Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!elsie!nih-csl!keith
From: keith@nih-csl.UUCP (keith gorlen)
Newsgroups: comp.arch
Subject: Re: Do every object based systems suffer from poor performance ?
Message-ID: <218@nih-csl.UUCP>
Date: Mon, 18-May-87 17:41:14 EDT
Article-I.D.: nih-csl.218
Posted: Mon May 18 17:41:14 1987
Date-Received: Wed, 20-May-87 03:47:16 EDT
References: <6055@shemp.UCLA.EDU> <1831@hplabsc.UUCP> <6894@alice.UUCP> <1837@hplabsc.UUCP>
Organization: NIH-CSL, Bethesda, MD
Lines: 89
Keywords: capability, object, 80286, 432
Summary: clarification of C++ virtual functions

In article <1837@hplabsc.UUCP>, kempf@hplabsc.UUCP (Jim Kempf) writes:
> In article <6894@alice.UUCP>, ark@alice.UUCP writes:
> 
> > Second, C++ does have dynamic lookup of methods at run time -- that's
> > what "virtual" is for:
> > 
> > 	class Foo {
> > 	public:
> > 		virtual void print();
> > 	};
> > 
> > 	class Bar: public Foo {
> > 	public:
> > 		void print();
> > 	};
> > 
> > 	Foo foo;
> > 	Bar bar;
> > 	Foo* fp1;
> > 	Foo* fp2;
> > 
> > 	fp1 = &foo;
> > 	fp2 = &bar;
> > 
> > Now calling foo.print() and bar.print() still take the same overhead as
> > a C function call, because the compiler can determine the type of foo
> > and bar statically. ...

In fact, the code generated for foo.print() and bar.print() uses the virtual
function pointer table (vtbl) just as fp1->print() and fp2->print() do.  You
must write Foo::foo.print() to inhibit the dynamic binding.  It is not
immediately obvious to me why this is.

> 
> But can I define another level:
> 
> class Fee : public Bar {
> public:
> 
> 	 void print();
> 
> };
> 
> and have this function override the one inherited from Bar?

Yes.

> Also, can I define a virtual function halfway down an inheritance
> chain and have it inherited?

If by this you mean "If I declare Foo::print() as a non-virtual member
function and Bar::print() as a virtual member function, will class Fee
inherit the definition of Bar::print()?" the answer is yes.

Where the C++ dynamic binding mechanism differs from that of
Smalltalk-80 is that the table used to locate the virtual member
functions ("methods" in Smalltalk parlance) for a class contains only
the methods actually defined in the class in Smalltalk, whereas in C++
the table contains pointers to virtual functions defined in the class
AND to those inherited from the base class, the base class's base
class, etc.  Thus, in Smalltalk calling a method involves searching
the tables from the class of the current object up toward the root of
the class hierarchy until the method is found.  In C++, you just index
into the table for the class of the current object.  C++ trades off
space for speed.  Also, C++ has nothing analogous to Smalltalk's
"super".

> (Apologies to those who are not interested in the esoterica of
> object oriented programming. Maybe this discussion should be
> moved to comp.lang?)

Yes, it should!




Subject: Re: Do every object based systems suffer from poor performance ?
Newsgroups: comp.arch
Keywords: capability, object, 80286, 432
Summary: Clarification of C++ virtual functions
References: <6055@shemp.UCLA.EDU> <1831@hplabsc.UUCP> <6894@alice.UUCP> <1837@hplabsc.UUCP>



-- 
	Keith Gorlen			phone: (301) 496-5363
	Building 12A, Room 2017		uucp: seismo!elsie!nih-csl!keith
	National Institutes of Health
	Bethesda, MD 20892