C / C++ / MFC

23-Nov-01 5:00

Hi, all. Below is a program i wrote for test. it contains a class named CBase.

#include <stdio.h>

class CBase{
public:
virtual void VFunction()
{
printf("Hello, VFunction.\n");
}
int _a;
};

int main()
{
CBase obj;
int** ppv;

ppv = (int**)&obj;

void (*pFunc)();
// Now i want pFunc to be assigned to the value of **ppv,
// but it can't be like pFunc = **ppv, right?

return 0;
}

so, what can i do for that? Thanks. Frown | :(

Michael Dunn23-Nov-01 8:04

23-Nov-01 8:04

What are you trying to test? The code doesn't make any sense. Once you do ppv = (int**)&obj;, nothing involving ppv will work right because &obj is not of type int**. The cast forces the assignment to compile, but trying to dereference ppv will cause all sorts of havoc.

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Chen Jiadong23-Nov-01 18:34

23-Nov-01 18:34

since ppv=(int**)&obj;, ppv point to the begin of obj object, cos' the obj contains a virtual function, it contains a virtual ptr(__vptr), and i'm using vc6, the microsoft complier set the vptr at the begin of the object, so ppv is pointed to the vptr when finishing the assignment.
The memory just like below:

----------
| vptr |
----------
|int _a |
----------
CBase obj;
(each column is 4 bytes)
Smile | :)

Michael Dunn23-Nov-01 20:36

23-Nov-01 20:36

I still don't understand what you're trying to test.

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Nish Nishant23-Nov-01 21:00

Nish Nishant

23-Nov-01 21:00

Michael Dunn wrote:
I still don't understand what you're trying to test.

You are not the only one Michael, you are not the only one.

Nish

Sonork ID 100.9786 voidmain

www.busterboy.org

Nish is a BIG fan of Goran Ivanisevic

Chen Jiadong24-Nov-01 0:03

24-Nov-01 0:03

the main purpose of my program is to call the virtual function by using the ppv pointer. the ppv is the virtual pointer points to the virtual table, and the virtual table contains the address(es) of the virtual function(s) in the class, so the *ppv is pointed to the virtual table, and the **ppv[0] points to the first virtual function address, **ppv[1] points to the second virtual function address.
Cool | :cool:

Michael Dunn24-Nov-01 8:59

24-Nov-01 8:59

That's not how you call virtual functions. You use a pointer to a C++ object, and call methods thru that pointer.

class CBase
{
public:
  virtual void VFunction()
    {
    printf("Hello, VFunction.\n");
    }
 
  int _a;
};
 
int main()
{
CBase obj, *pobj = &obj;
 
  pobj->VFunction();
}

However none of this makes a difference because without a hierarchy of derived classes, you're not using polymorphism, which is what virtual functions are for.

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Chen Jiadong24-Nov-01 16:27

24-Nov-01 16:27

Thank you very much. i knew this way of calling the virtual function. but now i want to implement that by another way. and i know it really works. i just don't know how to do the last step -- if you got the function address, how to assign it to a function pointer, just like:
void* address; // or int* address;
void (*pFunc)();
...
pFunc = address; // address contains a function address. but pFunc cannot be assign like this expression.
Roll eyes | :rolleyes:

Michael Dunn24-Nov-01 18:48

24-Nov-01 18:48

Oh, OK, you just want a pointer to point at a member function.

class CBase
{
public:
  void func()
    {
    printf("Hello, VFunction.\n");
    }
};
 
int main()
{
CBase obj, *pobj = &obj;
void (CBase::*pBaseMethod)() = CBase::func;  // pBaseMethod now points at the member function.
 
  obj.*pBaseMethod();  // .* for plain objects
  pobj->*pBaseMethod();  // ->* for pointers to objects.
}

The .* or ->* operator dereferences the function pointer.

--Mike--
http://home.inreach.com/mdunn/
While I can't be correct on all matters, I can make the reassuring claim that where I am inaccurate, I am at least definitively inaccurate.
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Chen Jiadong24-Nov-01 23:01