C / C++ / MFC

15-Nov-07 16:36

Hello everyone,

Sometimes a template class is defined to have only parameter and no type (e.g. class something) information. Like the below sample. My question is, why define template class in this way (e.g. I can implement in an alternative way to put the parameter into constructor)? If there is no type information (e.g. class something), there will be no benefits and reasons to use the template class. Any comments?

<br />
// include headers <vector> <iostream><br />
<br />
using namespace std;<br />
<br />
template <int size> class Foo {<br />
<br />
private:<br />
	int limit;<br />
<br />
public:<br />
	Foo()<br />
	{<br />
		limit = size;<br />
	}<br />
<br />
};<br />
<br />
<br />
int main (int argc, char** argv)<br />
{<br />
	Foo <100> f1; // limit member of instance f1 is 100<br />
<br />
	Foo <200> f2; // limit member of instance f2 is 200<br />
<br />
	return 0;<br />
}<br />

thanks in advance,
George

George L. Jackson15-Nov-07 17:33

15-Nov-07 17:33

Nontyped template parameters give the user of your template class the opportunity to determine the size of "something".

template <size_t SIZE>
class Foo
{
public:
Foo()
{
for (size_t i = 0; i < SIZE; ++i)
vals[i] = int();
}
 
// ...
 
private:
int vals[SIZE];
};

In this example, the user can determine the size of the vals array. Otherwise, you would have to hardcode the size or dynamically create it to that size if passed via a construtor.

"We make a living by what we get, we make a life by what we give." --Winston Churchill

George_George15-Nov-07 19:19

15-Nov-07 19:19

Thanks George L. Jackson,

Any benefits of using template class parameter compared with using constructor?

regards,
George

George L. Jackson16-Nov-07 0:55

16-Nov-07 0:55

Run-time vs. compile-time! C++ gives you choices on how you want to program. Nontype template parameters can be used as compile-time expressions. My example had a constant-expression created at compile time.

"We make a living by what we get, we make a life by what we give." --Winston Churchill

George_George18-Nov-07 3:47

18-Nov-07 3:47

Thanks George L. Jackson,

In the past days, I read some articles about compile time and run time expression. I think using compiler to calculate the value of some constant is just come tricks, and I do not see any practical usage. Please feel free to comment and correct me if you are not agree. Smile | :)

Example, factorial function

http://en.wikipedia.org/wiki/Template_metaprogramming[^]

regards,
George

George L. Jackson18-Nov-07 7:28

18-Nov-07 7:28

Template metaprogramming is interesting. But like everything else, it is not useful to you if you can not visualize it as so. I have found programming concepts not so useful at first glance to be very useful over time. Nevertheless, whether you use a pair of pliers or a wrench to tighten a nut, all that matters in the end, the work was completed and the customer is happy.

"We make a living by what we get, we make a life by what we give." --Winston Churchill

George_George18-Nov-07 22:35

18-Nov-07 22:35

Thanks George L. Jackson,

What do you think is the most useful function of template class with only parameter information without type information in practices? Could you share some experiences/samples please?

regards,
George

George L. Jackson19-Nov-07 14:53

19-Nov-07 14:53

Here are a couple of links that might be helpful: http://faculty.rivier.edu/bhiggs/web/cs582aweb/slides/classtemplates.ppt#268,14,Nontype Template Parameters[^] and http://safari.oreilly.com/0201734842/ch04[^].

I haven't written much template code that just used a nontype template parameter. I usually use it with a type: tempate <typename T, int MAXSIZE> class Foo {...};. Nevertheless, the C++ Standard Library has a bitset class, in the bitset header, that is defined thus:

namespace std {
   template <size_t Bite> class bitset;
}

"We make a living by what we get, we make a life by what we give." --Winston Churchill

George_George19-Nov-07 15:33

Upgrade Appliction that link with MFC42.dll to newer version

19-Nov-07 15:33

Thanks George L. Jackson,

Good sample!

regards,
George

xanagan66615-Nov-07 15:16

xanagan666

15-Nov-07 15:16

I have some old MFC applications compiled with VC++ 6.0.

I plan on buying VS 2005 standard edition soon.

The question is

1.if I recompile it using VS 2005,Will the newly create executable file use MFC8 or it still linking with MFC42.dll ?

2.Is MFC8 dll come with every Windows XP SP2 or I have to include it in application folder(Will it conflict with the system provided one) ?

Sorry for my bad english.

Thank in advance

Re: Upgrade Appliction that link with MFC42.dll to newer version

Luc Pattyn15-Nov-07 15:59

Re: Upgrade Appliction that link with MFC42.dll to newer version

15-Nov-07 15:59

xanagan666 wrote:
I plan on buying VS 2005 standard edition soon

AFAIK VS 2008 will be available by the end of the month...

don't know about your questions though, I'm a .NET guy.

