What are the implications of self move assignment for vectors in the C Standard Library?

Self Move Assignment in the C Standard Library

The C standard library guarantees certain behaviors for self move assignment, as defined in the standard.

Specifically, the standard states that for functions in the standard library with arguments of type T&&, the implementation may assume that the parameter is a unique reference to the argument. This means that the standard library may optimize its behavior based on the assumption that the parameter is a temporary rvalue.

In the case of self move assignment, the implementation is allowed to assume that the argument vector is a prvalue. This means that the standard library may perform optimizations that would not be possible if the argument was an lvalue.

As a result, self move assignment in the standard library is likely to result in the vector being left in a resource-less state, with a capacity of 0. If the vector already has a capacity of 0, the self move assignment will be a no-op.

Update

The latest draft of the C standard explicitly states that in the MoveAssignable requirements, the expression t = rv (where rv is an rvalue) only requires t to be equivalent to rv prior to the assignment if t and rv do not reference the same object. Additionally, rv's state is unspecified after the assignment.

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