Why do you need copy constructor in C++ when the compiler makes one itself?

Because the default copy constructor creates a copy of that object only: it copies any pointers that class contains instead of creating new memory and copying the content of teh pointer into that.

So assume you have a class called Learning that contains a two fields: a pointer to a Student, and a pointer to a Subject.
If you create a Learning object and assign it to a Student and a Subject ("Joe Smith", "Maths") stored in a Learning type variable called JoeSmith, then copy it into a new Learning type variable called "MikeHunt" using the default copy constructor then the two variable point at different Learning objects - which is what you want. However, because the default copy constructor only copied the pointers any changes you make to the Student or Subject objects from JoeSmith will also affect the Student and / or Subject that MikeHunt is referring to, and vice versa.

That isn't what you wanted, not at all: if Joe Smith pays his tuition fees and you record the payment as part of his Student information, you definitely don't want it affecting Mike Hunt's payments!

So you create a Learning copy constructor which creates new Student and Subject objects and copies the data into the new ones. Not Jow and Mike are independant and don't affect each other.

Make sense?