va_copy --移植到visual C++？
va_copy --移植到visual C++？
提问于 2009-02-17 18:50:59
A previous question展示了打印到字符串的一种很好的方法。答案涉及到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面的代码呢？大概，我需要做一个va_copy拷贝，因为vsnprintf会破坏性地修改传递的va_list。
回答 4
Stack Overflow用户
回答已采纳
发布于 2009-02-17 19:00:28
你应该能够只做一项常规的任务：
va_list apcopy = ap;从技术上讲，它是不可移植和未定义的行为，但它可以与大多数编译器和体系结构一起工作。在x86调用约定中，va_list只是指向堆栈的指针，可以安全地复制。
Stack Overflow用户
发布于 2012-11-30 02:17:25
对于Windows，您可以简单地自己定义va_copy：
#define va_copy(dest, src) (dest = src)Stack Overflow用户
发布于 2009-02-17 19:19:50
您可以做的一件事是，如果您不需要vformat()函数，可以将其实现移到format()函数中(未经测试)：
#include <stdarg.h>
#include <string.h>
#include <assert.h>
#include <string>
#include <vector>
std::string format(const char *fmt, ...)
{
va_list ap;
enum {size = 1024};
// if you want a buffer on the stack for the 99% of the time case
// for efficiency or whatever), I suggest something like
// STLSoft's auto_buffer<> template.
//
// http://www.synesis.com.au/software/stlsoft/doc-1.9/classstlsoft_1_1auto__buffer.html
//
std::vector<char> buf( size);
//
// where you get a proper vsnprintf() for MSVC is another problem
// maybe look at http://www.jhweiss.de/software/snprintf.html
//
// note that vsnprintf() might use the passed ap with the
// va_arg() macro. This would invalidate ap here, so we
// we va_end() it here, and have to redo the va_start()
// if we want to use it again. From the C standard:
//
// The object ap may be passed as an argument to
// another function; if that function invokes the
// va_arg macro with parameter ap, the value of ap
// in the calling function is indeterminate and
// shall be passed to the va_end macro prior to
// any further reference to ap.
//
// Thanks to Rob Kennedy for pointing that out.
//
va_start (ap, fmt);
int needed = vsnprintf (&buf[0], buf.size(), fmt, ap);
va_end( ap);
if (needed >= size) {
// 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.
buf.resize( needed + 1);
va_start (ap, fmt);
needed = vsnprintf (&buf[0], buf.size(), fmt, ap);
va_end( ap);
assert( needed < buf.size());
}
return std::string( &buf[0]);
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/558223
复制相关文章
相似问题
腾讯云开发者
