Question

Explain why memberwise assignment can cause problems with a class that contains a pointer member.

Short answer

Answer: Memberwise assignment can cause problems with a class containing a pointer member because it performs a shallow copy, which copies only the member values without allocating new memory for the pointers. This can lead to issues such as unintended aliasing, resource sharing, and double deletion of the same memory. To avoid these problems, a custom copy constructor and assignment operator (deep copy) should be implemented to handle memory allocation and resource management for the pointer members.

Step-by-step solution

Understanding Memberwise Assignment

Memberwise assignment refers to the default way in which objects of a class are copied or assigned in C++. The compiler automatically generates a default copy constructor and assignment operator for every class, which perform a shallow copy of members. In a shallow copy, only the member values are copied, not the actual objects to which pointers refer.

Introduction to Problems with Pointer Members

The problem arises when the class contains pointer members. When a pointer member is copied using memberwise assignment, a shallow copy is made, which means that the copied object's pointer member still points to the same memory location as the original object's pointer member. This can lead to various issues like unintended aliasing, resource sharing, and double deletion of the same memory.

Unintended Aliasing

Alias occurs when two or more different names refer to the same memory location. When a class containing a pointer member is copied using a shallow (memberwise) copy, the pointer members of the original and copied objects point to the same memory location. This can lead to unintended aliasing, where changes made to one object's pointer member directly affect the other object's pointer member.

Resource Sharing

When two objects share a pointer pointing to the same resource, they also share the resource, which can result in undesired modifications, and in turn, can lead to various logical errors or undefined behaviors in a program.

Double Deletion of Memory

If the class contains a destructor that deletes the pointer member, and two objects share the same pointer due to the memberwise assignment, then when both objects go out of scope or are deleted, the destructor will be called twice on the same memory, leading to a double deletion. Double deletion can result in undefined behavior, crashed programs, or memory corruption.

Solution to the Problem

To avoid these problems, we can implement a custom copy constructor and assignment operator (also known as deep copy) that specifically handle memory allocation and resource management for the pointer members. A deep copy ensures that each object has its own copy of the resources, avoiding the issues mentioned above.