Why does my function only work with lvalues?

Question

I have a function that returns a lowercase string:

constexpr auto string_to_lower_case(const std::string& string) {
    return string
        | std::views::transform(std::tolower)
        | std::views::transform([](const auto& ascii) { return static_cast<char>(ascii); });
}

and I expect that function return the same resultat when i will pass "SOME" or const std::string some("SOME"), but it's not. When I try to print out a result of string_to_lower_case("SOME"), I retrieve an empty console (the output of string_to_lower_case(some) is correct)

const std::string some("SOME");
for (const auto& ch : string_to_lower_case(some))
    std::cout << ch;

Look [here](https://stackoverflow.com/questions/63531431/passing-a-char-array-to-a-function-that-expects-a-const-stdstring-reference). The call with the char-array creates a temporary, that is gone when you want to output it — davidhigh, Jun 16 '22 at 11:57
Cannot reproduce in [GCC/Clang](https://godbolt.org/z/of85d4GKv). — 康桓瑋, Jun 16 '22 at 12:36
@康桓瑋 Add `-g -fsanitize=address,undefined` and it'll be more fun :-) — Ted Lyngmo, Jun 16 '22 at 12:41

Ted Lyngmo · Accepted Answer · 2022-06-16T13:00:55.960

Some issues:

The temporary std::string that is created when you call the function with a char[] goes out of scope when the function returns and is then destroyed. The view you return can't be used to iterate over the string after that.
You take the address of std::tolower which isn't allowed since it's not on the list of Designated addressable functions.
You don't convert the char used with std::tolower to unsigned char first. If char has a negative value, it'll cause undefined behavior.
Your second transformation seems redundant.

An alternative is to return an actual std::string instead of a view:

constexpr std::string string_to_lower_case(const std::string& string) {
    auto view = string
              | std::views::transform([](char ch){
                    return static_cast<char>(std::tolower(static_cast<unsigned char>(ch)));
                });
    return {view.begin(), view.end()};
}

Why does my function only work with lvalues?

1 Answers1