c++ - What can I do with a moved-from object?

Question

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?)

Answer 1

17.6.5.15 [lib.types.movedfrom]

Objects of types defined in the C++ standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state.

When an object is in an unspecified state, you can perform any operation on the object which has no preconditions. If there is an operation with preconditions you wish to perform, you can not directly perform that operation because you do not know if the unspecified-state of the object satisfies the preconditions.

Examples of operations that generally do not have preconditions:

• destruction
• assignment
• const observers such as get, empty, size

Examples of operations that generally do have preconditions:

• dereference
• pop_back

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