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.

Answer 2

I do not approve of you const_casting above and using strtok.

A std::string can contain embedded nulls but let's assume it will be all ASCII 32 characters before you hit the NULL terminator.

One way you can approach this is with a simple loop, and I will assume const char *.

bool all_spaces( const char * v )
{
   for ( ; *v; ++v )
   {
      if( *v != ' ' )
          return false;
   }
   return true;
}

For larger strings, you can check word-at-a-time until you reach the last word, and then assume the 32-bit word (say) will be 0x20202020 which may be faster.

Answer 3

Here's one that only uses STL (Requires C++11)

inline bool isBlank(const std::string& s)
{
    return std::all_of(s.cbegin(),s.cend(),[](char c) { return std::isspace(c); });
}

It relies on fact that if string is empty (begin = end) std::all_of also returns true

Here is a small test program: http://cpp.sh/2tx6

Answer 4

Why so much work, so much typing?

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

Answer 5

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.

Answer 6

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);