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C++ named requirements: CopyConstructible

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< cpp‎ | named req
 
 
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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.

#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 14882:2023):
  • 16.4.4.2 Template argument requirements [utility.arg.requirements]

[edit] See also

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