FAQ 5.08 How are classes better than the three basic building blocks of procedural software?

A class is better than a C-style struct, better than a module, and better than a C-style function.

A class can be thought of as a C-style struct that is also alive and intelligent. Take away all the member functions from a class, make everything public:, eliminate inheritance, and remove all the static member data and functions; the result is a traditional C-style struct. A C-style struct supports multiple instances, but it is difficult to encapsulate its implementations properly. Classes support both multiple instances and encapsulation. When a C-style struct dreams about growing up, it dreams of being a class.

A class can be thought of as a module that also provides natural support for multiple instances and inline functions. Take away from a class all the non-static member data and the non-static member functions and force all remaining static member functions to be non-inline, and the result is a traditional module. Modules support encapsulation, but creating multiple instances of a module is cumbersome. Classes support both encapsulation and multiple instances. When a module dreams about growing up, it dreams of being a class.

A class can be thought of as a C-style function that can maintain state between invocations in a thread-safe manner and can also provide multiple services. If there were exactly one instance of a class, and all its member functions except for exactly one public: member function were removed, the result would be a C-style function (the object's member data would correspond to static data that is local to the function). C-style functions support computation, but it's cumbersome to maintain state between calls in a thread-safe manner (using static local data is not immediately thread safe). However, classes support computation and can maintain state between calls in a thread-safe manner (each thread can make a separate local object of the class, so there is no conflict when multiple threads are accessing their own independent data). When a C-style function dreams about growing up, it dreams of being a class.