itoa was a non-standard helper function designed to complement the atoi standard function, and probably hiding a sprintf (Most its features can be implemented in terms of sprintf): http://www.cplusplus.com/reference/clibrary/cstdlib/itoa.html
Despite the fact some functions are not in the standard, as rightly mentioned by "onebyone" in one of his comments, most compiler will offer you an alternative (e.g. Visual C++ has its own _snprintf you can typedef to snprintf if you need it).
Use the C++ streams (in the current case std::stringstream (or even the deprecated std::strstream, as proposed by Herb Sutter in one of his books, because it's somewhat faster).
You're in C++, which means that you can choose the way you want it:
@Chris Kaminski: My one tests did show the sprintf was a 5 to ten times faster, which is confirmed my Herb Sutter's own measurements. if you have tests with different results, I'm interested.
@Chris Kaminski: If you study the c++ stream's interface, you'll understand why they are slower, even when output a simple integer: In C, you use your own buffer, possibly allocated in the stack, whereas in C++, the stringstream will use its own. In C, you can then reuse your buffer. In C++, you must extract the string from the stringstream, which is a std::string copy.
@fuzzyTew : Thanks for your concern, but I guess I'm familiar enough with both C API and C++ API to handle sprintf, and know when (and how) to use it safely, and when NOT to use it at all... :-D
@fuzzyTew: 1 In my post, I spoke about sprintf and its secure variants, not only sprintf. 2 Knowing where your code is compiled is not an impossible task (in my case, at worst, Windows/VC++, Solaris/CC and Linux/g++, at best, Windows/VC++ only). 3 You are describing a world where saboteurs trying to crash your code are the norm. My world is composed by normal developers, so losing time trying to protect my code from saboteurs by rewriting my "secure" version of every API is not productive. [...]
@fuzzyTew: [...] Conclusion Use the best tool at hand. And if the best tool is a sprintf hidden inside a wrapper class or function... Now, if you advocate the rewriting of sprintf as an answer for this question, feel free to write your own answer. I'm not sure the question author reads all the comments.