Where did the concept of Interfaces come from?

Question

In c#, we have interfaces. Where did these come from? They didn\'t exist in c++.

Answer 1

Interfaces came from computer science. Or, let's say, from common sense in programming. Interface is a logical group of methods of a class. C++ didn't need a separate language concept of "interface", because any class might be used as an interface -- just define a set of methods in it, make no implementation, call it like IExecutable and use:

class IExecutable
{
public:
    virtual void Execute() = 0;
};

class MyClass : public IExecutable
{
public:
    void Execute() { return; };
};

Some languages, called "dynamically typed", like Python, don't require to define interfaces at all, you just call a method you need, and run-time checks if it is possible ("If it walks like a duck and talks like a duck, it must be a duck").

C# clearly separates a concept of interfaces from classes, because it uses static typing... and multiple inheritance is prohibited in that language, but it is ok for a class to have one base class and another interface, or to implement several interfaces at a time.

public interface IPurring
{
    void Purr();
}

public class Cat : Animal, IPurring
{
    public Cat(bool _isAlive)
    {
        isAlive = _isAlive;
    }

    #region IPurring Members

    public void Purr()
    {
        //implement purring
    }

    #endregion
}