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From: hargrove@asc.slb.com (Jim Hargrove)
Newsgroups: comp.object
Subject: C++ vs. C
Message-ID: <1991Apr24.125926.5146@asc.slb.com>
Date: 24 Apr 91 12:59:26 GMT
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This note is prompted by several postings by Scott Guthery, beginning
with The Emperor Strikes Lethe. I am struck by how different SBG's
experience with object oriented programming differs from my own. I
began programming in C++ about 18 months ago. Prior to that, I worked
mainly in C, though I have used many different languages over the
years. 

I believe that I am writing better programs today after switching to
C++. Here is an example:  Several years ago, I was faced with the
problem of writing display routines for a number of different types
of "objects." Each of these objects could be a block of text, an
appointment page, a calendar, or a "folder" containing other objects.
I wrote the routine as follows:  

void    DisplayObject(Object    * Obj,
                      Window    *   W)
{
    switch(ClassOf(Obj))
    {
        case    Text:
            DisplayText(Obj,W);
            break;
        case    ApptPage:
            DisplayAppt(Obj,W);
            break;
        case    Calendar:
            DisplayCalendar(Obj,W);
            break;
        case    Folder:
            DisplayFolder(Obj,W);
            break
        default:
            Error("Invalid Object");
    }
}

That is a nice, simple routine. Of course we can improve it quite
easily, but there is little incentive to do so. This works; it's easy
to understand; and so long as we don't invent a large number of 
objects, it is relatively easy to maintain.  Today, I would write the
same routine in C++ as 

Obj->Display(W);

which is better both technically and aesthetically. I could have
written the routine the same way five years ago. I could have created
a table of routines containing the address of the function to call to
display each kind of object. The ClassOf function provides a
convenient index into the table. The routine would be simply:

(*DisplayFn[ClassOf(Obj)])(Obj,W);

In ANSI C, I could even dispense with the (*) notation. So, I have to
agree with Scott. There is really nothing new here. The syntax is a
bit different, granted, but we probably don't really have a "paradigm
shift." 

Or do we? The language we speak/write/code in influences the
way we think. In C, I was used to thinking of switch statements.
Indeed, this style of programming is taught extensively. Look at any
book on Window programming, for example. But the use of a virtual
function is much better. It is easier to maintain, since I never have
to modify a DisplayObject routine at all. Moreover, it is more
efficient. 

Notice that the code represented by the original DisplayObject
routine has virtually disappeared! All of the code necessary to
invoke the proper routine is generated by the compiler.  In C++, the
use of a virtual function in this case is obvious. 

If I were to code the routine using a switch statement [and dynamic
type checking :-)] it would look very strange. So, the mere use of an
object oriented langauge has led me to a better overall program. The
fact that the technique used, a jump vector, has been around since
the days of the earliest computers is largely irrelevant. C++ appears
to be a significantly better tool than C.

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|These opinions are my own, of course. But you know that.            |
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|Internet: hargrove@asc.slb.com                                      |
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        -- jwh
-- 

        -- jwh