The Standard C (and C++) function strtok() leaves a lot to
      be desired in terms of user-friendliness.  It's unintuitive, it
      destroys the character string on which it operates, and it requires
      you to handle all the memory problems.  But it does let the client
      code decide what to use to break the string into pieces; it allows
      you to choose the "whitespace," so to speak.
   
A C++ implementation lets us keep the good things and fix those annoyances. The implementation here is more intuitive (you only call it once, not in a loop with varying argument), it does not affect the original string at all, and all the memory allocation is handled for you.
It's called stringtok, and it's a template function. Sources are as below, in a less-portable form than it could be, to keep this example simple (for example, see the comments on what kind of string it will accept).
#include <string>
template <typename Container>
void
stringtok(Container &container, string const &in,
          const char * const delimiters = " \t\n")
{
    const string::size_type len = in.length();
          string::size_type i = 0;
    while (i < len)
    {
        // Eat leading whitespace
        i = in.find_first_not_of(delimiters, i);
        if (i == string::npos)
	  return;   // Nothing left but white space
        // Find the end of the token
        string::size_type j = in.find_first_of(delimiters, i);
        // Push token
        if (j == string::npos) 
	{
	  container.push_back(in.substr(i));
	  return;
        } 
	else
	  container.push_back(in.substr(i, j-i));
        // Set up for next loop
        i = j + 1;
    }
}
The author uses a more general (but less readable) form of it for parsing command strings and the like. If you compiled and ran this code using it:
   std::list<string>  ls;
   stringtok (ls, " this  \t is\t\n  a test  ");
   for (std::list<string>const_iterator i = ls.begin();
        i != ls.end(); ++i)
   {
       std::cerr << ':' << (*i) << ":\n";
   } You would see this as output:
:this: :is: :a: :test:
with all the whitespace removed.  The original s is still
      available for use, ls will clean up after itself, and
      ls.size() will return how many tokens there were.
   
As always, there is a price paid here, in that stringtok is not as fast as strtok. The other benefits usually outweigh that, however.
Added February 2001:  Mark Wilden pointed out that the
      standard std::getline() function can be used with standard
      istringstreams to perform
      tokenizing as well.  Build an istringstream from the input text,
      and then use std::getline with varying delimiters (the three-argument
      signature) to extract tokens into a string.