How to check if string ends with .txt

Question

I am learning basic C++, and right now I have gotten a string from a user and I want to check if they typed the entire file name (including .txt) or not. I have the string,

Answer 1

The easiest approach is probably to verify that the string is long enough to hold ".txt" at all and to see if the string can be found at the position size() - 4, e.g.:

bool ends_with_string(std::string const& str, std::string const& what) {
    return what.size() <= str.size()
        && str.find(what, str.size() - what.size()) != str.npos;
}