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

if(str.find_first_not_of(' ') != std::string::npos)
{
    // There's a non-space.
}

In C++11, the all_of algorithm can be employed:

// Check if s consists only of whitespaces
bool whiteSpacesOnly = std::all_of(s.begin(),s.end(),isspace);

Why so much work, so much typing?

bool has_only_spaces(const std::string& str) {
   return str.find_first_not_of (' ') == str.npos;
}