Class Redefinition in C++

1. Class Redefinition in C++
2. the redefinition of class <classname> Error in C++
3. Use Class Inheritance to Fix the redefinition of class Error in C++
4. Use the Composite Class Method to Fix the redefinition of class Error in C++
5. Conclusion

In this article, we’ll explore the redefinition of class error, its common occurrences, and two effective approaches—class inheritance and composite classes—to address and modify class behavior without direct redefinition.

Class Redefinition in C++

A class is an integral part of an object-oriented programming language, encompassing elements like data members and data functions. Data members are variables declared within a class, representing the properties or attributes of objects instantiated from that class, while member functions define the behavior or actions associated with the class.

We can use such elements in our programs by creating objects of this class. Consequently, a lot of functions that we use in programming have been defined in some classes in header files.

An interesting feature of this concept is redefining a class. It’s essential to note that direct redefinition of a class and its structure is not possible.

Polymorphism, one of the four key principles of object-oriented programming—alongside encapsulation, inheritance, and abstraction—enables the usage of objects of different types as if they were of the same type. Using these concepts, we can generate new classes while maintaining the structure of the base class.

In C++, class redefinition typically refers to attempting to define a class with the same name more than once within the same scope. This is strictly prohibited and results in a compilation error.

the redefinition of class <classname> Error in C++

Errors may occur when implementing this class redefinition concept. Specifically, the redefinition of class <classname> error occurs when the same class is defined twice in the program.

A common example of this error is shown below.

#include <iostream>

class DelftStack {
 public:
  void Func() { std::cout << "Hello from DelftStack!" << std::endl; }
};

class DelftStack {
 public:
  void Func() { std::cout << "Hello again from DelftStack!" << std::endl; }
};

int main() {
  DelftStack obj;
  obj.Func();
}

Output:

In the above example, we create a class DelftStack and proceed to redefine the same class later in the code. We can see that an error is thrown on the line of code where we again define the class.

Note that the error thrown is redefinition of 'class DelftStack'.

Another common example of redefinition of class error in C++ is when we try to include header files in a program. In C++, header files are used to declare the structure and interface of classes and variables, allowing them to be used in multiple source files.

If a given header file has a class defined already, then the same error is thrown if we try to create a class with the same name in the program.

Let us now discuss several methods to resolve the above-demonstrated error in C++.

Use Class Inheritance to Fix the redefinition of class Error in C++

A flexible way to implement the concept of redefining a class is to use inheritance. It is a concept that allows a class to inherit properties and elements like the data members and functions from another class.

In C++, one can achieve inheritance through different types, including public, private, and protected inheritance. Using inheritance, we can create classes that have the same structure and methods as the base class.

This is a very detailed topic, and we can also have a lot of restrictions based on the type of inheritance we implement. For this tutorial, we will demonstrate a very basic example using public inheritance.

Derived class can access public and protected members of the base class as if they were its own in such type of inheritance.

Let’s observe the concept mentioned above using an example.

#include <iostream>

using namespace std;

class DelftStack {
 public:
  void Func() { std::cout << "Hello from DelftStack!" << std::endl; }
};

class DelftStack2 : public DelftStack {
 public:
};

int main() {
  DelftStack2 obj;
  obj.Func();
}

Output:

In the above example, we create a class, DelftStack, and we use it as the base class for another class, DelftStack2. Using inheritance, we were able to inherit the public function Func() of the base class and use it with the object of the DelftStack2 class.

We then create an object obj of this class and access the func() function. This function is derived from the class DelftStack.

Use the Composite Class Method to Fix the redefinition of class Error in C++

A composite class can be utilized as well to fix the class redefinition in C++ errors. This is a class that contains objects of other classes as member variables.

This concept is often associated with the composite design pattern.

We can use the same method of using an instance of another class in a new class. This object can be used to access data members and methods from the other class.

Let’s understand this with an example.

#include <iostream>

using namespace std;

class DelftStack {
 public:
  void Func() { std::cout << "Hello from DelftStack!" << std::endl; }
};

class DelftStack2 {
 public:
  DelftStack d;

  void Func2() { d.Func(); }
};

int main() {
  DelftStack2 obj;
  obj.Func2();
}

Output:

In the above example, we demonstrate a very simple example of a composite class in C++. We create a class, DelftStack, and create an object of this class within another class, DelftStack2.

We use this object to access the Func() method from the DelftStack class from the DelftStack2 class. This function displays a message on the screen: Hello from DelftStack!.

Conclusion

In this tutorial, we discussed class redefinition in C++ and the error associated with this problem. Two methods were demonstrated which allow us to modify the behavior of a class without redefining it.

The first method was to use class inheritance in C++, where we create a base class that is inherited by another class in the program. We are able to access the function of the base class from the object of the base class in this example.

Remember that, for our example, we demonstrated a public inheritance. The use of private and protected inheritance also exists in C++, which limits the inheritance of certain data members and functions of the base class.

The second method introduces the use of composite classes in C++, where an object of one class is incorporated while creating another class. This enables the accessibility of functions from the first class to the second class.

If there is a need to redefine a class with different members or structures, then we will have to create a new class from scratch with the desired modifications.

One should also focus on having a class with a specific name only for the required purpose. Terminology is important in programming, and it also makes the programmer make the code more readable.