why should the parameter in copy construction be a reference

if not， it will lead to an endless loop！！！

 # include<iostream>
 using namespace std;
 class A
 {
 public:
     int var;

     A():var(){}

     A(A &a){this->var = a.var;cout << "copy\n";}

     void operator=(A b){cout << "assign\n";}

     A func(A a){return a;}
     //A func(A &a){ return a; }

     ~A(){cout << "destroy\n";}
 };

 int main()
 {
     A a, b, c;
     b.var = ;
     a = b;
     cout << "\n";
     c = a.func(b);
     system("pause");
 }

update:  1) class a = b; 2) a=c;

1) will call copy constructor, because a havent be constructed;

2) will call assignemnt constructor, because a has it's mem room, we should just modify it's value