The “Big Three” in C++ refer to three essential member functions in a class:

Destructor (~ClassName()): Cleans up resources (e.g., dynamic memory) when an object is destroyed.

Copy Constructor (ClassName(const ClassName &other)): Creates a new object as a copy of an existing object, ensuring proper duplication.

Copy Assignment Operator (ClassName& operator=(const ClassName &other)): Assigns the values of one object to another, managing resources and avoiding memory leaks.

The rule of three (also known as the law of the big three or the big three) is a rule of thumb in C++ (prior to C++11) that claims that if a class defines any of the following then it should probably explicitly define all three: destructor. copy constructor. copy assignment operator.