声明名称相同但类型不同的变量

我正在尝试制作支持IPV4或IPV6的东西。这两个代码是相似的，所以我尝试这样做。由于代码的唯一差异取决于这个sin变量，我怎么能只编译其中一个呢?假设IF语句中的"ipv"变量为真或假取决于用户输入

//FOR IPV4
//struct sockaddr_in sin;
//FOR IPV6
//struct sockaddr_in6 sin;
//IPV4

// IPV4 ---------------------------------------
if (ipv){
  struct sockaddr_in sin;   
  if ( (s = socket(AF_INET, SOCK_STREAM, 0 ) ) < 0) { 
    perror("Chyba pri vytvareni socketu");  
    return -1;
  }

  sin.sin_family = AF_INET;     
  sin.sin_port = htons(port_number);
  sin.sin_addr.s_addr  = INADDR_ANY;
}
// IPV6 ---------------------------------------
else{
  struct sockaddr_in6 sin;
  if ( (s = socket(AF_INET6, SOCK_STREAM, 0 ) ) < 0) { 
    perror("Chyba pri vytvareni socketu");  
    return -1;
  }
  sin.sin6_family = AF_INET6;     
  sin.sin6_port = htons(port_number);
  sin.sin6_addr = in6addr_any;
  sin.sin6_flowinfo = 0;
}   
if (bind(s, (struct sockaddr *)&sin, sizeof(sin) ) < 0 ) {
    printf("error on bindn"); return -1;  
}
if (listen(s, 5)) { 
    printf ("error on listenn"); 
return -1;
}
sinlen = sizeof(sin);
pid_t pid;
while (1) {

    /* accepting new connection request from client,
    socket id for the new connection is returned in t */
    if ( (t = accept(s, (struct sockaddr *) &sin, &sinlen) ) < 0 ) {
      printf("error on acceptn");  /* accept error */
      return -1;
    }
continues .... not important 

这段代码将给我:

server.cpp: In function ‘int main(int, char**)’:  
server.cpp:132:35: error: ‘sin’ was not declared in this scope
   if (bind(s, (struct sockaddr *)&sin, sizeof(sin) ) < 0 ) {
                               ^
server.cpp:145:19: error: ‘sin’ was not declared in this scope
   sinlen = sizeof(sin);
               ^
make: *** [all] Error 1

那么问题是我如何做到这一点，而不写相同的代码两次不同的"sin";