About Object Oriented in C++

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hi, I want to make my class B concrete which is derived from class A (Abstract class).

say class A have a pure virtual function "void SayHello()=0;" and class B is deriving from class A. is it possible to make class B concrete without providing pure virtual function "SayHello" definition?

Posted 28-Jan-13 23:15pm

Faisalabadians

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CPallini 29-Jan-13 6:24am

Nope.

4 solutions

Solution 1

Yes, in fact you cannot have any pure virtual functions in a concrete class so the overridden version of SayHello in B must be concrete for B to be concrete.

/Fredrik

Posted 28-Jan-13 23:22pm

Fredrik Bornander

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Faisalabadians 29-Jan-13 5:40am

can you please give me a little example?

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OriginalGriff · Accepted Answer · 2013-01-28T23:22:00

Solution 2

No.
All abstract properties and methods must be implemented in the concrete class in order to fulfil the contract.

Posted 28-Jan-13 23:22pm

OriginalGriff

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Faisalabadians 29-Jan-13 5:51am

then please help my, I have mentioned problem where class A is abstract and class B is deriving from class A then how to make class B concrete in this scenario?

OriginalGriff 29-Jan-13 6:00am

Implement the function! :laugh:

Faisalabadians 29-Jan-13 6:22am

:( you laughing :(

OriginalGriff 29-Jan-13 7:00am

I'm laughing because it is kinda obvious!

Faisalabadians 29-Jan-13 7:11am

I am new to C++

OriginalGriff 29-Jan-13 7:18am

That's fine - we all have to start somewhere! :)

Stefan_Lang · Accepted Answer · 2013-01-29T00:12:00

Solution 3

C++

class A { // abstract class
public:
   virtual void SayHello() = 0; // abstract member function
};

class B { // concrete class
public:
   void SayHello() { // concrete function
      std::cout << "Hello, I am B." << std::endl;
   }
};

Posted 29-Jan-13 0:12am

Stefan_Lang

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Faisalabadians 29-Jan-13 6:21am

bundle of thanks, please tell me one thing, if sayhello method accepts some arguments, then? to make class b concrete, it is necessary to implement the say hello function?

Stefan_Lang 29-Jan-13 7:25am

The prototype of the function must be the same, i. e. same number of arguments and same argument type. The function in class B may be declared virtual or not, that is optional. It is also possible, albeit not recommended, to use a different return type.

And yes, class B is only considered concrete if all of its or its base classes' virtual functions are 'implemented' (i. e. defined as not abstract). Otherwise, B is considered abstract. (which is also possible - you can always derive another class C from B and make that class concrete by providing the missing functions)

CPallini · Accepted Answer · 2013-01-29T00:26:00

Solution 4

Nope, see, for instance: What is a "pure virtual" member function?[^] at C++ FAQ.

Posted 29-Jan-13 0:26am

CPallini