# C++ named requirements: CopyConstructible

From cppreference.com
< cpp‎ | named req

Specifies that an instance of the type can be copy-constructed from an lvalue expression.

### Requirements

The type `T` satisfies CopyConstructible if

- The type `T` satisfies MoveConstructible, and

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)

### 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
    }

### References

Extended content

- C++23 standard (ISO/IEC 14882:2024):

  - 16.4.4.2 Template argument requirements [utility.arg.requirements]

### See also

is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11) | checks if a type has a copy constructor (class template)
copy_constructible (C++20) | specifies that an object of a type can be copy constructed and move constructed (concept)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/named_req/CopyConstructible&oldid=170058"