C++ named requirements: CopyConstructible - cppreference.com (original) (raw)
Specifies that an instance of the type can be copy-constructed from an lvalue expression.
[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 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 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] |