toStr revisited

After my initial toStr C++ exploration, I recent found myself reading about Boost’s lexical_cast, which is does something similar, albeit more general and with more verbosity. lexical_cast will not only convert nearly anything to a string, it will also do its best to convert anything to anything, via a string in the middle.

Upon finding that, I was considering rewriting toStr to use lexical_cast (the only reason to hang onto toStr at all would have been for brevity in my code), but then I somehow stumbled upon Herb Sutter’s article The String Formatters of Manor Farm, which talks about the performance of various int to string methods. As it is, int to string is what I use my toStr function for most of the time (followed by doubles probably), so this is the performance I’m most interested in.

From Herb’s article, I learned that lexical_cast was extremely slow. stringstream, which is what the current implementation of toStr uses, is also extremely slow (but not as bad as lexical_cast). On the other hand, snprintf is very fast. I verified this with some of my own tests on Solaris/SunStudio and Linux/GCC. Rather then write up my own performance tests, allow me to refer you to this write up, as well as back to Herb Sutter’s article that I reference above.

snprintf requires explicit formatting strings, but this isn’t an issue because I can specialize the template for certain types, like ints. And, if I know the type, then I also know the maximum length string that can be generated. For instance, a 32 bit int, is 12 digits (The ‘-‘ if negative, 10 digits for the main number, and one digit for the trailing \0), while a 64 bit int is 22 digits. I could also figure out the maximum size for floats and doubles, but I haven’t yet done so.

So I will now modify toStr to include the existing version while specializing to be about 17 times faster for ints.

static inline std::string toStr(T v)
std::stringstream s;
s << v; return s.str(); } template <>
inline std::string toStr(int v)
//max 64 value: 18446744073709551615
char tmp[22]; //derived from the max 64bit value + 1 for '-' and 1 for \0
snprintf(tmp, 22, "%i", v);
return std::string(tmp);