{rev begin}}
 
{{rev|until=c++11|
 
{{rev|until=c++11|
−
The expression {{c|v.~T()}} also must be valid, and, for lvalue {{tt|v}}, the expression {{tt|&v}} must have the type {{c|T*}} or {{c|const T*}} and must evaluate to the address of {{tt|v}}
+
The expression {{c|v.~T()}} also must be valid, and, for lvalue {{c|v}}, the expression {{c|&v}} must have the type {{tt|T*}} or {{c/core|const T*}} and must evaluate to the address of {{c|v}}.
 
}}
 
}}
 
{{rev end}}
 
{{rev end}}
  
 
===Notes===
 
===Notes===
−
Until C++11, classes that overloaded {{tt|operator&}} were not {{tt|CopyConstructible}} and thus weren't usable in the standard library containers. As of C++11, the standard library uses {{lc|std::addressof}} whenever the address of an object is needed.
+
Until C++11, classes that overloaded {{c/core|operator&}} were not {{named req/core|CopyConstructible}} and thus were not usable in the [[cpp/container|standard library containers]]. This is a design decision in C++98 (instead of a defect, see {{lwg|390}}).
 +
 
 +
Since C++11, the standard library uses {{lc|std::addressof}} whenever the address of an object is needed.
 +
 
 +
{{cot}}
 +
 
 +
Being a {{named req/core|CopyConstructible}} class implies {{lc|std::is_copy_constructible}} but not vice versa since {{lc|std::is_copy_constructible}} will only check for the ability to call the constructor with the correct arguments, and, e.g., not a {{named req|MoveConstructible}} requirement.
 +
 
 +
{{example
 +
|code=
 +
#include <type_traits>
 +
#include <utility>
 +
 
 +
struct S
 +
{
 +
    S() = default;
 +
    S(S&&) = delete;
 +
    S(const S&) = default;
 +
};
 +
static_assert(std::is_copy_constructible_v<S>);
 +
 
 +
int main()
 +
{
 +
    S s1;
 +
 
 +
    // Class `S` doesn't satisfy MoveConstructible requirement,
 +
    // hence doesn't satisfy CopyConstructible requirement
 +
    [[maybe_unused]] S s2{std::move(s1)}; // ill-formed, use of deleted function
 +
}
 +
}}
 +
{{cob}}
 +
 
 +
===References===
 +
{{cot}}
 +
{{ref std c++23}}
 +
{{ref std|section=16.4.4.2|title=Template argument requirements|id=utility.arg.requirements}}
 +
{{ref std end}}
 +
{{cob}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
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{{dsc inc | cpp/types/dsc is_copy_constructible}}
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{{dsc inc|cpp/types/dsc is_copy_constructible}}
 +
{{dsc inc|cpp/concepts/dsc copy_constructible}}
 
{{dsc end}}
 
{{dsc end}}
  
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[[de:cpp/named req/CopyConstructible]]
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{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
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[[es:cpp/named req/CopyConstructible]]
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−
[[fr:cpp/named req/CopyConstructible]]
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−
[[it:cpp/named req/CopyConstructible]]
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−
[[ja:cpp/named req/CopyConstructible]]
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−
[[pt:cpp/named req/CopyConstructible]]
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−
[[ru:cpp/named req/CopyConstructible]]
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−
[[zh:cpp/named req/CopyConstructible]]
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Latest revision as of 22:19, 3 March 2024

 
C++
 
C++ named requirements
 

Specifies that an instance of the type can be copy-constructed from an lvalue expression.

Contents

[edit] Requirements

The type T satisfies CopyConstructible if

Given

  • v, an lvalue expression of type T or const T or an rvalue expression of type const T,
  • u, an arbitrary identifier.

The following expressions must be valid and have their specified effects:

Expression Post-conditions
T u = v; The value of u is equivalent to the value of v.

The value of v is unchanged.

T(v) The value of T(v) is equivalent to the value of v.

The value of v is unchanged.

The expression v.~T() also must be valid, and, for lvalue v, the expression &v must have the type T* or const T* and must evaluate to the address of v.

(until C++11)

[edit] Notes

Until C++11, classes that overloaded operator& were not CopyConstructible and thus were not usable in the standard library containers. This is a design decision in C++98 (instead of a defect, see LWG issue 390).

Since C++11, the standard library uses std::addressof whenever the address of an object is needed.

Extended content

Being a CopyConstructible class implies std::is_copy_constructible but not vice versa since std::is_copy_constructible will only check for the ability to call the constructor with the correct arguments, and, e.g., not a MoveConstructible requirement.

Run this code
#include <type_traits>
#include <utility>
 
struct S
{
    S() = default;
    S(S&&) = delete;
    S(const S&) = default;
};
static_assert(std::is_copy_constructible_v<S>);
 
int main()
{
    S s1;
 
    // Class `S` doesn't satisfy MoveConstructible requirement,
    // hence doesn't satisfy CopyConstructible requirement
    [[maybe_unused]] S s2{std::move(s1)}; // ill-formed, use of deleted function
}

[edit] References

Extended content
  • C++23 standard (ISO/IEC 14882:2024):
  • 16.4.4.2 Template argument requirements [utility.arg.requirements]

[edit] See also

(C++11)(C++11)(C++11)
 checks if a type has a copy constructor
(class template) [edit]
(C++20)
 specifies that an object of a type can be copy constructed and move constructed
(concept) [edit]
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