C语言之函数可变参数

先上一段代码：

#include<cstdarg>
#include<iostream>
#include<string>
using namespace std;
void error(char* format,...){//至少要有一个参数，后边的...表示参数可变
    va_list ap;
    int d,flag;
    char c, *s;
    va_start(ap,*format);//从args参数开始后面都是可变参数，va_start,va_end都为宏
    while (*format){        
               switch (*format) {
               case 's':              /* string */
                   if(!flag) break;
                   s = va_arg(ap, char *);
                   cout<<s;flag=0;
                   break;
               case 'd':              /* int */
                    if(!flag) break;
                   d = va_arg(ap, int);
                   cout<<d;flag=0;
                   break;
               case 'c':              /* char */
                   /* need a cast here since va_arg only
                      takes fully promoted types */
                   if(!flag) break;
                   c = (char) va_arg(ap, int);
                   cout<<c;flag=0;
                   break;
               case '%':
                    flag=1;
                      break;
               default:
                   putchar(*format);
               }
               format++;
               
        }
        putchar('
');
    /*for(i=0;i<args;i++){//遍历所有其他参数
        //printf("%d",va_arg(ap,int));
    }*/
    va_end(ap);//destroy va_list;
}
int main(){
    char s[]="rich";
    error("I am %s,I have %d %c",s,100,'$');
    //getchar();
    return 0;
}
其中error()函数像printf一样打印出格式化字符串。
#include <stdarg.h>
头文件中定义了一下一些宏，注意不是函数
       void va_start(va_list ap, last);
       type va_arg(va_list ap, type);
       void va_end(va_list ap);
       void va_copy(va_list dest, va_list src);
有趣的是va_arg宏，每次处理后ap指向下一个参数，请看man手册：
va_arg()
       The va_arg() macro expands to an expression that has the type and value
       of  the  next  argument in the call.  The argument ap is the va_list ap
       initialized by va_start().  Each call to va_arg() modifies ap  so  that
       the  next  call returns the next argument.  The argument type is a type
       name specified so that the type of a pointer to an object that has  the
       specified type can be obtained simply by adding a * to type.

       The  first use of the va_arg() macro after that of the va_start() macro
       returns the argument after last.   Successive  invocations  return  the
       values of the remaining arguments.

       If  there  is  no  next argument, or if type is not compatible with the
       type of the actual next argument (as promoted according to the  default
       argument promotions), random errors will occur.

       If  ap is passed to a function that uses va_arg(ap,type) then the value
       of ap is undefined after the return of that function.