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When should we write own Assignment operator in C++?

In C++, the default assignment operator provided by the language can be sufficient for many situations. However, in certain cases, it may be necessary to write your own custom assignment operator. Below are some scenarios where writing your own assignment operator can be useful:

Dynamic memory allocation:

If a class uses dynamic memory allocation (e.g., using the new keyword), the default assignment operator can lead to shallow copying of memory. Shallow copying can result in memory leaks, dangling pointers, or other memory-related issues. To avoid these problems, a custom assignment operator can be implemented to ensure proper copying and release of dynamic memory.

Here's an example:

class MyClass {
private:


    int* data;
    int size;


public:
    MyClass(int size) : size(size) {
        data = new int[size];
    }


    // copy constructor
    MyClass(const MyClass& other) {
        size = other.size;
        data = new int[size];
        for (int i = 0; i < size; i++) {
            data[i] = other.data[i];
        }
    }


    // destructor
    ~MyClass() {
        delete[] data;
    }


    // custom assignment operator
    MyClass& operator=(const MyClass& other) {
        if (this != &other) {
            delete[] data;
            size = other.size;
            data = new int[size];
            for (int i = 0; i < size; i++) {
                data[i] = other.data[i];
            }
        }
        return *this;
    }
};

Explanation:

  • This program defines a class called MyClass, which manages a dynamically allocated array of integers.
  • The class provides a constructor, a copy constructor, a destructor, and a custom assignment operator.
  • The constructor takes an integer parameter size, which specifies the size of the array. The constructor dynamically allocates an array of integers with the given size.
  • The copy constructor is used to create a new object of the class based on an existing object.
  • It takes a const reference to another MyClass object other as its parameter.
  • It allocates a new array of integers with the same size as the other object and copies the contents of the other object's array into the new array.
  • The destructor is called when an object of the class is destroyed. It deletes the dynamically allocated array of integers.
  • The custom assignment operator is used to copy the contents of one object to another object of the same class.
  • It takes a const reference to another MyClass object other as its parameter. The operator checks if the current object and the other object are the same.
  • If they are not the same, the operator deletes the current object's array and allocates a new array with the same size as the other object's array.
  • It then copies the contents of the other object's array into the new array.

Program output:

This program does not have any output as it only defines a class MyClass with a constructor, copy constructor, destructor, and custom assignment operator.

Resource management:

In cases where a class uses external resources (such as a file or network connection), a custom assignment operator can be used to manage those resources properly. For example, a custom assignment operator can be used to close the existing resource before reassigning it to a new value.

Here's an example:

class Connection {
private:
    int socket;




public:
    Connection() {
        socket = create_socket();
    }


    ~Connection() {
        close_socket(socket);
    }


    // custom assignment operator
    Connection& operator=(const Connection& other) {
        if (this != &other) {
            close_socket(socket);
            socket = other.socket;
        }
        return *this;
    }
};

Explanation:

  • This program defines a class called "Connection" which manages a socket connection.
  • In the private section of the class, there is an integer variable called "socket", which stores the socket file descriptor.
  • In the public section, there is a constructor that creates a new socket by calling the "create_socket()" function and assigns the returned file descriptor to the "socket" variable.
  • There is also a destructor that closes the socket connection by calling the "close_socket()" function with the "socket" variable as its argument.
  • In addition, the class defines a custom assignment operator that allows one Connection object to be assigned to another.
  •  This operator checks if the two objects are not the same (i.e., not the same memory address), closes the current socket connection of the object being assigned to, and then assigns the "socket" variable of the current object to be the same as the "socket" variable of the object being assigned to.
  • This program demonstrates how to create a C++ class that manages a socket connection and defines a custom assignment operator to ensure proper handling of socket resources when the object is copied or assigned.

Program output:

This code does not produce any output when compiled and executed because it only defines a C++ class called "Connection".

The class contains a constructor, a destructor, and a custom assignment operator, but it does not have any functions that would produce output to the console or any other form of output.

Inheritance:

In a class hierarchy, the derived classes may need to have a custom assignment operator to handle the assignment of their own member variables as well as those inherited from the base class. The custom assignment operator can ensure that the member variables of the derived class are assigned properly.

Here's an example:

class BaseClass {
protected:
    int baseVar;


public:
    BaseClass(int x) : baseVar(x) {}


    // custom assignment operator
    virtual BaseClass& operator=(const BaseClass& other) {
        if (this != &other) {
            baseVar = other.baseVar;
        }
        return *this;
    }
};


class DerivedClass : public BaseClass {
private:
    int derivedVar;


public:
    DerivedClass(int x, int y) : BaseClass(x), derivedVar(y) {}


    // custom assignment operator
    DerivedClass& operator=(const DerivedClass& other) {
        if (this != &other) {
            BaseClass::operator=(other);
            derivedVar = other.derivedVar;
        }
        return *this;
    }
};

Explanation:

  • This program defines two classes: BaseClass and DerivedClass.
  • DerivedClass is derived from BaseClass using the public access specifier, which means that the public and protected members of BaseClass are inherited as public and protected members of DerivedClass, respectively.
  • BaseClass has a single protected member variable called baseVar, and a constructor that takes an integer argument and initializes baseVar with that value.
  • Additionally, BaseClass defines a virtual custom assignment operator which allows for objects of derived classes to be assigned to objects of the BaseClass.
  • DerivedClass has two member variables: baseVar (inherited from BaseClass) and derivedVar.
  • The constructor takes two integer arguments and initializes the baseVar using the BaseClass constructor and derivedVar with the provided argument.
  •  It also defines a custom assignment operator, which calls the BaseClass assignment operator using the BaseClass::operator= syntax and then assigns the derivedVar with the value of the other object's derivedVar.
  • Both classes have custom assignment operators, which are called when an object of the class is assigned to another object.
  • These operators ensure that the object being assigned to is not the same object as the object being assigned from, and if they are different, the assignment operator copies the member variables from the right-hand side object (other) to the left-hand side object (this), ensuring that the object's state is correctly updated.
  • Overall, this program demonstrates the use of inheritance and custom assignment operators in C++ classes.

Program output:

This code does not produce any output when compiled and executed because it only defines two C++ classes called "BaseClass" and "DerivedClass".

The BaseClass and DerivedClass classes define member variables and custom assignment operators, but do not have any functions that would produce output to the console or any other form of output.

Conclusion:

In summary, a custom assignment operator in C++ can be useful in cases where the default operator is insufficient or when resource management, memory allocation, or inheritance requires special attention. It can help avoid issues such as memory leaks, shallow copies, or undesired behaviour due to differences in object states. However, writing a custom assignment operator requires careful attention to detail and can be complex. It should only be done when necessary, and only after considering the alternatives, such as disabling assignment or using a move constructor.