Is taking a reference from a temporary valid C++ code?

Question

I used the following syntactic sugar:

for (auto& numberString: {"one", "two", "three", "four"}) { /* ... */}

Is this valid code? AFAIK, based on this question, this should be illegal, yet the code runs as expected. I don't think my understanding is correct on the matter.

As far as I know, only literals should not have memory addresses, yet the linked question is talking about temporaries and r-values.

Answer 1

Yes, this code is valid.

Keep in mind that (for C++17), the compiler will semantically replace the range-based for loop by the construct

{

    auto && __range = {"one", "two", "three", "four"};
    for (auto __begin = begin(__range), __end = end(__range); __begin != __end; ++__begin)
    {

        auto& numberString = *__begin;
        /* ... */
    }

}

You see, the lifetime of the initializer_list is extended to the lifetime of __range inside of the outermost scope in the replacement.

Note however that you still can easily cause undefined behavior if the range expression contains a temporary itself:

struct some {
   auto get_list() { return {"one", "two", "three", "four"}; }
};

some foo() { return some{ }; }

for(auto& numberString : foo().get_list()) { /* ... */ }

The above code will result in a dangling reference in <= C++20. Only in C++23, the lifetime of the temporary created by foo() will get extended such that it becomes valid. See also https://en.cppreference.com/w/cpp/language/range-for