C++ named requirements: MoveConstructible (since C++11) (original) (raw)
Specifies that an instance of the type can be constructed from an rvalue argument.
[edit] Requirements
The type T satisfies MoveConstructible if
Given
rv, an rvalue expression of typeT,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. Run this code #include 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 14882:2024): 16.4.4.2 Template argument requirements [utility.arg.requirements] |