标签:c++ sprintf snprintf stdstring format
场景:
1. C语言有自己的sprintf函数,但是这个函数有个缺点,就是不知道需要创建多大的buffer, 这时候可以使用snprintf函数来计算大小,只要参数 buffer为NULL, count为0即可.
2. 这里实现std::string自己的sprintf也是用了snprintf的特性,先计算大小,再创建空间,之后存入std::string.
3. 还使用了C的可变参数特性.
std::wstring Format(const wchar_t *format,...) { va_list argptr; va_start(argptr, format); int count = _vsnwprintf(NULL,0,format,argptr); va_end(argptr); va_start(argptr, format); wchar_t* buf = (wchar_t*)malloc(count*sizeof(wchar_t)); _vsnwprintf(buf,count,format,argptr); va_end(argptr); std::wstring str(buf,count); free(buf); return str; }
typedef char * va_list;
#define _INTSIZEOF(n) ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) ) #define _crt_va_start(ap,v) ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) ) #define _crt_va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) ) #define _crt_va_end(ap) ( ap = (va_list)0 )
snprintf 源码实现是通过计算%的个数来判断参数个数的.
参考:
http://blog.csdn.net/echoisland/article/details/6086406
https://msdn.microsoft.com/en-us/library/1kt27hek.aspx
https://msdn.microsoft.com/en-us/library/2ts7cx93.aspx
If buffer is a null pointer and count is zero, len is returned as the count of characters required to format the output, not including the terminating null. To make a successful call with the same argument and locale parameters, allocate a buffer holding at least len + 1 characters.
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[C/C++标准库]_[初级]_[如何实现std::string自己的Format(sprintf)函数]
标签:c++ sprintf snprintf stdstring format
原文地址:http://blog.csdn.net/infoworld/article/details/46707485