va_start()va_end()函數的應用:
1:當無法列出傳遞函數的所有實參的類型和數目時,可用省略号指定參數表
void foo(...);
void foo(parm_list,...);
2:函數參數的傳遞原理
函數參數是以資料結構:棧的形式存取,從右至左入棧.
eg:
#include
void fun(int a, ...)
{
int *temp = &a;
temp++;
for (int i = ; i < a; ++i)
{
cout << *temp << endl;
temp++;
}
}
int main()
{
int a = ;
int b = ;
int c = ;
int d = ;
fun(, a, b, c, d);
system("pause");
return ;
}
Output:
3:擷取省略号指定的參數
在函數體中聲明一個va_list,然後用va_start函數來擷取參數清單中的參數,使用完畢後調用va_end()結束。
4.va_start使argp指向第一個可選參數。va_arg傳回參數清單中的目前參數并使argp指向參數清單中的下一個參數。va_end把argp指針清為NULL。函數體内可以多次周遊這些參數,但是都必須以va_start開始,并以va_end結尾。
執行個體:
#include <stdio.h>
#include <strings.h>
#include <stdarg.h>
int demo(char *fmt, ...);
int main()
{
demo("DEMO", "This", "is", "a", "demo!", "");
return ;
}
int demo(char *fmt, ...)
{
va_list argp;
int argno = ;
char *para;
va_start(argp, fmt);
while ()
{
para = va_arg(argp, char *);
if (strcmp(para, "") == )
break;
printf("Parameter #%d is: %s/n", argno, para);
argno++;
}
va_end(argp);
return ;
}
//vstart.c
#include <stdio.h>
#include <strings.h>
#include <stdarg.h>
int demo(char *fmt, ...);
int main()
{
demo("DEMO", "This", "is", "a", "demo!", "");
return ;
}
int demo(char *fmt, ...)
{
va_list argp;
int argno = ;
char *para;
va_start(argp, fmt);
while ()
{
para = va_arg(argp, char *);
if (strcmp(para, "") == )
break;
printf("Parameter #%d is: %s/n", argno, para);
argno++;
}
va_end(argp);
return ;
}
編譯運作:
[[email protected] test]# gcc -o vstart vstart.c
[[email protected] test]# ./vstart
Parameter #0 is: This
Parameter #1 is: is
Parameter #2 is: a
Parameter #3 is: demo!
[[email protected] test]#
注意:va_arg()的格式:
type va_arg(va_list ap, type);
是以:
int d;
char c, *s;
d = va_arg(ap, int);
c = (char) va_arg(ap, int);
s = va_arg(ap, char ); / string */
執行個體2:
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
int foo(char *fmt, ...);
int main()
{
char *a = "ast";
int b = ;
char c = 'x';
foo("%s,%d,%c/n", a, b, c);
return ;
}
int foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch (*fmt++)
{
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s/n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d/n", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char)va_arg(ap, int);
printf("char %c/n", c);
break;
}
va_end(ap);
return ;
}
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
int foo(char *fmt, ...);
int main()
{
char *a = "ast";
int b = ;
char c = 'x';
foo("%s,%d,%c/n", a, b, c);
return ;
}
int foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch (*fmt++)
{
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s/n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d/n", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char)va_arg(ap, int);
printf("char %c/n", c);
break;
}
va_end(ap);
return ;
}
編譯運作:
[[email protected] test]# gcc -o start start.c
[[email protected] test]# ./start
string ast
int 224
char x
[[email protected] test]#
注意foo()格式:
foo(“%s,%d,%c/n”,a,b,c);
int vsnprintf
(
char *buffer, // 輸出參數
size_t count, // 最大可寫字元數
const char *format,// 格式字元串
va_list argptr // 參數清單指針
);
// crt_vsnprintf.cpp
#include <stdio.h>
#include <wtypes.h>
void FormatOutput(LPCSTR formatstring, ...)
{
int nSize = ;
char buff[];
memset(buff, , sizeof(buff));
va_list args;
va_start(args, formatstring);
nSize = vsnprintf(buff, sizeof(buff)-, formatstring, args); // C4996
// Note: vsnprintf is deprecated; consider vsnprintf_s instead
printf("nSize: %d, buff: %s/n", nSize, buff);
}
int main()
{
FormatOutput("%s %s", "Hi", "there");
FormatOutput("%s %s", "Hi", "there!");
FormatOutput("%s %s", "Hi", "there!!");
}
Output
nSize : , buff : Hi there
nSize : , buff : Hi there!
nSize : -, buff : Hi there!