dev / null（或Windows下的NUL）并打印到该文件以检索大小的效果非常好（它比打印到字符串更快）：

#if！ defined（_MSC_VER）

   printf_dummy_file = fopen（/ dev / null，wb）;

#else

   printf_dummy_file = fopen（NUL，wb）;

#endif

...

n = vfprintf（printf_dummy_file，format， args）;

fopen部分应该做一次（你可以使用静态变量，或者使用C ++的全局变量的构造函数）。

I need to allocate a sufficient buffer for format function vswprintf(). When doing the same thing with ANSI string, I'm using:

vsnprintf( NULL, NULL, pszFormat, args );

which returns me required size of a buffer. But it seems that unicode version of this function doesn't have this functionality. When I execute:

vswprintf( NULL, NULL, pszFormat, args );

result value is always -1.

Only solution which I found is using a large static buffer for calculation required size. But I don't like this solution:

static const int nBuffSize = 1024;
static XCHAR evalBuff[nBuffSize];
int nSize = vswprintf( evalBuff, nBuffSize, pszFormat, args );
if ( nSize != -1 )
{
 return nSize;
}
else
{
 throw XXX;
}

Is there any way how to measure required buffer size for unicode strings?

Regards Ludek

Opening a dummy file to /dev/null (or NUL under windows) and printing to that file to retrieve the size works very well (it is faster than printing to a string):

#if !defined(_MSC_VER)
   printf_dummy_file = fopen("/dev/null", "wb");
#else
   printf_dummy_file = fopen("NUL", "wb");
#endif

...

n = vfprintf(printf_dummy_file, format, args);

The "fopen" part should be done once (you can use a static variable, or the constructor of a global variable if you are using C++).