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

this months tips:
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George L. Jackson15-Nov-07 16:35

Re: Upgrade Appliction that link with MFC42.dll to newer version

15-Nov-07 16:35

In addition to what the nice ".NET guy" said:

xanagan666 wrote:
1.if I recompile it using VS 2005,Will the newly create executable file use MFC8 or it still linking with MFC42.dll ?

It will require the new MFC DLL. However, you will need to check out all the breaking changes since that version. Most likely you won't have a clean recompile.

xanagan666 wrote:
2.Is MFC8 dll come with every Windows XP SP2 or I have to include it in application folder(Will it conflict with the system provided one) ?

From my experience, there is no gurantee that that DLL will be installed on a Windows XP SP2 system. However, I have not really check if this DLL is installed when the OS or service pack is installed.

"We make a living by what we get, we make a life by what we give." --Winston Churchill

Luc Pattyn15-Nov-07 17:31

Re: Upgrade Appliction that link with MFC42.dll to newer version

15-Nov-07 17:31

xanagan66615-Nov-07 18:13

xanagan666

15-Nov-07 18:13

Thank everyone for your answer.

NamedPipe Error in CreateFile

ginjikun15-Nov-07 14:54

15-Nov-07 14:54

Hi all!
hope somebody can help.

I am using named pipe for communication. however i am encountering an error when calling CreateFile. GetLastError returns "The system cannot find the file specified."
it was working fine but after sometime CreateFile fails. I am not sure what is wrong. the named pipe server is still running.
also will there be a problem if CreateFile is called concurrently on the same named pipe server?

thanks for any help!

old

Llasus15-Nov-07 14:59

Llasus

15-Nov-07 14:59

It could be the name of the pipe that is wrong that is why it can't find it. Or maybe the named pipe still has not been created when the CreateFile was executed. Posting the code snippet could help for us to know what the problem is.

ginjikun15-Nov-07 15:12

15-Nov-07 15:12

Hi Llasus! thanks for the quick reply.

the named pipe is correct and it was working fine. i have been running it for around 2hours and after that it failed on CreateFile.
the code is similar to the sample in msdn for multithreaded namedpipe. though i have limited the pipe server instances to 25. after each successful connection it will read the data, send a reply by WriteFile then disconnect the named pipe.
for the client it will just call CreateFile then write the data and wait for the reply and close the handle.

hope to hear from u again.

old

Llasus15-Nov-07 15:50

Llasus

15-Nov-07 15:50

So only one named pipe inside the server exists and at maximum 25 clients can access the server right?

ginjikun wrote:
after each successful connection it will read the data, send a reply by WriteFile then disconnect the named pipe.

Or will the server create a named pipe for each client?

ginjikun15-Nov-07 16:17

15-Nov-07 16:17

Hi again!

Yes, there is only 1 pipe server creating 25 instances to enable at least 25 clients to connect.

something like this for the server
for (;;)
{
CreateNamedPipe(
...,
25, // instances
...
);

// wait for clients to connect
ConnectNamedPipe(...);
...
ReadFile(...);
WriteFile(...);
DisconnectNamedPipe(...);
CloseHandle(...);
}

the client...
CreateFile(...);
WriteFile(...);
ReadFile(...);
CloseHandle(...);

again, it was working fine for a few hours then suddenly i get the error "The system cannot find the file specified." i am not sure what happened. the namedpipe server is still running.
thanks again!

old

Luc Pattyn15-Nov-07 16:38

15-Nov-07 16:38

Hi,

my named-pipe experience is limited, but I have some questions/ideas anyway:

- does it fail completely (all subsequent clients get no service) once it starts to fail?
- how many connections succeed before it fails the first time? 25? much more?
- how many clients did terminate abnormally before it started to fail? around 25?
I mean: maybe you are debugging the client and abort it, rather than let it finish and
close the handles and thing like that.
- you should consider adding some logging in the server (and the clients) so you can see
what is the first thing that fails, when it is, and possibly show error messages
from GetLastError() and such.

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

this months tips:
- use PRE tags to preserve formatting when showing multi-line code snippets
- before you ask a question here, search CodeProject, then Google

ginjikun15-Nov-07 16:59

15-Nov-07 16:59

Hi Luc! thanks for the reply!

yes, once it fails, it fails completely. no more CreateFile call is successful.
i am not sure how many successful connection were made. there was a lot since i was running it for a few hours already and every 1 minute i am sending some data.
no clients so far based on the logging terminated abonormally.
though i noticed in the logging that there was a concurrent call to CreatFile then it started to fail.
that is why i am asking what the effect is if 2 concurrent calls to CreateFile was made on the same pipe server.

thanks so much!

old

Luc Pattyn15-Nov-07 17:17