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

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< cpp‎ | named req
(+other variants)
(Example: CopyConstructible is false, is_copy_constructible is true)
 
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{{cpp/concept/title|CopyConstructible}}
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{{cpp/named req/title|CopyConstructible}}
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{{cpp/concept/sidebar}}
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{{cpp/named req/navbar}}
  
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Specifies that an instance of the type can be copy-constructed (copied).  
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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===
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The type {{tt|T}} satisfies {{named req/core|CopyConstructible}} if
  
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The type must implement the following functions:
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* The type {{tt|T}} satisfies {{named req|MoveConstructible}}, and
  
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{{member | {{dcl small|Type::}}Type | 2=
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Given
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* {{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}},
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* {{c|u}}, an arbitrary identifier.
  
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{{ddcl | notes={{mark|One of the variants is sufficient}} | 1=
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The following expressions must be valid and have their specified effects:
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Type::Type( Type& other );
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{|table class=wikitable
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Type::Type( const Type& other );
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|-
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Type::Type( volatile Type& other );
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!Expression||Post-conditions
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Type::Type( const volatile Type& other );
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|-
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|{{c|1=T u = v;}}
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|The value of {{c|u}} is equivalent to the value of {{c|v}}.
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The value of {{c|v}} is unchanged.
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|-
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|{{c|T(v)}}
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|The value of {{c|T(v)}} is equivalent to the value of {{c|v}}.
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The value of {{c|v}} is unchanged.
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|}
 +
 
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{{rev begin}}
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{{rev|until=c++11|
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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}}.
 
}}
 
}}
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{{ddcl | Type::Type( const Type& other );}}
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{{rev end}}
  
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[[cpp/language/copy_constructor|Copy constructor]]: constructs an instance of a type with the contents of {{tt|other}}. The internal state of {{tt|other}} is not modified.
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===Notes===
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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}}).
  
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The following expressions must have the specified effects:
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Since C++11, the standard library uses {{lc|std::addressof}} whenever the address of an object is needed.
  
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{{tdcl list begin}}
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{{cot}}
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{{tdcl list hitem | Expression | Effects}}
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{{tdcl list item | {{cpp|1=Type a = v;}} | {{tt|a}} is equivalent to {{tt|v}}, where {{tt|rv}} is an instance of {{tt|Type}}. {{tt|v}} must be unchanged.}}
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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.
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{{tdcl list item | {{cpp|Type(v);}} |  a temporary object of type {{tt|Type}} is equivalent to {{tt|v}}, where {{tt|v}} is an instance of {{tt|Type}}. {{tt|v}} must be unchanged. }}
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{{tdcl list end}}
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{{example
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|code=
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#include <type_traits>
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#include <utility>
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struct S
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{
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    S() = default;
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    S(S&&) = delete;
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    S(const S&) = default;
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};
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static_assert(std::is_copy_constructible_v<S>);
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int main()
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{
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    S s1;
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    // Class `S` doesn't satisfy MoveConstructible requirement,
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    // hence doesn't satisfy CopyConstructible requirement
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    [[maybe_unused]] S s2{std::move(s1)}; // ill-formed, use of deleted function
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}
 
}}
 
}}
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{{cob}}
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===References===
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{{cot}}
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{{ref std c++23}}
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{{ref std|section=16.4.4.2|title=Template argument requirements|id=utility.arg.requirements}}
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{{ref std end}}
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{{cob}}
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===See also===
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{{dsc begin}}
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{{dsc inc|cpp/types/dsc is_copy_constructible}}
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{{dsc inc|cpp/concepts/dsc copy_constructible}}
 +
{{dsc end}}
 +
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

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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