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 only replicates the values of all data members, including pointer addresses, which can lead to two objects sharing the same memory location. This can result in memory leaks, dangling pointers, and undefined behaviors. To avoid these issues, one should override the assignment operator and implement a "deep copy" that creates new memory allocation for the destination object, ensuring separate memory locations for both objects.

Step-by-step solution

Understanding memberwise assignment

Memberwise assignment is an operation that replicates the values of all data members from one object into another object of the same class type. This is also known as "shallow copy", as it only copies the values of the object's data members and not the objects they may reference.

Issue with pointer data members

When a class has a pointer data member, a memberwise assignment will copy the pointer's address without allocating a new memory for the destination object. This leads to two objects having the same pointer that points to the same memory location. This can cause some problems such as: 1. Memory leak: If later on, both objects modify the memory content using their pointer and without releasing the previously allocated memory, it will result in a memory leak. 2. Dangling pointers: If one of the objects deletes the memory pointed by the pointer, the other object will have a dangling pointer, which can cause undefined behaviors and crashes.

Proper handling of pointer data members

To avoid the issues caused by memberwise assignment, one should override the assignment operator (=) and implement a "deep copy". A deep copy creates a new memory allocation for the destination object, so the source and destination objects have separate memory locations, eliminating the risk of memory leaks or dangling pointers.

Example

Let's consider a simple example class with a pointer member: class MyClass { public: int* data; MyClass(int size) { data = new int[size]; } ~MyClass() { delete[] data; } }; If we use default memberwise assignment to copy objects of this class, we might face problems: MyClass obj1(10); MyClass obj2(5); obj2 = obj1; // Memberwise assignment Here, obj1's data pointer is overwritten to obj2's data pointer without releasing obj2's previously allocated memory, causing a memory leak. When both objects go out of scope and their destructors are called, they will try to delete the same memory region, causing a double deletion issue. To solve this problem, we need to override the assignment operator and implement deep copy: class MyClass { public: int* data; int size; MyClass(int _size) : size(_size) { data = new int[size]; } ~MyClass() { delete[] data; } MyClass& operator=(const MyClass& other) { if (this == &other) { return *this; } delete[] data; size = other.size; data = new int[size]; for (int i = 0; i < size; ++i) { data[i] = other.data[i]; } return *this; } }; Now with the overridden assignment operator, when we use MyClass obj2 = obj1, we will create a new object with its memory space, preventing any possible issues caused by memberwise assignment with pointer members.