OOP in C!

[Prashanth Ellina]

Hi,

I have a feeling that OOP can be done in C also. I have used a
structure to hold member variables and function pointers. The
structure is used as a class to create new 'objects'. But I hit a
problem. How do I access these 'member variables' from the function
that is pointed to by the function pointer in the structure.

I would really appreciate help with this and a code sample will be
wonderful.

Thanks in advance,
Prashanth Ellina

[Malcolm]

"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > wrote in message

You have never provided a single reason
why you believe that these are not callback functions.
I submit that the reason is that you can't
and that you know that you can't.
They are not callback functions because they are not passed by a high-level
function to a lower-level one, and then called.

The vast majority of C function pointers are used to implement callback
functions, but not all of them.
You are simply trolling.
Please go away troll.

Can people (Not just E. Robert Tisdale) please stop filling the ng with
"troll alerts", accusations of trolling, etc. Most of the regs are not
interested.

[E. Robert Tisdale]

Something that calls itself Christian Bau wrote:

A function pointer is by definition a callback function.

You are a liar.
I never said any such thing.
I merely asked for an example of a function pointer
that is not used as a callback function.
So far, you have failed to provide one.

[Christian Bau]

In article <ca**********@n ntp1.jpl.nasa.g ov>,
"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > wrote:

Something that calls itself Christian Bau wrote:

A function pointer is by definition a callback function.

In typical Tisdale manner, a falsified quote.

You are a liar.
I never said any such thing.
I merely asked for an example of a function pointer
that is not used as a callback function.
So far, you have failed to provide one.

Go away, idiot.

[Christian Bau]

In article <ca**********@n ews5.svr.pol.co .uk>,
"Malcolm" <ma*****@55bank .freeserve.co.u k> wrote:

"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > wrote in message

You have never provided a single reason
why you believe that these are not callback functions.
I submit that the reason is that you can't
and that you know that you can't.

They are not callback functions because they are not passed by a high-level
function to a lower-level one, and then called.

The vast majority of C function pointers are used to implement callback
functions, but not all of them.

You are simply trolling.
Please go away troll.

Can people (Not just E. Robert Tisdale) please stop filling the ng with
"troll alerts", accusations of trolling, etc. Most of the regs are not
interested.

I think this is a misunderstandin g.

It is not an accusation. It is the troll Tisdale, in this case replying
to Arthur J. O'Dwyer, and he is friendly enough to alert Arthur that
this is a post from a troll.

[Malcolm]

"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > wrote

I merely asked for an example of a function pointer
that is not used as a callback function.
So far, you have failed to provide one.

OK here's an example.

/*
return the maximum value of an stdlib maths function that takes one scalar
as its argument.
*/
double maximum( double (*fptr)(double x) )
{
if( fptr == sin)
return 1.0;
if(fptr == sqrt)
return DBL_MAX;
if(fptr == asin)
return 3.14 * 2.0;

/* add extra for the other function in the maths library */
}

[Me]

Please don't waist your time trying to be OO using C...

Just move to C++ man...

The code I am porting right now does that and it sucks....

Write OO in an OO language and get AWAY from non OO except for drivers and
such.

On 7 Jun 2004 20:50:54 -0700, Gustavo Cipriano Mota Sousa
<gu*********@in f.ufg.br> wrote:

tinybyte <ti************ ******@virgilio .it> wrote in message
news:<pa******* *************** ******@virgilio .it>...

On Thu, 03 Jun 2004 16:34:44 -0700, Prashanth Ellina wrote:

> I would really appreciate help with this and a code sample will be
> wonderful.

Have a look at this online book, I think it will teach you a real
deal about OOP in ANSI C.

http://www.planetpdf.com/codecuts/pdfs/ooc.pdf

I started reading the book, anyone know where can i download the code
examples?

