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copied!<pre><code>#include <string> const char digit_pairs[201] = { "00010203040506070809" "10111213141516171819" "20212223242526272829" "30313233343536373839" "40414243444546474849" "50515253545556575859" "60616263646566676869" "70717273747576777879" "80818283848586878889" "90919293949596979899" }; std::string& itostr(int n, std::string& s) { if(n==0) { s="0"; return s; } int sign = -(n<0); unsigned int val = (n^sign)-sign; int size; if(val>=10000) { if(val>=10000000) { if(val>=1000000000) size=10; else if(val>=100000000) size=9; else size=8; } else { if(val>=1000000) size=7; else if(val>=100000) size=6; else size=5; } } else { if(val>=100) { if(val>=1000) size=4; else size=3; } else { if(val>=10) size=2; else size=1; } } size -= sign; s.resize(size); char* c = &s[0]; if(sign) *c='-'; c += size-1; while(val>=100) { int pos = val % 100; val /= 100; *(short*)(c-1)=*(short*)(digit_pairs+2*pos); c-=2; } while(val>0) { *c--='0' + (val % 10); val /= 10; } return s; } std::string& itostr(unsigned val, std::string& s) { if(val==0) { s="0"; return s; } int size; if(val>=10000) { if(val>=10000000) { if(val>=1000000000) size=10; else if(val>=100000000) size=9; else size=8; } else { if(val>=1000000) size=7; else if(val>=100000) size=6; else size=5; } } else { if(val>=100) { if(val>=1000) size=4; else size=3; } else { if(val>=10) size=2; else size=1; } } s.resize(size); char* c = &s[size-1]; while(val>=100) { int pos = val % 100; val /= 100; *(short*)(c-1)=*(short*)(digit_pairs+2*pos); c-=2; } while(val>0) { *c--='0' + (val % 10); val /= 10; } return s; } </code></pre> <p>This will blow up on systems that disallow unaligned memory accesses (in which case, the first unaligned assignment via <code>*(short*)</code> would cause a segfault), but should work very nicely otherwise.</p> <p>One important thing to do is to minimize the use of <code>std::string</code>. (Ironic, I know.) In Visual Studio, for example, most calls to methods of std::string are not inlined, even if you specify /Ob2 in compiler options. So even something as trivial as a call to <code>std::string::clear()</code>, which you might expect to be very fast, can take 100 clockticks when linking CRT as a static library, and as much as 300 clockticks when linking as a DLL.</p> <p>For the same reason, returning by reference is better because it avoids an assignment, a constructor and a destructor.</p>

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