Interface vs class with pure virtual methods

Question

I wanted to write shell extensions for windows in plain C++, but then I got confused by the keyword interface. In many articles I read that I can create interfaces in C++ by writing classes containing only virtual methods without any code. For example:

class IIsThisAnInterface_QuestionMark {
    virtual MyMethod (
        int firstParameter,
        double secondParameter) = 0;
    virtual AnotherMethod (
        wchar_t *firstParameter) = 0;
}

But the author of this article defined interfaces by using the interface keyword. So my question is: How to correctly define interfaces in C++? (Becuase I grew up in C#'s world, I know interfaces as constructs specifying methods for classes that are implementing these interfaces.)

There's no `interface` keyword in C++. The header included in that example probably defined it as a macro. — T.C., Jul 14 '14 at 16:48
this was the first site in google when i searched for "c++ interface" http://stackoverflow.com/questions/318064/how-do-you-declare-an-interface-in-c — iedoc, Jul 14 '14 at 16:49
That `interface` keyword is part of Component Object Model (COM). You may wish to add the COM tag and update the question. — SE_net4 the downvoter, Jul 14 '14 at 16:53
AFAIK the `interface`-stuff is specific to the `COM`-Interface stuff. Do not know whether you need this for shell extensions. — Tobias, Jul 14 '14 at 16:55
http://msdn.microsoft.com/en-us/library/50h7kwtb.aspx there is a microsoft extension that adds a keyword `__interface`. — Alexander Oh, Jul 14 '14 at 16:55
1st: I didn't know that there is no `interface` keyword in C++. And you are right, in the `ObjBase.h` header is following macro defined: `#define interface struct`. 2nd: I believe you can mark this question as duplicate, because I was asking how to correctly define an interface. (I already marked it as a duplicate) — Cubi73, Jul 14 '14 at 17:00
@Tobias Shell extensions are little programs extending the ability of windows. They have to be written as a COM component. — Cubi73, Jul 14 '14 at 17:11

score 2 · Accepted Answer · answered Jul 14 '14 at 16:58

C++ doesn't strictly provide interfaces in the way that some languages do. The C++ mechanism is to provide a class with one or more pure virtual methods that declare the desired interface. Strictly speaking such a class is just an abstract class, but one could consider calling it an interface that child classes would then implement.

Interface vs class with pure virtual methods

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