va_copy——移植到visual C++?

问题描述

上一个问题 展示了一种打印到字符串的好方法.答案涉及 va_copy:

A previous question showed a nice way of printing to a string. The answer involved va_copy:

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.
   s ize_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);

   if (needed <= size) {
       // 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]);
   }

}

我遇到的问题是上面的代码不能移植到 Visual C++，因为它不提供 va_copy(甚至 __va_copy).那么，有谁知道如何安全地移植上述代码?据推测，我需要做一个 va_copy 副本，因为 vsnprintf 破坏性地修改了传递的 va_list.

The problem I'm having is that the above code doesn't port to Visual C++ because it doesn't provide va_copy (or even __va_copy). So, does anyone know how to safely port the above code? Presumably, I need to do a va_copy copy because vsnprintf destructively modifies the passed va_list.

推荐答案

你应该能够摆脱只是做一个常规的任务:

You should be able to get away with just doing a regular assignment:

va_list apcopy = ap;

从技术上讲，它是不可移植和未定义的行为，但它适用于大多数编译器和体系结构.在 x86 调用约定中，va_list 只是指向堆栈的指针，可以安全复制.

It's technically non-portable and undefined behavior, but it will work with most compilers and architectures. In the x86 calling convention, va_lists are just pointers into the stack and are safe to copy.

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