这是我喜欢的习惯用法，它使功能与“sprintf”相同，但返回 std::string，并且不受缓冲区溢出问题的影响。该代码是我正在编写的开源项目的一部分（BSD 许可证），因此每个人都可以随意使用它。
#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]);
}
}
编辑：当我编写这段代码时，我不知道这需要 C99 一致性，并且 Windows（以及旧版 glibc）具有不同的 vsnprintf 行为，其中失败时返回 -1，而不是确定多少空间的度量是需要的。这是我修改后的代码，大家可以看一下吗？如果您认为可以，我将再次编辑以使其成为列出的唯一成本：
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];
}
}
