C / C++ / MFC

Stephen Hewitt18-Oct-07 14:42

Stephen Hewitt

18-Oct-07 14:42

Chill out! It's a public forum.

Steve

George_George18-Oct-07 4:06

18-Oct-07 4:06

Thanks James,

Could you describe in more details please what benefits we could get if we have a derived class please? Compared with non-virtual destructor?

regards,
George

David Crow18-Oct-07 2:51

David Crow

18-Oct-07 2:51

George_George wrote:
What is the purpose of virtual destructor?

To ensure that the proper derived class destructor is called if an object of a derived class is deleted through a pointer to a base class. If a class has any virtual functions, it should have a virtual destructor.

George_George wrote:
If currently, no derived class?

Can you guarantee that your class will never be derived from? If so, then a virtual destructor is not necessary.

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

George_George18-Oct-07 4:14

18-Oct-07 4:14

Thanks DavidCrow,

DavidCrow wrote:
Can you guarantee that your class will never be derived from? If so, then a virtual destructor is not necessary.

Your reply makes senses.

DavidCrow wrote:
To ensure that the proper derived class destructor is called if an object of a derived class is deleted through a pointer to a base class. If a class has any virtual functions, it should have a virtual destructor.

Could you show a simple sample (or some pseudo code) why *ensure that the proper derived class destructor is called* please? Compared with non-virtual destructor?

regards,
George

toxcct18-Oct-07 4:15

toxcct

18-Oct-07 4:15

George_George wrote:
Your reply makes senses

that's why you gave him '3' ?

[VisualCalc][Binary Guide][CommDialogs] | [Forums Guidelines]

Nemanja Trifunovic18-Oct-07 2:54

Nemanja Trifunovic

18-Oct-07 2:54

George_George wrote:
What is the purpose of virtual destructor? If currently, no derived class?

If your class is meant to be derived from and used in a polimorphic manner (a base pointer to a derived class' object), virtual destructor is a must.

On the other hand, if the class is a "concrete type" that is never meant to be derived from, virtual destructor is not required.

Programming Blog
utf8-cpp

George_George18-Oct-07 4:17

18-Oct-07 4:17

Thanks Nemanja,

I think you mean if the destructor is virtual, then if we delete an object of derived class through based class pointer, then the derived class destructor is invoked? Right?

If the destructor is non-virtual, then if we delete an object of derived class through based class pointer, then the base class destructor is invoked? Right?

regards,
George

Off Topic, Don't rate this !

David Crow18-Oct-07 4:26

David Crow

18-Oct-07 4:26

Rather than ask these basic questions, why don't you spend five minutes with the debugger to see what gets called when. That will hopefully solve all of your mysteries.

toxcct18-Oct-07 4:27

toxcct

18-Oct-07 4:27

just a question, how old are you ?

[VisualCalc][Binary Guide][CommDialogs] | [Forums Guidelines]

Nemanja Trifunovic18-Oct-07 5:42

Nemanja Trifunovic

18-Oct-07 5:42

George_George wrote:
If the destructor is non-virtual, then if we delete an object of derived class through based class pointer, then the base class destructor is invoked? Right?

Strictly speaking, if you do that, it will result in undefined behavior, which means that different compilers may do different things.

Programming Blog
utf8-cpp

George_George18-Oct-07 19:19

How to make setup Borland C++ Builder 6

18-Oct-07 19:19

Thanks Nemanja,

I think deleting object from base class pointer is common in C++ code, and if you think the approach has potential issues, could you describe what do you think is the best practices please?

regards,
George

upamir18-Oct-07 1:42

upamir

18-Oct-07 1:42

Have a good day

for Borland C++ Builder 6, setup for windows
how can i find the files(my program needs them) which doesnt exists on the other computers for setup.

Re: How to make setup Borland C++ Builder 6

Nelek18-Oct-07 2:41

Nelek

18-Oct-07 2:41

I am programming with VC++, and I wanted to check it out too. Someone recommended me to use the "dependancy walker" and in my case wored very good.

Greetings.

--------
M.D.V.

If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?

Help me to understand what I'm saying, and I'll explain it better to you

Wink | ;)

Re: How to make setup Borland C++ Builder 6

upamir20-Oct-07 23:21

upamir

20-Oct-07 23:21

Have a good day

thanks for your interest

i work on the depen. walker and trying to understatnd.,
i couldnt find the files
could you help me how to use the depen. walker

Is there an alternative solution to this? [Solved]

Nelek18-Oct-07 0:51

Nelek

18-Oct-07 0:51

Hi all,

the other day someone asked about intializing a buffer (this post)[^]).

Today I have had more or less the same problem, but with a pointer. The case is:

I have a fixed area of data available to write data in a PLC, but the data itself can be variable (Area = 4096 Bytes, data = [64 ~ 4096]). The length depends on the actual project opened in the application. As the data in the PLC are permanent and a project may be shorter than another, I want to ensure empty values in the rest of the destiny that are over the length of the actual project. (I hope you can understand what I mean).

