c++，关于构造和析构函数，帮忙看看解决办法

c++，关于构造和析构函数，帮忙看看
#include<iostream>
using namespace std;
class MyA
{
public:
MyA(int i=5)
{
a=i;
cout<<"Constructor a="<<a<<endl;
}
MyA(MyA&P);
void setA(int i)
{a=i;}
int getA()
{return a;}
~MyA()
{
cout<<"Destructor a="<<a<<endl;
}
private:
int a;
};
MyA fun(MyA&);
void main()
{
MyA m1,m2(8);
cout<<"m1.a="<<m1.getA()<<endl;
cout<<"m2.a="<<m2.getA()<<endl;
m2=fun(m1);
cout<<"m1.a"<<m1.getA()<<endl;
cout<<"m2.a"<<m2.getA()<<endl;
}
MyA fun(MyA&Q)
{
Q.setA(88);
cout<<"Q.a="<<Q.getA()<<endl;
return Q;
}
编译时没有错误，但组建时提示有错误，怎么回事

[解决办法]
把
void setA(int i)
{a=i;}
int getA()
{return a;}中的a改为this.a试试
[解决办法]
MyA(MyA&P);
没定义
[解决办法]

C/C++ code

#include "stdafx.h"#include<iostream>using namespace std;class MyA{public:    MyA(int i=5)    {        a=i;        cout<<"Constructor a="<<a<<endl;    }    MyA(MyA&P)    {        a = P.a;    }    void setA(int i)    {        a=i;    }    int getA()    {        return a;    }    ~MyA()    {        cout<<"Destructor a="<<a<<endl;    }private:    int a;};MyA fun(MyA&);void main(){    MyA m1,m2(8);    cout<<"m1.a="<<m1.getA()<<endl;    cout<<"m2.a="<<m2.getA()<<endl;    m2=fun(m1);    cout<<"m1.a"<<m1.getA()<<endl;    cout<<"m2.a"<<m2.getA()<<endl;}MyA fun(MyA& Q){    Q.setA(88);    cout<<"Q.a="<<Q.getA()<<endl;    return Q;}