COPY CONSTRUCTOR

[C Pranav]

What is a copy constructor and when is it called?

[SECMoG1T]

AFAIK, a copy constructor is used to create copies of the class type objects, for example when we pass a class type obj as a parameter to a functions by value or when we initialize newly created objects from existing objects.

class Foo
{
   Foo(const Foo& F_obj)
    ///members
}

void do_something(Foo obj);

int main()
{

  Foo obj;
  Foo obj1=obj;//copy constr called

  do_something(obj);///cpy cnstr called 

Note: copy constructors usually takes their params by const reference or *at-least* normal lvalue reference coz if they took non_ref parameters they would be required to construct their own arguments which would lead to an infinite recursion though standard compilers will not allow you to define such constrs.

[AbstractionAnon]

If you don't supply a copy constructor, the compiler will generate one if needed. However, the compiler generated copy constructor only does a shallow copy of members.

If you need the copy constructor to do more than a shallow copy, then you need to provide your own constructor.

Consider if class Foo contains pointer to an object allocated with new Bar. After a shallow copy, you have two instances of Foo each with a pointer to the same instance of Bar. If both instances of Foo now try to delete Bar, you're going to have a problem because only one instance exists.

[C Pranav]

thanks guys....

[ButchCavendish]

Copy constructor is used to copy class objects data into other

[dhayden]

Consider

class MyClass {
...
};

void func(MyClass c)
{
...
}

int main()
{
    MyClass c1;
    func(c1);
}

By default, C++ passes parameters by value, so when you call func(), the compiler makes a copy of MyClass instance. It does that via the Copy Constructor.

There are other places where copy constructors are used, but this is a good example.

[ne555]

> By default, C++ passes parameters by value
no, they are passed by whatever way you have specified in the prototype of the function.


@OP: the questions at the end of the chapter are for self-evaluation. Consider reviewing the material if you can't answer them.

[dhayden]

no, they are passed by whatever way you have specified in the prototype of the function.

Yes that's a much clearer way to put it. I was thinking along the lines of "if you just give the type of the parameter" but that is quite different from "default." Thanks.