CALLBACK Phenomenom

Hi All!

I wish to know what exactly are those callback functions. I've seen so much of it, esp. with Tree/ListViews. Please explain it in lamens terms.

Will be very much appreciated.
Thank you.

Yours,
Anonymous.

If you are familiar with function pointers, that is all CALLBACK functions are. If you are not, then you need to be if you are going to use them.

A CALLBACK function is a function with particular parmeters that you implement and pass the address to the function that needs the CALLBACK function.

For example

The following code uses a CALLBACK function of type FONTENUMPROC, and it calls this function until it returns a 0 or there are no more fonts that fit the particular catagory. This is a typical way of enumerating the fonts on a computer.

int CSomeClass::EnumerateFonts()
{
LOGFONT lf;
FONTENUMPROC pCallback;

HDC hDC = ::GetDC(NULL);
memset(&lf, 0x0, sizeof(LOGFONT));
lf.lfCharSet = DEFAULT_CHARSET;
pCallback = ::EnumFontsCallBack;

if ( !::EnumFontFamiliesEx(hDC, &lf, pCallback, (LPARAM) this, (DWORD) 0) )
{
return 0;
}

m_IsEnumerated = true;
// Set default font
CString str;

str.LoadString(IDS_DEFAULT_FONT);
GetFontIndex(str);

return 1;
}

static int AFXAPI EnumFontsCallBack(const LOGFONT *pLF, const TEXTMETRIC *pTM, DWORD fontType, LPARAM lParam)
{
// Do not support UNICODE
if ( pLF->lfCharSet > SYMBOL_CHARSET )
{
return 1;
}

...
Do Something with the font
...

return 1;
}




Wayne

I would like to understand ueses for this callback functions. Why would someone use a callback function? Is there a substitute method
to use other than a callback function. May be the answer to this question help me understand callbacks.
Thank you

I am not very good at explaining but here goes.

Don't let the word CALLBACK confuse you. If you see the word just think function pointer. It is a windows defined term, that's all. As a matter of fact it is just defined as __stdcall in the windows header files.
If you are going to use a windows function that requires you to have a CALLBACK function, there is no workaround. The previous example is a good one to break apart. I call a function called ::EnumFontFamiles where in the third parameter it expects a CALLBACK function of type FONTENUMPROC. This means you would have to write a function that is of type FONTENUMPROC and pass the address to this function. The reason why you do this is because the ::EnumFontFamiles function will keep calling your function pointer until you send back a zero or it runs out of fonts. So in your function you can do whatever you want to with the fonts that come in. Put them in a list box or whatever.
It sounds to me you do not know why somebody would use function pointers. If this is not the case then disregard the last section.
I think I explain better with examples so bear with me.

The way I learned function pointers is in the following snipet of code:


#include <iostream.h>

// This function multiplies two integers
int Multiply(int x, int y)
{
return x * y;
}

// This function adds two integers
int Add(int x, int y)
{
return x + y;
}

// This function subtracts two integers
int Subtract(int x, int y)
{
return x - y;
}

// This is a function pointer that takes two integers
// and returns an int
int (*DoOperation) (int x, int y);

// I could also do this
//#define CALLBACK
//CALLBACK int (*DoOperation) (int x, int y);

void main(int argc, char *argv[])
{
int x = 5, y = 2;

// pass the address of the function Multiply
DoOperation = Multiply;
cout << DoOperation(x, y) << endl;

// pass the address of the function Add
DoOperation = Add;
cout << DoOperation(x, y) << endl;

// pass the address of the function Subtract
DoOperation = Subtract;
cout << DoOperation(x, y) << endl;
}




As you can see function pointers are extremely powerful. You can see that this is limitless. I hope this helps a little bit. The best way and I think the only way is to type away. It is one thing when somebody tells you how to do something, but another when you figure it out yourself.

Wayne