C / C++ / MFC

Can I group function (method) within Struct in ANSI C?

19-Sep-04 3:11

I can only assume you mean when the compiler is expecting an l-value on the left-hand side of an assignment:

i=10; is fine so long as i was not defined as a constant.

More obviously:

10=3; will give you an l-value error because 10 is not an l-value and therefore cannot be assigned to anything else.

10 is 10, and cannot be anything else! Big Grin | :-D

I hope this helps,

Zoltan

Link260018-Sep-04 19:05

Link2600

18-Sep-04 19:05

Can I group function (method) within Struct in ANSI C?
I constantly listen people talking about it. Many have
stated that it can be done.

If it can be done, is it recommended to do so?

Thanks

-----------------------------
C++ without virtual functions is not OO. Programming with classes but without dynamic binding is called "object based", but not "object oriented".

Re: Can I group function (method) within Struct in ANSI C?

Michael Dunn18-Sep-04 19:16

Re: Can I group function (method) within Struct in ANSI C?

18-Sep-04 19:16

You can have function pointers in a struct, which creates the same syntax as C++:

int myfunc() { return 1; }
 
typedef struct Foo
{
int a;
int (*pfunc)();
} Foo;
 
Foo f;
 
f.pfunc = myfunc;
f.a = f.pfunc();  /* call thru function pointer */

--Mike--
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Link260018-Sep-04 19:23

Link2600

18-Sep-04 19:23

Thanks for your reply.

I did mean function pointers within Stuct. I mean function within
Struct. Some people are stating that Object-oriented can be done
in ANSI C. I just don't think so. But I'm not really sure.
Can it be done?

-----------------------------
C++ without virtual functions is not OO. Programming with classes but without dynamic binding is called "object based", but not "object oriented".

Re: Can I group function (method) within Struct in ANSI C?

cmk18-Sep-04 21:00

cmk

18-Sep-04 21:00

Literal reply : No.
Assumed reply : Yes.

ANSI C is not an OO language.
The concept of excapsulation is not supported.
A pure C compiler would fail to compile a struct with a method.
There are few, if any, pure C compilers around in use anymore.

C++ made an effort to be backwards compatible with C.
In doing so it made some modifictions to some of the C syntax to make it fit better with the C++ paradigm.

One such change was to give struct the same abilities as class. That is, struct definitions can include methods.

The only real difference, in C++, between struct and class is the default access control.
struct : by default everything is public.
class : by default everything is private.

So, a C++ compiler would have no problem with a struct with a method.

As for being recommended ... a topic sure to start a religous war.

Personally i use struct for simple records.
I will only provide constructor, reset/init methods.
For me, seeing a struct is an indicator that the object is simple/dumb.
I use classes for all other records.

...cmk

Save the whales - collect the whole set

Re: Can I group function (method) within Struct in ANSI C?

Anonymous18-Sep-04 18:10

Anonymous

18-Sep-04 18:10

it would probably be a good idea to delete one of these posts no need for 2

static constant template member problems

Andras Zoltan18-Sep-04 12:17

18-Sep-04 12:17

I have a template class inside which I wish to embed some static constants to speed up some time-critical code. Consider this:

template <class T, int I> class X<br />
{<br />
private:<br />
  static const icopy = i;<br />
  static const iminus1 = i-1;<br />
  static const itimes8 = i<<3;<br />
  int array[I];<br />
public:<br />
  (..)<br />
};

I is assigned by the code using the template to the size of T, which is fine when the compiler uses it to create the array with num elements equal to I.

However, all of the static constants appear to be getting initialised to 1, even when I is explicitly set to 4 or 8.

I wish to make these constants static in order to reduce memory overhead, therefore a solution to this problem is kinda essential to me? Am I mis-using the language? Or is it an example of VS.Net 2003's C++ compiler's non-conformance?

Confused | :confused:

Please help!

Thanks in advance...

Michael Dunn18-Sep-04 12:56

18-Sep-04 12:56

I and i are different variables entirely. And making them static probably won't reduce memory usage. The values will all be known at compile time, so the compiler will be able to directly insert the values (unless you take the address of iminus1 or one of the other members).

--Mike--
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Andras Zoltan19-Sep-04 3:08

19-Sep-04 3:08

Hi Mike,

Thank you for your reply -

Michael Dunn wrote:
I and i are different variables entirely. And making them static probably won't reduce memory usage

I was being stupid in my code example - I am trying to use the template argument I to assign my static constants in order to reduce computational overhead in any functions that need to use them. I accidentally put i instead because I was half asleep when I posted the question! Sleepy | :zzz:

In theory, therefore, if I use the template argument, which will be a compile-time constant of, say, 4, to calculate the other constants, how can icopy be getting set to 1 and not 4?

Thanks

Michael Dunn19-Sep-04 11:50

19-Sep-04 11:50

Unfortunately, I'm not seeing the problem you are. Here's the code I tested on VC 7.1:

#include <iostream>
#include <tchar.h>
using namespace std;
 
template <class T, int I> class X
{
public:
    X() {}
    static const icopy = I;
    static const iminus1 = I-1;
    static const itimes8 = I<<3;
 
    int array[I];
};
 
int _tmain(int argc, _TCHAR* argv[])
{
X<int, sizeof(int)> x1;
 
    cout << "x1.icopy = " << x1.icopy
         << "\nx1.iminus1 = " << x1.iminus1
         << "\nx1.itimes8 = " << x1.itimes8 << '\n';
 
	return 0;
}

Output:

x1.icopy = 4
x1.iminus1 = 3
x1.itimes8 = 32

Andras Zoltan20-Sep-04 12:25

20-Sep-04 12:25

Mike,

Sometimes, it just helps when someone forces you to look at your code again eh!!? Whilst we've been discussing this, I converted the template class so that it simply used standard members and discovered that there was a general error in some of my logic that was making it appear that these static constants were getting initialised to 1. Of course, however, they weren't - as you've demonstrated and I've just verified.

What didn't help - and I'm not going mad on this one - is that the VC++ debugger picks up these values at run-time as 1 (perhaps understandably, since their values are dependent on the template args), regardless of what value they actually have, therefore I had just assumed that the compiler was stupid, and not me. I should have remembered that the console project is my friend when it comes to debugging values.

Thanks to your help, I have now converted it back to static constants and have received a fifth of a second speed increase over the course of 250,000 operations that will use this class - not much, but enough for me to be happy Big Grin | :-D

!

Thanks very much for your help,

Zoltan

I'm not as evil as my name suggests

Michael Dunn20-Sep-04 12:36