C++ return a std::string &

Question

std::string &func(int vlu)
{
    std::string str;
    str = std::to_string(vlu) + "something";

    return str;
}

the function above is unsafe clearly.
Following is another version.

std::string &func(int vlu)
{
    return std::to_string(vlu) + "something";
}  

I have some questions:
the compiler(gcc), in the second version, doesn't give me any warning. Is it safe? I just think that compiler(or something?) will create a temporary variable to hold the return of expression std::to_string(vlu) + "something". So the second version is unsafe too. and I right?

Answer 1

No, neither program is safe. In both cases the returned reference is dangling and the behaviour of using the reference will be undefined.

The second program is also ill-formed, because it attempts to bind an lvalue reference (that is returned) to an rvalue (the temporary). The second program might be considered "less unsafe", since a compiler might choose to not compile it.

To fix the function: Don't attempt to return a reference, when the purpose is to return a new object. Return an object instead.