Effective construction std::string from std::unordered_set

I have an unordered_set of chars

std::unordered_set<char> u_setAlphabet;

Then I want to get a content from the set as std::string. My implementation now looks like this:

std::string getAlphabet() {
    std::string strAlphabet;
    for (const char& character : u_setAlphabet)
        strAlphabet += character;
    return strAlphabet;
}

Is this a good way to solve this task? The additions of signle chars to string seems not to be optimal for large u_setAlphabet (multiple reallocs?). Is there any other method to it?