Ranged base for loop over temporary reference C++11

Question

I'm wondering if the following is valid:

#include <iostream>
#include <vector>

std::vector<int>& getVec()
{
    static std::vector<int> vec{1, 2, 3, 4, 5};
    return vec;
}

int main()
{
    for (const auto& i : getVec())
    {
        std::cout << "i = " << i << std::endl;
    }
    return 0;
}

Basically I'm unsure about the lifetime of the temporary from getVec(). I've looked at both this post and this one, but am in a bit of a different situation as my function returns a reference to static data. Specifically, I'm wondering if scenario violates the following exception in the stated rule:

A temporary bound to a reference parameter in a function call [...]

or if this is indeed safe code. I think it is safe, but just wanted to be sure.

There is no temporary vector here. You are just iterating over `getVec`'s `vec`. — François Andrieux, May 31 '18 at 19:09
Ok, I was a bit confused and though maybe the range based for loop was creating a temporary reference to the `vector` returned in `getVec`. Thanks for the clarification. Post an answer if you'd like and I'll mark it as the answer. — zeus_masta_funk, May 31 '18 at 19:15
the range based for loop basically just calls `begin()` and `end()` and exapnds to an iterator based loop. see eg [here](https://en.cppreference.com/w/cpp/language/range-for) — 463035818_is_not_an_ai, May 31 '18 at 19:25

score 2 · Accepted Answer · answered May 31 '18 at 19:32

Yes, this is fully valid and well-defined.

The range-based for loop in your question is defined to be equivilent to the following, for imaginary variables range, begin, and end:

auto&& range = getVec();
auto begin = std::begin(range);
auto end = std::end(range);
for (; begin != end; ++begin)
{
    const auto& i = *begin;
    {
        std::cout << "i = " << i << std::endl;
    }
}

After reference collapsing rules are applied, the type of range becomes std::vector<int>&. That means no temporaries are ever created. The loop iterates over the static vector defined in getVec.

If getVec instead returned by value, the type of range would be std::vector<int>&&, and lifetime extension would be applied. That would extend the lifetime of the temporary object to that of the reference, and everything would still be totally valid.

Ranged base for loop over temporary reference C++11

1 Answers1