Copy constructors

y do we use pass by referance in copy constructors...
can we use pass by value there...

i.e.
the syntax of copy constructor is
class a{
public:
a(a &b){//copy constructor
//body
}
}
y we cant use pass by value here...like a(a b)...????

i am saying just in layman terms...

if suppose u do it by call by value,that itself needs a copy constructor...so it goes on recursively.
it becomes necessary to use reference/pointers to solve this

bingooo RamP...
bahi phodu ho to Ramp jaisa...