Pointer to function won't

Point to a function, that is.

I have a class called viewPort, which is a simple 2D raster graphics class. I want there to be several plotting functions (such as overlay, XOR, blend, etc) so I thought I'd use a function pointer.

Here is an abridged code portion

Code:

---

class viewPort
{
.
.
.
public:
  void plot (int x, int y, int col);
  void (*plotH) (int x, int y, int col);
};

.
.
.

viewPort::viewPort (int width, int height, HWND hwnd)
{
.
.
.
plotH = plot;
.
.
.
}

---

The line in bold generates a compile-time error: cannot convert from (__thiscall viewPort::*) (int, int, int) to (__cdecl *) (int, int, int).

I really don't know what's wrong. I've used function pointers before with no problems - but not with class member functions. I looked function pointers up in C++ Primer and it makes no mention of any exceptions when pointing to member functions.

As far as I can see, this is a non-error. It's a pointer in viewPort to a void (int, int, int) and I'm trying to point it to a void (int, int, int) which is also in the viewPort class.

And to add insult to injury, when I try to do what it wants and typecast to (__cdecl *) it says it doesn't recognise __cdecl DESPITE it changing colour when I type it.

I'm totally stuck.

We did this about ten hours ago.

......... Sorry.

Do I need the class:: prefix?

Does adding the &, as mentioned in the thread, work for you?

I'm on my friend's machine at the moment, but I'll try both these things.

I've never had to use the & operator in this situation before (but, like I said, I've never tried it with member functions and I use a notoriously non-standard compiler - MS VC++ 6).

OK so I tried these things and it didn't work.

Now it's:

Code:

---

void (viewPort::*plotH) (int x, int y, DWORD col);
        void plot (int x, int y, DWORD col);

plotH = &viewPort::plot;

---

And for every invocation through the pointer it says "term does not evaluate to a function" - regardless as to whether I use the & operator to assign the pointer.

Then I tried adding "const = 0" then it just said pure virtual specifier can only be used for functions.

This works for me:

Code:

---

class bob {
        bob();
        int x, y, z;
        static int func(int a, int b, int c);
        int (*funcp)(int a, int b, int c);
};

int bob::func(int a, int b, int c) {
        return a*b+c;
}

bob::bob() {
        x = 3;
        y = 4;
        z = 5;
        funcp = &bob::func;
}

---

I believe that there is no such thing as a pointer to a non-static member function, except perhaps using the ever-popular .* or ->* notation, but I'm not finding that in the standard right this minute.

Edit: Alright, it's a pointer to member thing. This is the example from the standard:

Quote:

---

Originally Posted by ISO C++, section 8.3.3

[Example:

Code:

---

class X {
public:
    void f(int);
    int a;
};
class Y;
int X::* pmi = &X::a;
void (X::* pmf)(int) = &X::f;
double X::* pmd;
char Y::* pmc;

---

declares pmi, pmf, pmd and pmc to be a pointer to a member of X of type int, a pointer to a member of X
of type void(int), a pointer to a member of X of type double and a pointer to a member of Y of type
char respectively. The declaration of pmd is well-formed even though X has no members of type
double. Similarly, the declaration of pmc is well-formed even though Y is an incomplete type. pmi and
pmf can be used like this:

Code:

---

X obj;
//...
obj.*pmi = 7; // assign 7 to an integer member of obj
(obj.*pmf)(7); //call a function member of obj with the argument 7

---

—end example]

---

Perhaps try a more up-to-date compiler before we start blaming the compiler and look for workarounds? There are several free ones out there, you know, including VS 2008 Express, a much much more standards compliant compiler.

If it makes you feel better, I was getting the same errors using, oddly enough, VS 2008 Express. So I figured maybe it was a real error.

This works for me:

Code:

---

class bob
{
        bob();
        int x, y, z;
        int func(int a, int b, int c); // Removed static
        int (bob::* funcp)(int a, int b, int c);
};

int bob::func(int a, int b, int c)
{
        return a * b + c;
}

bob::bob()
{
        x = 3;
        y = 4;
        z = 5;
        funcp = &bob::func;
        (this->*funcp)(1, 2, 3);
}

int main()
{

}

---

Quote:

---

Originally Posted by samGwilliam

I have a class called viewPort, which is a simple 2D raster graphics class. I want there to be several plotting functions (such as overlay, XOR, blend, etc) so I thought I'd use a function pointer.

---

Can I make a suggestion: Instead of having a pointer to a function, have a pointer to a class (a "baseplot class", which has pure virtual memberfunction(s) to do the actual plotting) - not only does it make the syntax much easier to handle, but it's also "the correct way to solve this type of problem in an Object Oriented Design).

--
Mats

Thank you. I want to get it to work as a function pointer just on principle more than anything but I like the plot class idea too.

OK, so Elysia's solution worked but I've got to say the anality of the syntax seems unnecessary to me.

I'm sure Stroustroup had his reasons though...

There's another problem now.

I've fixed the problem with the viewPort class, now I want a derived class (called vSpec) to be able to provide its own plotting method.

Code:

---

plotH = &vSpec::aPlot;

---

This generates:

Code:

---

error C2440: '=' : cannot convert from 'void (__thiscall vSpec::*)(int,int,unsigned long)' to 'void (__thiscall viewPort::*)(int,int,unsigned long)'

---

Which is understandable, so I tried to typecast with:

Code:

---

plotH = (viewPort::*) &vSpec::aPlot;

---

And it then says:

Code:

---

error C2059: syntax error : '<tag>::*'

---

To do what I want must be possible right? Am I typecasting wrongly?

