The Rule of Three

The Stack Overflow question “What is the Rule of Three?” addresses a fundamental concept in C++ programming related to resource management and object copying. The Rule of Three states that if a class requires a user-defined destructor, copy constructor, or copy assignment operator, it likely needs all three. This principle ensures proper handling of resources, especially dynamic memory, to avoid issues such as memory leaks, double deletions, and dangling pointers.

Why the Rule of Three is Important

When a class manages resources such as dynamic memory, the compiler-generated versions of these functions typically perform shallow copies, which copy the memory addresses rather than the actual data. This can lead to multiple objects pointing to the same memory location, causing problems like double deletion when destructors are called on these objects.

Example

Consider the following class that allocates dynamic memory:

class MyClass {
public:
    MyClass(int size) : _size(size), _data(new int[size]) {}

    ~MyClass() {
        delete[] _data;
    }

    MyClass(const MyClass& other) : _size(other._size), _data(new int[other._size]) {
        std::copy(other._data, other._data + other._size, _data);
    }

    MyClass& operator=(const MyClass& other) {
        if (this == &other) return *this;

        delete[] _data;
        _size = other._size;
        _data = new int[_size];
        std::copy(other._data, other._data + _size, _data);
        return *this;
    }

private:
    int _size;
    int* _data;
};

Conclusion

The Rule of Three ensures that if one of these special member functions is defined to manage resources properly, the other two should be defined as well to maintain consistency and prevent resource management issues. This practice leads to more robust and reliable code.