Copy Constructor

[skcpp]

Why is that when declaring a copy constructor, the object being passed as argument to that constructor is (1) passed by reference using "&", and (2) the instantiated object is typecast as "const"?

class MyClass {
   public:
      MyClass( int len); // simple constructor
      MyClass( const MyClass &obj); // copy constructor
      ~MyClass(); // Destructor
};

[keskiverto]

You don't have to make a copy, when you use reference. Consider:

MyClass( MyClass obj ) {
 // make *this copy of obj
}

The obj is copy constructed, because it is a by value parameter.
If you can create obj, then you already know how to copy construct MyClass objects?

Reference avoids that recursion.


For const, consider:

class MyClass {
  MyClass( MyClass& obj );
};

const MyClass alpha;
MyClass bravo( alpha ); // error 

You can't have non-const reference to const object. Therefore, without the const you can't copy constant objects.

[skcpp]

Thanks keskiverto. Thanks for your patience as I am an newbie. Two follow up questions if you don't mind please:

(1) In "const MyClass &obj", is obj being passed by value or by reference (i.e. its address)?
(2) Aren't we typecasting obj to be of type MyClass, and if so, why use const on top?

[seeplus]

1) by reference is not by address (although internally it may be done using pointers). Yes, obj is passed by reference (not by address). To pass by address it would be MyClass* obj

2) const means that the contents of obj can't be changed in the function. Hence the passed-in value can be const or could be an r-value.

[keskiverto]

(2) Aren't we typecasting obj to be of type MyClass, and if so, why use const on top?

No, not "typecasting". Creating a reference. Look at:

void func( MyClass one, MyClass& two, const MyClass& three, const MyClass* ptr );

const MyClass obj;
func( obj, obj, obj, &obj );

During that function call the parameters are created like this:

MyClass  one = obj; // by value parameter, copy construction
MyClass& two = obj; // by reference. ERROR: obj is const
const MyClass& three = obj;  // by reference. ok
const MyClass* ptr   = &obj;  // by value. ok