Your code, as written, works. You’re probably trying to achieve something unrelated, but similar:
std::string c = "hello" + "world";
This doesn’t work because for C++ this seems like you’re trying to add two char pointers. Instead, you need to convert at least one of the char* literals to a std::string. Either you can do what you’ve already posted in the question (as I said, this code will work) or you do the following:
std::string c = std::string("hello") + "world";