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

Wouldn't it be easier to do:

bool has_only_spaces(const std::string &str)
{
    for (std::string::const_iterator it = str.begin(); it != str.end(); ++it)
    {
        if (*it != ' ') return false;
    }
    return true;
}

This has the advantage of returning early as soon as a non-space character is found, so it will be marginally more efficient than solutions that examine the whole string.