Memory leak is - dynamically allocating memory and forgeting to free it. In C++, using a new operator to allocate a chunk of memory, and forgetting to delete it. There are several reasons this could occur. Some of them are,
1. Allocate a chunk of memory and assign the starting address to a pointer variable, and later, reassing another address to the same pointer without freeing its original address
2. Allocate memory for an array and not using proper delete method (delete[]) to free the memory
3. Unhandled exceptions - allocating memory and deleting it at the end of the program, but the program abnormally terminates (crashes) before the delete code line is executed.

Some of the ways of avoiding memory leaks within a class object are
1. Allocate all dynamic memory required for the class in the constructor, and delete it in the distructor
2. Use TRY/CATCH blocks around all statements where there is a possiblity of a crash
3. Override new operator for your class and Track memory allocations and pointers. Override delete operator and untrack the deleted/freed memory. In the destructor, check to see if there are any tracked memory locations (not freed) and free them up.
But beyond all these, it's still a good programming style that avoids memory leaks, because beyond objects, there is a program where you instantiate and run these objects. If you are allocating memory dynamically in the parent program, then carefully make sure you delete those chunks at all exit points (or centralize your exit point).