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C++ named requirements: MoveConstructible (since C++11)

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MoveConstructible
(C++11)
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Specifies that an instance of the type can be constructed from an rvalue argument.

Contents

[edit] Requirements

The type T satisfies MoveConstructible if

Given

  • rv, an rvalue expression of type T,
  • u, an arbitrary identifier.

The following expressions must be valid and have their specified effects.

Expression Post-conditions
T u = rv; The value of u is equivalent to the value of rv before the initialization.

The new value of rv is unspecified.

T(rv) The value of T(rv) is equivalent to the value of rv before the initialization.

The new value of rv is unspecified.

[edit] Notes

A class does not have to implement a move constructor to satisfy this type requirement: a copy constructor that takes a const T& argument can bind rvalue expressions.

If a MoveConstructible class implements a move constructor, it may also implement move semantics to take advantage of the fact that the value of rv after construction is unspecified.

Extended content

Being a MoveConstructible class implies std::is_move_constructible but not vice versa since std::is_move_constructible will only check for the ability to call the constructor with the correct arguments, not a post-condition value.

#include <iostream>
 
struct S
{
    int n;
    S(int in) : n{in} {}
    S(S&& other) { n = other.n + 1; }
};
static_assert(std::is_move_constructible_v<S>);
 
int main()
{
    S v{1};
    std::cout << "v.n = " << v.n << '\n';
    S u = std::move(v);
 
    // Class `S` doesn't satisfy a MoveConstructible requirement
    // The value of `u` is NOT equivalent to the value of `v` before the `u` initialization
    std::cout << "u.n = " << u.n << '\n';
}

Output:

v.n = 1
u.n = 2

[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 can be constructed from an rvalue reference
(class template) [edit]
specifies that an object of a type can be move constructed
(concept) [edit]