Difference between revisions of "cpp/named req/CopyConstructible"
From cppreference.com
(→See also: + copy_constructible) |
(Added a note about LWG issue #390.) |
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{{cpp/named req/navbar}} | {{cpp/named req/navbar}} | ||
| − | Specifies that an instance of the type can be copy-constructed from an [[cpp/language/ | + | Specifies that an instance of the type can be copy-constructed from an [[cpp/language/value category|lvalue expression]]. |
===Requirements=== | ===Requirements=== | ||
| Line 10: | Line 10: | ||
Given | Given | ||
| − | * {{ | + | * {{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}} |
| − | * {{ | + | * {{c|u}}, an arbitrary identifier |
| − | + | ||
| − | + | ||
| + | The following expressions must be valid and have their specified effects: | ||
{|table class=wikitable | {|table class=wikitable | ||
|- | |- | ||
| Line 20: | Line 19: | ||
|- | |- | ||
| {{c|1=T u = v;}} | | {{c|1=T u = v;}} | ||
| − | | The value of {{ | + | | The value of {{c|u}} is equivalent to the value of {{c|v}}. |
| − | The value of {{ | + | The value of {{c|v}} is unchanged. |
|- | |- | ||
| {{c|T(v)}} | | {{c|T(v)}} | ||
| − | | The value of {{ | + | | The value of {{c|T(v)}} is equivalent to the value of {{c|v}}. |
| − | The value of {{ | + | 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 {{ | + | 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 {{ | + | 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. | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/types/dsc is_copy_constructible}} | + | {{dsc inc|cpp/types/dsc is_copy_constructible}} |
| − | {{dsc inc | cpp/concepts/dsc copy_constructible}} | + | {{dsc inc|cpp/concepts/dsc copy_constructible}} |
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Revision as of 18:47, 8 February 2023
Specifies that an instance of the type can be copy-constructed from an lvalue expression.
Requirements
The type T satisfies CopyConstructible if
- The type
Tsatisfies MoveConstructible, and
Given
- v, an lvalue expression of type
Tor 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 |
(until C++11) |
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.
See also
| (C++11)(C++11)(C++11) |
checks if a type has a copy constructor (class template) |
| (C++20) |
specifies that an object of a type can be copy constructed and move constructed (concept) |
