Abstract Class vs Interface in C

Question

Possible Duplicate    How do you declare an interface in C          This is a general question about C    As you know  there is no clear distinction between interface and abstract class in C   unlike Java and C   When would it be more preferrable to use an interface instead of an abstract class in C    Could you give some examples

User · Answer

Pure Virtual Functions are mostly used to define:

a) abstract classes

These are base classes where you have to derive from them and then implement the pure virtual functions.

b) interfaces

These are 'empty' classes where all functions are pure virtual and hence you have to derive and then implement all of the functions.

Pure virtual functions are actually functions which have no implementation in base class and have to be implemented in derived class.

User · Answer

Please don t put members into an interface  though it s correct in phrasing  Please don t  delete  an interface    class IInterface          Public      Virtual  IInterface                   Class ClassImpl   public IInterface                  Int main          IInterface  pInterface   new ClassImpl              delete pInterface     Wrong in OO Programming  correct in C

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An abstract class would be used when some common implementation was required  An interface would be if you just want to specify a contract that parts of the program have to conform too  By implementing an interface you are guaranteeing that you will implement certain methods  By extending an abstract class you are inheriting some of it s implementation  Therefore an interface is just an abstract class with no methods implemented  all are pure virtual

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I assume that with interface you mean a C   class with only pure virtual methods  i e  without any code   instead with abstract class you mean a C   class with virtual methods that can be overridden  and some code  but at least one pure virtual method that makes the class not instantiable  e g    class MyInterface   public       Empty virtual destructor for proper cleanup   virtual  MyInterface         virtual void Method1     0    virtual void Method2     0       class MyAbstractClass   public    virtual  MyAbstractClass       virtual void Method1      virtual void Method2      void Method3       virtual void Method4     0     make MyAbstractClass not instantiable      In Windows programming  interfaces are fundamental in COM  In fact  a COM component exports only interfaces  i e  pointers to v-tables  i e  pointers to set of function pointers   This helps defining an ABI  Application Binary Interface  that makes it possible to e g  build a COM component in C   and use it in Visual Basic  or build a COM component in C and use it in C    or build a COM component with Visual C   version X and use it with Visual C   version Y  In other words  with interfaces you have high decoupling between client code and server code   Moreover  when you want to build DLL s with a C   object-oriented interface  instead of pure C DLL s   as described in this article  it s better to export interfaces  the  mature approach   instead of C   classes  this is basically what COM does  but without the burden of COM infrastructure    I d use an interface if I want to define a set of rules using which a component can be programmed  without specifying a concrete particular behavior  Classes that implement this interface will provide some concrete behavior themselves   Instead  I d use an abstract class when I want to provide some default infrastructure code and behavior  and make it possible to client code to derive from this abstract class  overriding the pure virtual methods with some custom code  and complete this behavior with custom code  Think for example of an infrastructure for an OpenGL application  You can define an abstract class that initializes OpenGL  sets up the window environment  etc  and then you can derive from this class and implement custom code for e g  the rendering process and handling user input      Abstract class for an OpenGL app     Creates rendering window  initializes OpenGL      client code must derive from it     and implement rendering and user input  class OpenGLApp   public    OpenGLApp      virtual  OpenGLApp                Run the app       void Run            lt ---- This behavior must be implemented by the client ---- gt        Rendering   virtual void Render     0        Handle user input       returns false to quit  true to continue looping    virtual bool HandleInput     0         lt --------------------------------------------------------- gt    private            Some infrastructure code               void CreateRenderingWindow      void CreateOpenGLContext      void SwapBuffers         class MyOpenGLDemo   public OpenGLApp   public    MyOpenGLDemo      virtual  MyOpenGLDemo          Rendering   virtual void Render        implements rendering code       Handle user input   virtual bool HandleInput       implements user input handling             some other stuff

User · Answer

interface were primarily made popular by Java  Below are the nature of interface and its C   equivalents    interface can contain only body-less abstract methods  C   equivalent is pure virtual methods  though they can cannot have body interface can contain only static final data members  C   equivalent is static const data members which are compile time constants Multiple interface can be implemented by a Java class  this facility is needed because a Java class can inherit only 1 class  C   supports multiple inheritance straight away with help of virtual keyword when needed   Because of point 3 interface concept was never formally introduced in C    Still one can have a flexibility to do that   Besides this you can refer Bjarne s FAQ on this topic

[c++] Abstract Class vs Interface in C++

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