C / C++ / MFC

8-Oct-01 21:30

Hello All,

Forgive me if this isn't the right place to post this, but I'm new to this community.

As the subject reads, I would like to pass a ponter to a function as an argument to another function. I'm pretty comfortable with that, but here's the catch: the function is a an method of an object.

Is this possible?

Any suggestions pointning me in the right direction would be apreciated.

Thanks in advance,
mark

Andrew Torrance8-Oct-01 21:53

Andrew Torrance

8-Oct-01 21:53

Unless its a static function , you will get errors as there is no this pointer . Therefore you either pass an object , or pass a pointer to a static function.

Steen Krogsgaard8-Oct-01 23:08

8-Oct-01 23:08

The syntax for at typedef of a pointer to a member is:
(a member of CMyClass returning an int and taking a long and a void* as arguments, just an example)

typedef int (CMyClass::* LPFNMYCLASSMEMBER)(long, void*);
...

//your function prototype:
bool YourFunction(LPFNMYCLASSMEMBER pfnMember)
{
   ...
   // get a pointer to the object on which you will call pfnMember
   CMyClass* pClass = something;
   // make the call
   int iRetVal = pClass->*pfnMember(aLongParam, aVoidPtrParam);
   ...
}

As I have shown you will have to have a valid object on which you can call the member function (unless it's a static member function which has no this pointer). So you cannot use non-static member functions as callbacks, for instance.

Cheers
Steen.

"To claim that computer games influence children is rediculous. If Pacman had influenced children born in the 80'ies we would see a lot of youngsters running around in dark rooms eating pills while listening to monotonous music"

mightymark9-Oct-01 19:25

9-Oct-01 19:25

Thank you Steen (and Andrew).

I'll let you know how it works out when I try to implement it.
However, could you pleas explain what you mean about "...non-static member functions as callbacks?"

thx,
mark

Steen Krogsgaard10-Oct-01 1:33

10-Oct-01 1:33

A good example is for timers (this is not what you want to do, but it illustrates the problem).

class CTestClass
{
   public:
      int ANonStaticMember(void* pData);
      static int AStaticMember(void* pData);

      CString m_csNonStaticData;
      static CString m_csStaticData;
}

CString CTestClass::m_csStaticData = ""; // initialization of static data

int CTestClass::ANonStaticMember (void* pData)
{
   // here we can access non-static data member
   m_csNonStaticData = *(CString*)pData;
   // and we can access static data member
   m_csStaticData = *(CString*)pData;
}

int CTestClass::AStaticMember (void* pData)
{
   // here we can only access static data member
   m_csStaticData = *(CString*)pData;
   // it would be an error to access the non-static member
}

void Foo()
{
   // call SetTimer with static member function as callback is OK
   SetTimer(NULL, 1000, &CTestClass::AStaticMember);
   // calll SetTimer with non-static member as callback is error
   SetTimer(NULL, 1000, &CTetsClass::ANonStaticMember); // ERROR
}

The above code will probably not compile, but I hope you get the idea. The key issue here is that a non-static member function is passed a pointer to the object on which it should act (the this pointer) as it first (and hidden) parameter. The function using the callback function will have to know what object to call the function on to be useful.

But I guess, if you pass a pointer to a non-static member function as a callback to a function you write yourself, and if you in that function calls the callback function on a valid object, it could be OK. Didn't think of that.

Frown | :(

Cheers
Steen.

"To claim that computer games influence children is rediculous. If Pacman had influenced children born in the 80'ies we would see a lot of youngsters running around in dark rooms eating pills while listening to monotonous music"

mightymark10-Oct-01 20:18

10-Oct-01 20:18

Thanks for the example-that makes sense.

"...and if you in that function calls the callback function on a valid object, it could be OK. Didn't think of that."

This is probably what I will do for what I'm working on, but it is very good to know that non-static calback wouldn't otherwise work.

--mark

mightymark9-Oct-01 20:27

9-Oct-01 20:27

Might you know why I'm getting an error message like:"term does not evaluate to a function?"

typedef void (BinarySearchTree::*BSTmemptr) (const &int);

main()
{
BinarySearchTree<int,string> t(error); //intitializes a BST
BSTmemptr fnc; //declare pointer to member funct

fnc=BinarySearchTree::remove; //assign funct to pointer
t.*fnc(9); //attempt to call funct--error is for this line
//....
//....
}

thanx again.

Steen Krogsgaard10-Oct-01 1:23

10-Oct-01 1:23

Operator preceedence. Evaluation of the function operator "()" goes before evaluation of ".*". Put t.*fnc in paranthesis and everything works:

fnc=BinarySearchTree::remove;
(t*.fnc)();

For reference, search VC++ docs at MSDN Lib for "pointer to member", see the article "Expressions with Pointer-to-Member Operators".

Cheers
Steen.

"To claim that computer games influence children is rediculous. If Pacman had influenced children born in the 80'ies we would see a lot of youngsters running around in dark rooms eating pills while listening to monotonous music"

mightymark10-Oct-01 20:21

10-Oct-01 20:21

Once again, you're right.

Thank you for your patience--it's is sincerely appreciated. I'll also refer to the article.

How long have you been coding for?

Steen Krogsgaard10-Oct-01 22:31