Home/ Questions/Q 6254665
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-24T14:13:12+00:00

Does the standard define precisely what I can do with an object once it

  • 0

Does the standard define precisely what I can do with an object once it has been moved from? I used to think that all you can do with a moved-from object is do destruct it, but that would not be sufficient.

For example, take the function template swap as defined in the standard library:

template <typename T>
void swap(T& a, T& b)
{
    T c = std::move(a); // line 1
    a = std::move(b);   // line 2: assignment to moved-from object!
    b = std::move(c);   // line 3: assignment to moved-from object!
}

Obviously, it must be possible to assign to moved-from objects, otherwise lines 2 and 3 would fail. So what else can I do with moved-from objects? Where exactly can I find these details in the standard?

(By the way, why is it T c = std::move(a); instead of T c(std::move(a)); in line 1?)

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Moved-from objects exist in an unspecified, but valid, state. That suggests that whilst the object might not be capable of doing much anymore, all of its member functions should still exhibit defined behaviour — including operator= — and all its members in a defined state- and it still requires destruction. The Standard gives no specific definitions because it would be unique to each UDT, but you might be able to find specifications for Standard types. Some like containers are relatively obvious — they just move their contents around and an empty container is a well-defined valid state. Primitives don’t modify the moved-from object.

    Side note: I believe it’s T c = std::move(a) so that if the move constructor (or copy constructor if no move is provided) is explicit the function will fail.

    • 0