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(0){}

    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 = 5;
    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