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copied!<p>Here's the idiom I like for making functionality identical to 'sprintf', but returning a std::string, and immune to buffer overflow problems. This code is part of an open source project that I'm writing (BSD license), so everybody feel free to use this as you wish.</p> <pre><code>#include <string> #include <cstdarg> #include <vector> #include <string> std::string format (const char *fmt, ...) { va_list ap; va_start (ap, fmt); std::string buf = vformat (fmt, ap); va_end (ap); return buf; } std::string vformat (const char *fmt, va_list ap) { // Allocate a buffer on the stack that's big enough for us almost // all the time. size_t size = 1024; char buf[size]; // Try to vsnprintf into our buffer. va_list apcopy; va_copy (apcopy, ap); int needed = vsnprintf (&buf[0], size, fmt, ap); // NB. On Windows, vsnprintf returns -1 if the string didn't fit the // buffer. On Linux & OSX, it returns the length it would have needed. if (needed <= size && needed >= 0) { // It fit fine the first time, we're done. return std::string (&buf[0]); } else { // vsnprintf reported that it wanted to write more characters // than we allotted. So do a malloc of the right size and try again. // This doesn't happen very often if we chose our initial size // well. std::vector <char> buf; size = needed; buf.resize (size); needed = vsnprintf (&buf[0], size, fmt, apcopy); return std::string (&buf[0]); } } </code></pre> <p>EDIT: when I wrote this code, I had no idea that this required C99 conformance and that Windows (as well as older glibc) had different vsnprintf behavior, in which it returns -1 for failure, rather than a definitive measure of how much space is needed. Here is my revised code, could everybody look it over and if you think it's ok, I will edit again to make that the only cost listed:</p> <pre><code>std::string Strutil::vformat (const char *fmt, va_list ap) { // Allocate a buffer on the stack that's big enough for us almost // all the time. Be prepared to allocate dynamically if it doesn't fit. size_t size = 1024; char stackbuf[1024]; std::vector<char> dynamicbuf; char *buf = &stackbuf[0]; va_list ap_copy; while (1) { // Try to vsnprintf into our buffer. va_copy(ap_copy, ap); int needed = vsnprintf (buf, size, fmt, ap); va_end(ap_copy); // NB. C99 (which modern Linux and OS X follow) says vsnprintf // failure returns the length it would have needed. But older // glibc and current Windows return -1 for failure, i.e., not // telling us how much was needed. if (needed <= (int)size && needed >= 0) { // It fit fine so we're done. return std::string (buf, (size_t) needed); } // vsnprintf reported that it wanted to write more characters // than we allotted. So try again using a dynamic buffer. This // doesn't happen very often if we chose our initial size well. size = (needed > 0) ? (needed+1) : (size*2); dynamicbuf.resize (size); buf = &dynamicbuf[0]; } } </code></pre>

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