A previous question展示了一种打印到字符串的好方法。答案涉及 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没有移植到 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 复制,因为 vsnprintf 破坏性地修改了传递的 va_list。
最佳答案
你应该能够摆脱只做一个常规的任务:
va_list apcopy = ap;
它在技术上是不可移植和未定义的行为,但它适用于大多数编译器和架构。在 x86 调用约定中,va_list
只是指向堆栈的指针,可以安全复制。
关于c++ - va_copy -- 移植到 Visual C++?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/558223/