Difference between revisions of "cpp/named req/CopyConstructible"
(Added a note about LWG issue #390.) |
(Example: CopyConstructible is false, is_copy_constructible is true) |
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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|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 | + | * {{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: | ||
| Line 18: | Line 18: | ||
!Expression||Post-conditions | !Expression||Post-conditions | ||
|- | |- | ||
| − | | {{c|1=T u = v;}} | + | |{{c|1=T u = v;}} |
| − | | The value of {{c|u}} is equivalent to the value of {{c|v}}. | + | |The value of {{c|u}} is equivalent to the value of {{c|v}}. |
The value of {{c|v}} is unchanged. | The value of {{c|v}} is unchanged. | ||
|- | |- | ||
| − | | {{c|T(v)}} | + | |{{c|T(v)}} |
| − | | The value of {{c|T(v)}} is equivalent to the value of {{c|v}}. | + | |The value of {{c|T(v)}} is equivalent to the value of {{c|v}}. |
The value of {{c|v}} is unchanged. | The value of {{c|v}} is unchanged. | ||
|} | |} | ||
| Line 37: | Line 37: | ||
Since C++11, the standard library uses {{lc|std::addressof}} whenever the address of an object is needed. | 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=== | ||
Latest revision as of 22:19, 3 March 2024
Specifies that an instance of the type can be copy-constructed from an lvalue expression.
Contents |
[edit] 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) |
[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 |
|---|
|
[edit] 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) |
