while compiler encounters following situation it uses the CC
1.when assigning one object to another object of same class 
type

2.When an object is passed to the called function.....

3.when a function returns the object type and assigns it to 
the another object..........

1. "assigning" - don't use this word, it should be 
initializing an object using another object of same type
2. When the object is passed to function - "by value"
3. when a function returns the object type - "by value"
here doesn't matter if you are assigning that returned 
value to to another object or not.

Whenever u want to copy the members of one object to another
object of the same class in a C++ program.

hi fellows..try this code
class A {
public:
A() { }
A(const A& rhs)
{
cout<<"inside copy constructor..";
}
};

int main()
{
std::vector<A> aVec(10);
}

The answer to this is another reason a copy constructor 
will be called.