Efficient way to check if std::string has only spaces

Question

I was just talking with a friend about what would be the most efficient way to check if a std::string has only spaces. He needs to do this on an embedded project he is worki

Answer 1

Hm...I'd do this:

for (auto i = str.begin(); i != str.end() ++i)
    if (!isspace(i))
       return false;

Pseudo-code, isspace is located in cctype for C++.

Edit: Thanks to James for pointing out that isspace has undefined behavior on signed chars.