va_copy - 移植到VISUAL C ++? [英] va_copy -- porting to visual C++?
问题描述
<一个href=\"http://stackoverflow.com/questions/69738/c-how-to-get-fprintf-results-as-a-stdstring-w-o-sprintf#69911\">A previous问题显示印刷一个很好的方式为字符串。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,上述code不端口到Visual C ++,因为它不提供va_copy(甚至__va_copy)。因此,没有人知道如何安全端口上面code? presumably,我需要做一个va_copy副本,因为vsnprintf破坏性修改通过va_list的。
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
您应该能够只是做了常规的分配就完事了:
You should be able to get away with just doing a regular assignment:
va_list apcopy = ap;
这是技术上不可移植和不确定的行为,但它会与大多数编译器与架构工作。在x86调用约定,的va_list
是公正的指针进栈,并安全地复制。
It's technically non-portable and undefined behavior, but it will work with most compilers and architectures. In the x86 calling convention, va_list
s are just pointers into the stack and are safe to copy.
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