Why Should We Not Inherit std::vector in C++?

In C++, you may sometime want to inherit vector in your class to make the use of already present functionality. But inheriting from std::vector is generally discouraged due to various technical and design issues. In this article, we will explore why inheriting from std::vector is problematic. We will also look at some practical alternatives for extending its functionality.

Problem with Inheriting std::vector in C++

Following are the main reasons why inheriting from std vector is not feasible:

1. std::vector is Not Designed for Inheritance

std::vector is part of the Standard Template Library. It is designed to be a flexible and efficient container for managing a sequence of elements. But it is not designed for one to derive from it. It is designed to be used in a container, in a composition manner.

2. Protected Members and Virtual Destructors

std::vector does not have protected members, which means it doesn't expose internal implementation details that you might want to override or extend. It lacks a virtual destructor. Without a virtual destructor, if you inherit from std::vector and create objects dynamically, deleting a derived class object through a base class pointer (e.g., std::vector*) will lead to undefined behaviour.

3. Inheritance Can Lead to Unexpected Behaviour

Inheriting from std::vector and overriding its member functions can lead to unexpected and undefined behaviour because the internal logic of std::vector assumes its own implementation. Extending std::vector may result in compatibility issues with standard algorithms and other parts of the STL, which expect a standard-compliant std::vector.

4. Breaking Encapsulation

By inheriting from std::vector, you may accidentally expose internal data structures and behaviour of std::vector, thus losing encapsulation that std::vector has provided. This can result in maintenance problems because changes in the implementation of std::vector might break your derived class.

Alternatives to Extend the Functionality of std::vector using Composition

Composition involves creating a new class that contains a std::vector as a member, allowing you to add new functionalities while maintaining encapsulation. This is a "has-a" relationship.

Example:

A Car class might have an Engine class as a member because a car "has-a" engine.

#include <iostream>
#include <vector>

using namespace std;

// MyContainer class that uses composition with vector
class MyContainer {
private:
    // Private member vector to hold data
    vector<string> data;

public:
    // Method to add elements to the vector
    void addElement(const string& element)
    {
        data.push_back(element);
    }

    // Method to print all elements in the vector
    void printElements() const
    {
        for (const auto& element : data) {
            cout << element << " ";
        }
        cout << endl;
    }
};

int main()
{
    MyContainer container;
    container.addElement("Geeks");
    container.addElement("For");
    container.addElement("Geeks");
    container.printElements();
    return 0;
}

Geeks For Geeks 

It's a good alternative, as

• Encapsulation: Keeps the internal implementation (the std::vector) hidden.
• Flexibility: Allows adding new methods specific to the class without modifying the std::vector.
• Maintainability: Changes to the underlying data structure can be made without affecting the interface.

Conclusion

While inheriting from std::vector might seem convenient, it introduces several risks and complications that are better avoided. Using composition and other best practices can help you extend the functionality of STL containers in a safer and more maintainable way.