Assign a value to an rvalue reference returned from function

Question

#include <utility>
template <typename Container>
decltype(auto) index(Container &&arr, int n) {
    return std::forward<Container>(arr)[n];
}

Make a function call :

#include <vector>
index(std::vector {1, 2, 3, 4, 5}, 2) = 0;

When function calling finished, the object std::vector {1, 2, 3, 4, 5} will be destroyed, assigning a value to a deallocated address would cause undefined behaviour. But the above code works well and valgrind detected nothing. Maybe the compile helps me make another invisible variable like

auto &&invisible_value {index(std::vector {1, 2, 3, 4, 5}, 2)};
invisible_value = 9;

If my guess is incorrect, I want to know why assigning a value to an rvalue reference returned from function is worked and when the temporary object index(std::vector {1, 2, 3, 4, 5}, 2) will be destroyed.

This idea originated from 《Effective Modern C++》, Item3 : Understand decltype.

Answer 1

You said "When function calling finished, the object vector {1, 2, 3, 4, 5} will be destroyed" but that is untrue. The temporary created for the function call is not deleted until the statement ends, i.e. the next line of code. Otherwise imagine how much code would break that passes c_str() of a temporary string.