--
Using Opera's revolutionary e-mail client: http://www.opera.com/m2/

[E. Robert Tisdale]

Malcolm wrote:

OK here's an example. cat example.c #include <math.h>
#include <float.h>

// return the maximum value of an stdlib maths function
// that takes one scalar as its argument.

double maximum(double (*fptr)(double x)) {
if( fptr == sin)
return 1.0;
if(fptr == sqrt)
return +DBL_MAX;
if(fptr == asin)
return 3.14 * 2.0;
return -DBL_MAX;

// add extra for the other function in the maths library
}
gcc -Wall -std=c99 -pedantic -c example.c

Contrived and silly but correct.

[Richard Tobin]

In article <ca**********@n ntp1.jpl.nasa.g ov>,
E. Robert Tisdale <E.************ **@jpl.nasa.gov > wrote:

I merely asked for an example of a function pointer
that is not used as a callback function.

A function is a callback when it calls *back*. So if you pass a
function pointer to a library, and the library calls it, then it's
reasonable to call it a callback.

Many uses of function pointers aren't like that.

For example, an interpreter that uses a table to map function names
to function pointers.

Or a struct that contains a function pointer representing a method
(you might consider it a callback if it's set "from outside", but not
if it's just set by the constructor).

Or a function pointer used in an expression, such as

return (size < threshhold ? quickalloc : slowalloc)(size , heap);

Ot a function pointer used for calls to dynamically loaded functions
(obviously not pure standard C).

-- Richard

[Alan Balmer]

On Tue, 8 Jun 2004 22:41:17 +0100, "Malcolm"
<ma*****@55bank .freeserve.co.u k> wrote:

"E. Robert Tisdale" <E.************ **@jpl.nasa.gov > wrote

I merely asked for an example of a function pointer
that is not used as a callback function.
So far, you have failed to provide one.

OK here's an example.

/*
return the maximum value of an stdlib maths function that takes one scalar
as its argument.
*/
double maximum( double (*fptr)(double x) )
{
if( fptr == sin)
return 1.0;
if(fptr == sqrt)
return DBL_MAX;
if(fptr == asin)
return 3.14 * 2.0;

/* add extra for the other function in the maths library */
}

And, of course, function pointers are a well-known approach to
implementing state machines.

--
Al Balmer
Balmer Consulting
re************* ***********@att .net

[E. Robert Tisdale]

Richard Tobin wrote:

E. Robert Tisdale wrote:

I merely asked for an example of a function pointer
that is not used as a callback function.
A function is a callback when it calls *back*.
So, if you pass a function pointer to a library
and the library calls it,
then it's reasonable to call it a callback.

Agreed.
Many uses of function pointers aren't like that.

For example, an interpreter that uses a table
to map function names to function pointers.
What about Stephen Sprunk's parsing example upthread?

"Since the command selector is a string,
it can't be used in a switch statement,
and keeping the list of commands and their respective parsers
in an array also makes it very easy to add and remove commands
without touching the core logic.
And, since the list of commands is stored outside said logic,
it also allows different users
to get a different list of valid commands
(or the same commands pointing to different parsers)
just by passing in a different array to the same core logic."

Because the function table

struct parser_record parser_table_us er[] = {
{ "FOO", parse_foo },
{ "BAR", parse_bar },
{ "BAZ", parse_baz },
{ NULL, NULL}
};

can be defined in the calling program and passed to parse_main,
the functions (parse_foo, parse_bar and parse_baz)
are callback functions.
Or a struct that contains a function pointer representing a method
(you might consider it a callback if it's set "from outside",
but not if it's just set by the constructor).

It is a callback function
if you pass a reference (pointer) to that struct to another function
which calls it through the pointer set by the constructor.

The bottom line is that pointers to callback functions can be used
to implement run-time polymorphism in C as well as C++.
The code written to operate on object of a "base" type
will still work with objects "derived" from the base type
because it can callback the functions implemented for
the derived type through the function pointers
in the virtual function table (jump table).