NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/3734

Text File | 1996-08-06 | 2.7 KB | 67 lines

Newsgroups: comp.lang.c++,comp.lang.c
Path: txnews.amd.com!news
From: Bret Patterson <faustus>
Subject: Re: Performance: C vs. C++
Content-Type: text/plain; charset=us-ascii
Message-ID: <[e-mail]>
Sender: [e-mail]
Nntp-Posting-Host: fuggles
Content-Transfer-Encoding: 7bit
Organization: Advanced Micro Devices, Inc., Austin, Texas, USA
References: <[e-mail]> <4da9pn$[e-mail]> <4dnpl2$[e-mail]> <[e-mail]>
Mime-Version: 1.0
Date: Thu, 25 Jan 1996 19:16:09 GMT
X-Mailer: Mozilla 1.12 (X11; I; HP-UX A.09.05 9000/715)
X-Url: news:[e-mail]

Harri Halttunen <[e-mail]> wrote:
>> Not quite.  C is quite different from C++ from the performance point of
>> view.  The difference is not only in virtual functions and exceptions
>> but in class construction, destruction, etc... and most significantly in
>> Runtime Type Checking.  However, because machines are getting faster by
>> the day, this sort of "inefficiency" is hardly noticable.
>
>All that is true (at least in the some point of view), but we have to remember
>that usually in real applications this kind of comparison doesn't matter. This
>is because fetching something from a database or file takes many times longer
>than, for example, a virtual function call or to handle an excepion. Or even
>check a type of an object at runtime.

Except what most people use virtual functions for is to avoid large switch
statements and make code more easily changed, more readable etc.

I would say a*WELL written* C++ program is faster than a *Well written* C  program that does the
exact same thing. Notice *Well written* because most c++ programmers are in their infancy
and blame their bad coding on the language.

Here is an example to illustrate why virtual functions do not cause extra overhead.

C program:

switch (object->type)
{
    case 1 : DragonAttack();
    case 2 : OrcAttack();
    ...
};

and a C++:
    object->Attack(); // which internally g++ does a object->virtualTable[Function#]();

Big deal. So c++ does a virtual table lookup, but in order for C to have this ability
it has to use a switch statement or ifelse construct. I would say with good compiler
optimization C++ is going to be faster and more readable.

Of course C++ uses 4bytes for the pointer while in C you would use a char to represent
type. But if the extra 3 bytes per object is a major problem then you wouldn't want that
extra byte in C either. You would use another method.

>I use C++ mainly, because it supports code and idea reuse better than other
>languages.

The only thing of C++ that I know causes alot of overhead is exceptions. But other forms
of error checking are available and exceptions can also be optimized/minimized.

Bret