With a normal buffer like in the post of the other day is:

BYTE Buffer [BUF_LENGTH] = {0};

but a pointer created with new is fulled of "I" = 240 = 0xCD. As I want to have "0", I am now using:

nBufLength = nTotalArea - nValidWrittenData;
BYTE* pBuffer = NULL;
pBuffer = new BYTE [nBufLength];
for (int i = 0; i < nBufLength; i++)
	*(pBuffer + i) = 0;

Which other possibilities can I use / do you recommend to write "0" in the whole buffer?

-- modified at 7:18 Thursday 18th October, 2007

image processing toolkits | batch image processing

C^hris L^osinger18-Oct-07 0:56

C^hris L^osinger

18-Oct-07 0:56

there's also ZeroMemory and memset, but they do the same thing your for loop does.

Re: Is there an alternative solution to this? [SOLVED]

Nelek18-Oct-07 1:07

Nelek

18-Oct-07 1:07

Thanks.

sps-itsec4618-Oct-07 1:25

18-Oct-07 1:25

Paranoid people like me tend to use SecureZeroMemory[^], "as the compiler can optimize a call to ZeroMemory by removing it entirely".

Makes me think of this article: Optimization: Your Worst Enemy[^] Wink | ;)

cheers!
mykel

OMM: "Let us be thankful we have an occupation to fill. Work hard, increase production, prevent accidents and be happy."

James R. Twine18-Oct-07 1:38

James R. Twine

18-Oct-07 1:38

   SecureZeroMemory(...) should not really be used for first initialization of a buffer, because if you use memset(...) right before you use the buffer, the call to memset(...) will not be optimized away.

   You use SecureZeroMemory(...) in situations where you are clearing a buffer AFTER its use. For example, if you decode/decrypt something into a buffer, and you want to make sure that the plaintext is not lying around in memory somewhere so you clear the buffer before deallocating it (or exiting the function).

   It is situations where the last thing you do to the buffer is call memset(...) on it where it can get optimized away.

   Peace!

-=- James
Please rate this message - let me know if I helped or not!<HR>If you think it costs a lot to do it right, just wait until you find out how much it costs to do it wrong!
Avoid driving a vehicle taller than you and remember that Professional Driver on Closed Course does not mean your Dumb Ass on a Public Road!
See DeleteFXPFiles

sps-itsec4618-Oct-07 5:27

18-Oct-07 5:27

You're right, it should definitely be used for clearing e.g. passwords from memory.

But where did you get the info that an initial memset(...) / ZeroMemory(...) is never optimized away? Furthermore I cannot see any info on MSDN that it's a no-go to do the first initialization with SecureZeroMemory(...). Just a speed issue - saving 42 CPU cycles? Wink | ;)

cheers!
mykel

OMM: "Let us be thankful we have an occupation to fill. Work hard, increase production, prevent accidents and be happy."

James R. Twine18-Oct-07 12:09

James R. Twine

18-Oct-07 12:09

   The optimization that can remove the last memset(...) is based on whether or not the memory being memset(...) is ever read after being written to (by memset(...)).

   In cases of initialization, you can safely use memset(...) because you will be reading the memory at some point after the initial call to memset(...) (well, at least you would be in general).

   By using memset(...) to clear after use, when you are all done with the memory, if you do not read from that memory again, the optimizer assumes that if you never read what you wrote, you did not need to write it, and can optimize away the call.

   Cycle counting only makes sense when you know your target hardware. Without knowing the actual differences in what SecureZeroMemory(...) actually has to do behind the scenes (e.g. does it have to read each and every byte/32-bit value it writes to), it is safer to assume that if you do not need the assurances that it provides, you do not need to use the function.

   Peace!

sps-itsec4625-Oct-07 12:55

25-Oct-07 12:55

Makes perfectly sense... thanks for the answer! Rose | [Rose]

No FUD anymore when initializing memory!

But tell me one more thing: Is this behavior your personal experience or where did you get that deep knowledge of code optimization?

cheers!
mykel

OMM: "Let us be thankful we have an occupation to fill. Work hard, increase production, prevent accidents and be happy."

James R. Twine25-Oct-07 13:52

James R. Twine

25-Oct-07 13:52

   This kind of knowledge and wisdom is based on what I have learned, what I have done, and both together.

   Most of my professional development has been with server-side development for financial-related products, so I learned very quickly that the stuff you get away with on the client side just does not cut it on the server side. Taking .5 seconds longer than necessary to do something when the market is moving away from you can cost real $$$ in the real world.

   So I spend much of my professional development life researching things like CPU architecture and code-level optimization techniques so that when milliseconds count, people can count on me.

   Peace!

sps-itsec4626-Oct-07 6:41