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

Luc Pattyn

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 | :)

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

George L. Jackson15-Nov-07 16:35