c++ function pointer

[Marco]

Hi,

I would like implement a class with a method Draw(),
..I would like that method Draw() change with another method when the
state of class change.
I think to use function pointer to do it, but I don't know how use this
in a class?
Can someone do a example?

[Victor Bazarov]

Re: c++ function pointer

Marco wrote:[color=blue]
> I would like implement a class with a method Draw(),[/color]

class a_class {
void Draw();
};
[color=blue]
> .I would like that method Draw() change with another method when the
> state of class change.[/color]

What does it mean "method Draw() change"? How can a method change?
[color=blue]
> I think to use function pointer to do it, but I don't know how use this
> in a class?[/color]

I don't think there is a need of any pointer. Just implement your 'Draw'
so that it checks the "state of class" and behaves accordingly:

void a_class::Draw() {
if (state_didnt_ch ange) // whatever that means
; // do something here
else // the state has changed
; // do something different
}
[color=blue]
> Can someone do a example?[/color]

I can do a example alright. Just give me the example and I'll do it.

V

[n2xssvv g02gfr12930]

Re: c++ function pointer

Marco wrote:[color=blue]
> Hi,
>
> I would like implement a class with a method Draw(),
> .I would like that method Draw() change with another method when the
> state of class change.
> I think to use function pointer to do it, but I don't know how use this
> in a class?
> Can someone do a example?
>[/color]
Marco,

Why not create a base class with the Draw() method being declared as
virtual and use a pointer to the base class. The actual Draw() function
can then be defined in descendant classes, (see below).

class Shape
{
public:
virtual void Draw(void);

[osmium]

Re: c++ function pointer

"Marco" writes:
[color=blue]
> I would like implement a class with a method Draw(),
> .I would like that method Draw() change with another method when the
> state of class change.
> I think to use function pointer to do it, but I don't know how use this
> in a class?
> Can someone do a example?[/color]

Do you know about polymorphism in C++? Perhaps that will help you do what
you want. If you want to stick with the function pointer approach, here is
a sample of the rather nasty syntax. But note that the function pointer is
in main and not in the class. But there is a way to get access to the state
variable should you wish to. I can show you how to get it too, but first I
think you should knowingly reject the polymorphic approach.

#include <iostream>

class C
{
public:
void draw();
private:
int state; // igonred in this test program
};
//--------------
void C::draw()
{
std::cout << "draw called\n";
}
//=============== ===
int main()
{
C object;
typedef void( C::*PMF) (); // for clarity
PMF pmf; // pointer to member function
pmf = &C::draw; // select desired member function
(object.*pmf) (); // call it

std::cin.get();
}

[Alf P. Steinbach]

Re: c++ function pointer

* Marco:[color=blue]
>
> I would like implement a class with a method Draw(),
> .I would like that method Draw() change with another method when the
> state of class change.
> I think to use function pointer to do it, but I don't know how use this
> in a class?
> Can someone do a example?[/color]

Use a pointer to an instance of a class with a virtual member function.


--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[HappyHippy]

Re: c++ function pointer

Marco wrote:[color=blue]
> Hi,
>
> I would like implement a class with a method Draw(),
> .I would like that method Draw() change with another method when the
> state of class change.
> I think to use function pointer to do it, but I don't know how use this
> in a class?
> Can someone do a example?
>[/color]

You can do something like that:

#include <iostream>


class CMYClass
{
public:
enum E_STATE
{
E_ST1,
E_ST2,
E_ST3
};
CMYClass():
m_pDrawFnc(&CMY Class::draw_S1)
{
}

void draw()
{
(this->*m_pDrawFnc)() ;
}

void changeState(E_S TATE eState)
{
switch(eState)
{
case E_ST1:
m_pDrawFnc = &CMYClass::draw _S1;
return;

case E_ST2:
m_pDrawFnc = &CMYClass::draw _S2;
return;

case E_ST3:
m_pDrawFnc = &CMYClass::draw _S3;
return;

default:
//invalid argument!
return;
}
}

private:
void draw_S1()
{
std::cout<<"dra w_S1"<<std::end l;
}
void draw_S2()
{
std::cout<<"dra w_S2"<<std::end l;
}
void draw_S3()
{
std::cout<<"dra w_S3"<<std::end l;
}

typedef void (CMYClass::*dra w_fnc)();

