#include <iostream>
using namespace std;
class test;
void main(){
    test my;
    my.show();
}
class test{
public:
   int x;
   test(int a=0){
      x=a;
    }
   void show(){
       cout<<x<<endl;
    }
};   
编译出错，error C2079: 'my' uses undefined class 'test'
求解释，感谢！