这个，大家能解释么？

[ganwg07]

问题：
#include <iostream>
using namespace std;
class A{
int n;
int k;
int m;
public:
A(){}
A(int n,int k,int m):n(n),k(k),m(m){}
A& operator++(){ //++i
m++;
return *this;
}
A& operator++(int){ //i++
A a(*this);
m++;
return a;
}
friend ostream& operator<<(ostream& o,const A& a);
friend istream& operator>>(istream& i,A& a);
};
ostream& operator<<(ostream& o,const A& a){
o<<a.n<<"-"<<a.k<<"-"<<a.m<<endl;

return o;
}
int main(int argc, char *argv[])
{
A a(10,20,30);

cout<<++a; //10 20 31
cout<<a++; //10 20 31
a++;
cout<<a++; //10 20 33
return 0;
}
结果为：10-20-31
10-4205168-31
10-4205168-31

这个程序的结果怎么会有 随机数呢
这个 a.k 怎么会不同呢？
请说说，谢谢！

[pengzhixi]

#include<iostream>
#include<ostream>
#include<istream> 
using namespace std; 
class A{ 
    int n; 
    int k; 
    int m; 
    public: 
    A(){} 
    A(int n,int k,int m):n(n),k(k),m(m){} 
    A& operator++(){  //++i 
        m++; 
        return *this; 
    } 
    A operator++(int){  //i++ 
        A b=A(*this);   //改了一下这里，同时将返回值的类型改了下。
         m++;
         return b;
    } 
  friend ostream& operator<<(ostream & o,const A& a); 
  friend istream& operator>>(istream& i,A& a); 
}; 
ostream& operator <<(ostream& o,const A& a){ 
      o <<a.n <<"-" <<a.k <<"-" <<a.m <<endl; 

          return o; 
    } 
int main(int argc, char *argv[]) 
{ 
    A a(10,20,30); 

    cout <<++a;  //10 20 31 
    cout <<a++;  //10 20 31 
    a++; 
    cout <<a++;  //10 20 33
    system("pause"); 
    return 0; 
}

再试下

[pengzhixi]

#include<iostream>
#include<ostream>
#include<istream> 
using namespace std; 
class A{ 
    int n; 
    int k; 
    int m; 
    public: 
    A(){} 
    A(int n,int k,int m):n(n),k(k),m(m){} 
    A& operator++(){  //++i 
        m++; 
        return *this; 
    } 
    A operator++(int){  //i++ 或者直接将返回类型改成返回一个类也行
         A a(*this);
         m++;
         return a;
    } 
  friend ostream& operator<<(ostream & o,const A& a); 
  friend istream& operator>>(istream& i,A& a); 
}; 
ostream& operator <<(ostream& o,const A& a){ 
      o <<a.n <<"-" <<a.k <<"-" <<a.m <<endl; 

          return o; 
    } 
int main(int argc, char *argv[]) 
{ 
    A a(10,20,30); 

    cout <<++a;  //10 20 31 
    cout <<a++;  //10 20 31 
    a++; 
    cout <<a++;  //10 20 33
    system("pause"); 
    return 0; 
}