Quote:

---

Originally Posted by samGwilliam

To do what I want must be possible right?

---

It depends on what you mean by "what I want". If you mean make this function pointer point to that function, then the answer is no, you can't; a member function of viewPort and a member function of vSpec are not compatible types. That is, there is no way to make their parameter list match, since the implicit this parameters are of different types.

If you mean override this function call, then that's what virtual functions are born to do.

Well that's just stupid isn't it?

It's a derived class, so the base classes members are also the derived classes members. The classes are related somehow and the signatures are the same.

Guess I'll just have to abandon the project.

Quote:

---

Originally Posted by samGwilliam

It's a derived class, so the base classes members are also the derived classes members.

---

This sentence makes no sense whatsoever. Why would you think a viewPort is also a vSpec? (And for that matter, it doesn't work the other way around either: a vSpec isn't a viewPort -- it may be built on top of a viewPort, yes, but they are not actually the same thing.) If you mean that a vSpec contains all the same things as a viewPort, yes that's true, but the whole point of vSpec::aPlot is that it can access all the new stuff in vSpec and thus claiming that it should somehow be the same as a viewPort:: function is just silly. My understanding is that you think this should work, somehow:

Code:

---

class viewPort {
//... stuff ...
void aPlot(int, int, unsigned long);
};

class vSpec : public viewPort {
//... stuff ...
void aPlot(int, int, unsigned long);
};

viewPort vpobject;
void (viewPort::*funcp)(int, int, unsigned long);
funcp = dynamic_cast<viewPort::*>(&vSpec::aPlot); //this is the line you want to use (for some appropriate cast)
vpobject.*funcp(2,3,4L); //what the heck is this going to DO?

---

If this isn't what you want, well I don't know what you want then, because there's no other reason to have a pointer to member function.

Quote:

---

Originally Posted by samGwilliam

The classes are related somehow

---

True...

Quote:

---

Originally Posted by samGwilliam

and the signatures are the same.

---

And back to madness.

Quote:

---

Originally Posted by samGwilliam

Guess I'll just have to abandon the project.

---

My suggestion would be to temporarily put the project on hold and read a book, or at least a couple of chapters.

Well it allowed: viewPort *v = new vSpec; so there is some degree of interchangabliliy. It makes perfect sense to be able to do what I want to do and I think it's crappy that it's not allowed.

Quote:

---

Originally Posted by samGwilliam

Well it allowed: viewPort *v = new vSpec; so there is some degree of interchangabliliy. It makes perfect sense to be able to do what I want to do and I think it's crappy that it's not allowed.

---

There is:

Code:

---

class viewPort {
//... stuff ...
virtual void aPlot(int, int, unsigned long);
};

class vSpec : public viewPort {
//... stuff ...
void aPlot(int, int, unsigned long);
};

viewPort * vpptr = new vSpec();
vpptr->aPlot(2,3,4L);

---

Notice that you don't have a pointer to a function, you have a pointer to the object, which is the key, since the object itself knows what kind it is, and the compiler doesn't as you're lying to the compiler about what class vpptr points to.

Again, my suggestion is to read the chapters about polymorphism.

But vSpec needs the proper plot function too. I just wanted to be able to extend the repertoire of plot methods in the derived class.

Quote:

---

Originally Posted by samGwilliam

But vSpec needs the proper plot function too. I just wanted to be able to extend the repertoire of plot methods in the derived class.

---

It has it. (Assuming by "proper plot function" you mean it's own.)

Another solution:

Code:

---

class viewPort
{
.
.
.
public:
  static void plot (viewPort* self, int x, int y, int col);
  void (*plotH) (viewPort* self, int x, int y, int col);
};

.
.
.

viewPort::viewPort (int width, int height, HWND hwnd)
{
.
.
.
plotH = &plot;
.
.
.
}

---

When you declare a function in a class (f(x)) it's really (f(self, x)). where self is a pointer in memory of the data to act upon.

In simple terms 'static' say's leave as is, unfortunately if you want to act on data upon the memory location, you'll need to supply it yourself. I think there is also another way but I don't remember. There is a strategy paradigm, I know little about though.

When calling within the viewPort class

Code:

---

void viewPort::somefunction(){
    plotH(this, x, y, col);
}

---

When calling from somewhere else

Code:

---

int main(){
    viewPort view;
    viewPort *pview;
    pview = &view;
    // initialize and stuff
    view.plotH(&view, x, y, col);
    // or if you have a pointer
    pview->plotH(pview, x, y, col);
    // ...
}

---

The MORE I read the this, the MORE I think my suggestion is the right one - don't use function pointers, use virtual functions in a class. Not only does it simplify the syntax of the whole lot, but it will "magically work" without much effort, instead of trying to make something that work that doesn't.

Of course, Benzakhar's solution will work - you can pass a pointer to the object, and then call a static function. But this is basically just "doing your own virtual functions" - you are reinventing the wheel, except your wheel is a octagon in approximation of a circle, so your ride will be a bit bumpy.

--
Mats

I agree with mats - polymorphism should do fine. Either static or dynamic.
Another "way" around your problem is also templates, because it can be any type, you can pass "any" member function pointer. But I don't think your problem requires this. It can be a more advanced solution.