E_STATE m_eState;
draw_fnc m_pDrawFnc;
};

int main()
{
CMYClass obj;
obj.draw();
obj.changeState (CMYClass::E_ST 2);
obj.draw();
obj.changeState (CMYClass::E_ST 3);
obj.draw();

return 0;
}

[osmium]

Re: c++ function pointer

"Marco" writes:
[color=blue]
> I would like implement a class with a method Draw(),
> .I would like that method Draw() change with another method when the
> state of class change.
> I think to use function pointer to do it, but I don't know how use this
> in a class?
> Can someone do a example?[/color]

I think I was seduced by your talk of a function pointer in my earlier post.
As I read your post literally, and ignoring your speculation, it seems you
simply want something like this.

#include <iostream>

class C
{
public:
C() {state = 0;}
void draw();
void change_state() { state = 1;}
void draw1() {std::cout << "draw1 called\n";}
void draw2() {std::cout << "draw2 called\n";}
private:
int state;
};
//---------------------
void C::draw()
{
if (state == 0)
draw1();
else
draw2();
}
//=============== ==
int main()
{
C c;
c.draw1();
c.change_state( );
c.draw();

std::cin.get();
}

[Marco]

Re: c++ function pointer

Thanks it's right what I need.

[osmium]

Re: c++ function pointer

"osmium" <r124c4u102@com cast.net> wrote in message
news:3sfechFng0 usU1@individual .net...
[color=blue]
> C c;
> c.draw1();[/color]

Oops! Shoule be c.draw();
[color=blue]
> c.change_state( );
> c.draw();[/color]

[Andrej Hristoliubov]

Re: c++ function pointer

?[color=blue]
>
> I can do a example alright. Just give me the example and I'll do it.
>
> V[/color]
Vitya is right again!!!

Do listen to Victor, I neither had seen this in production not
Victor's code. And I know Victor for quite some time now (Vitya is it
25 or 30 years in total is the duration of our acquaintance and
bestfriendship? ).


hi to babulya....

[gary]

Re: c++ function pointer


On Fri, 28 Oct 2005 15:48:27 -0400
HappyHippy <karenman@mail. ru> wrote:
[color=blue]
> Marco wrote:[color=green]
> > Hi,
> >
> > I would like implement a class with a method Draw(),
> > .I would like that method Draw() change with another method when the
> > state of class change.
> > I think to use function pointer to do it, but I don't know how use this
> > in a class?
> > Can someone do a example?
> >[/color]
>
> You can do something like that:
>
> #include <iostream>
>
>
> class CMYClass
> {
> public:
> enum E_STATE
> {
> E_ST1,
> E_ST2,
> E_ST3
> };
> CMYClass():
> m_pDrawFnc(&CMY Class::draw_S1)
> {
> }
>
> void draw()
> {
> (this->*m_pDrawFnc)() ;
> }
>
> void changeState(E_S TATE eState)
> {
> switch(eState)
> {
> case E_ST1:
> m_pDrawFnc = &CMYClass::draw _S1;
> return;
>
> case E_ST2:
> m_pDrawFnc = &CMYClass::draw _S2;
> return;
>
> case E_ST3:
> m_pDrawFnc = &CMYClass::draw _S3;
> return;
>
> default:
> //invalid argument!
> return;
> }
> }
>
> private:
> void draw_S1()
> {
> std::cout<<"dra w_S1"<<std::end l;
> }
> void draw_S2()
> {
> std::cout<<"dra w_S2"<<std::end l;
> }
> void draw_S3()
> {
> std::cout<<"dra w_S3"<<std::end l;
> }
>
> typedef void (CMYClass::*dra w_fnc)();
>
> E_STATE m_eState;
> draw_fnc m_pDrawFnc;
> };
>
> int main()
> {
> CMYClass obj;
> obj.draw();
> obj.changeState (CMYClass::E_ST 2);
> obj.draw();
> obj.changeState (CMYClass::E_ST 3);
> obj.draw();
>
> return 0;
> }[/color]

I think the resolution is still not adequate.
If the collection of states you need is greater than E_STATE, what should you do?
I think using a integer variable or array is simple, every bit of the variable represents a state or simplely the value of the variable represents states