Question

I have a question about declaration of class.

I read the following lines in a paper: A C++ class declaration combines the external interface of an object with the implementation of that interface.

So, what is a external interface in C++? are there concept of interface in C++? how to understand this: A C++ class declaration combines the external interface of an object with the implementation of that interface. ?

Answer 1

I think it’s referring to the API — i.e. the signatures of the public functions — as contrasted to the implementation — i.e. private functions and data members.

class T
{
   // Here are some private things.
   // You might say that these are part of
   //  the class's implementation.

   int x;
   int y;

   void foo();
   void bar();

public:
   // And here are some public things, that
   //  constitute the class's API or "interface",
   //  which is the mechanism by which you may
   //  interact with objects of this type.

   void baz();
   void boz();
};

Notice how they are all present, right there, in the single class definition, and they must be. “Combined”.

Ultimately, though, you’d have to ask the author of the paper as the terminology level used is up to them. Conceivably they could be talking about inheritance heirarchies and abstract base classes… though I can’t find a way to rationalise the assertion that C++ class definitions “combine” those in any real sense.