Programming Forums > Application Development > C++

grumpy · Feb 18th, 2006, 9:24 PM

And, one other issue is that Java disallows inheritence from multiple classes (whether abstract or not). That's why Java treats interfaces differently from abstract classes .... they sought to eliminate multiple inheritence of classes from the language (because full blown implementation of MI of classes significantly increases difficulty implementing a compiler and can introduce serious gotchas for simple-minded programmers who misuse it). But then they wanted to allow implementation of multiple interfaces. Hence, they had to treat interfaces differently from abstract classes.

C++, by supporting multiple inheritence, does not need to make the distinction between abstract classes and interfaces (unlike Java, an interface can be implemented as a particular type of abstract class).

One effect of this is that, in C++, it is possible for an interface class to provide implementation of its methods. While it is not possible to instantiate an interface class (as for any abstract class), it is possible to provide basic functionality along with the interface that can be used in derived classes. In practice there are better ways to do that in C++ (a discussion for another day), but the fact it is possible in C++ and not in Java is one discriminator between those two languages.

titaniumdecoy · Feb 18th, 2006, 11:27 PM

Quote:

Originally Posted by grumpy

One effect of this is that, in C++, it is possible for an interface class to provide implementation of its methods. While it is not possible to instantiate an interface class (as for any abstract class), it is possible to provide basic functionality along with the interface that can be used in derived classes. In practice there are better ways to do that in C++ (a discussion for another day), but the fact it is possible in C++ and not in Java is one discriminator between those two languages.

It is possible to "provide basic functionality" in a Java abstract base class, as the code below demonstrates. As long as methods within an abstract class are not specifically declared to be abstract, they are inherited (a()) and can be overloaded (b()).

(Toggle Plain Text)

class Test {
	public static void main(String[] args) {
		Derived d = new Test().new Derived();
		d.a();
		d.b();
		d.c();
	}
	abstract class Base {
		void a() { System.out.print("A "); }
		void b() { System.out.print("B "); }
		abstract void c();
	}
	class Derived extends Base {
		void b() { System.out.print("b "); }
		void c() { System.out.print("c "); }
	}
}

This code will print "A b c".

grumpy · Feb 19th, 2006, 12:03 AM

True but, AFAIK, it is not possible for class Base to provide an implemention of abstract method c(). In C++, it is possible. It is also not possible in Java to extend more than one abstract class.

titaniumdecoy · Feb 19th, 2006, 12:43 AM

If you wanted the class Base to provide an implementation of c() you would not declare it as abstract. This may be different in C++, which I believe is what you mean to say.

grumpy · Feb 19th, 2006, 1:33 AM

Yep. C++ allows one to provide an implementation for all member functions of a class, even if they are "abstract" or (in C++ speak) "pure virtual". Providing an implementation of a pure virtual function is optional (as long as you don't explicitly call it, as in an example I gave earlier), but it is certainly possible.

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Feb 18th, 2006, 9:24 PM	#21
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,208 Rep Power: 5	And, one other issue is that Java disallows inheritence from multiple classes (whether abstract or not). That's why Java treats interfaces differently from abstract classes .... they sought to eliminate multiple inheritence of classes from the language (because full blown implementation of MI of classes significantly increases difficulty implementing a compiler and can introduce serious gotchas for simple-minded programmers who misuse it). But then they wanted to allow implementation of multiple interfaces. Hence, they had to treat interfaces differently from abstract classes. C++, by supporting multiple inheritence, does not need to make the distinction between abstract classes and interfaces (unlike Java, an interface can be implemented as a particular type of abstract class). One effect of this is that, in C++, it is possible for an interface class to provide implementation of its methods. While it is not possible to instantiate an interface class (as for any abstract class), it is possible to provide basic functionality along with the interface that can be used in derived classes. In practice there are better ways to do that in C++ (a discussion for another day), but the fact it is possible in C++ and not in Java is one discriminator between those two languages.

Feb 19th, 2006, 12:03 AM	#23
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,208 Rep Power: 5	True but, AFAIK, it is not possible for class Base to provide an implemention of abstract method c(). In C++, it is possible. It is also not possible in Java to extend more than one abstract class.

Feb 19th, 2006, 12:43 AM	#24
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 843 Rep Power: 3	If you wanted the class Base to provide an implementation of c() you would not declare it as abstract. This may be different in C++, which I believe is what you mean to say.

Feb 19th, 2006, 1:33 AM	#25
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,208 Rep Power: 5	Yep. C++ allows one to provide an implementation for all member functions of a class, even if they are "abstract" or (in C++ speak) "pure virtual". Providing an implementation of a pure virtual function is optional (as long as you don't explicitly call it, as in an example I gave earlier), but it is certainly possible.

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Programming Forums > Application Development > C++

Java-style interfaces in C++