How do I repass a function pointer in C++ - c++

Question

Firstly, I am very new to function pointers and their horrible syntax so play nice.
I am writing a method to filter all pixels in my bitmap based on a function that I pass in. I have written the method to dereference it and call it in the pixel buffer but I also need a wrapper method in my bitmap class that takes the function pointer and passes it on. How do I do it? What is the syntax? I'm a little stumped.
Here is my code with all the irrelevant bits stripped out and files combined (read all variables initialized filled etc.).
struct sColour
{
unsigned char r, g, b, a;
};
class cPixelBuffer
{
private:
sColour* _pixels;
int _width;
int _height;
int _buffersize;
public:
void FilterAll(sColour (*FilterFunc)(sColour));
};
void cPixelBuffer::FilterAll(sColour (*FilterFunc)(sColour))
{
// fast fast fast hacky FAST
for (int i = 0; i < _buffersize; i++)
{
_pixels[i] = (*FilterFunc)(_pixels[i]);
}
}
class cBitmap
{
private:
cPixelBuffer* _pixels;
public:
inline void cBitmap::Filter(sColour (*FilterFunc)(sColour))
{
//HERE!!
}
};

Answer 1

If I understand what you want:
inline void cBitmap::Filter(sColour (*FilterFunc)(sColour))
{
_pixels->FilterAll( FilterFunc);
}
Often dealing with function pointers can be made easier to read if you use a typedef for the function pointer type (yours actually isn't too bad on its own - they can get much worse very easily):
struct sColour
{
unsigned char r, g, b, a;
};
typedef
sColour (*FilterFunc_t)(sColour); // typedef for a FilterFunc
class cPixelBuffer
{
private:
sColour* _pixels;
int _width;
int _height;
int _buffersize;
public:
void FilterAll(FilterFunc_t FilterFunc);
};
void cPixelBuffer::FilterAll(FilterFunc_t FilterFunc)
{
// fast fast fast hacky FAST
for (int i = 0; i < _buffersize; i++)
{
_pixels[i] = (*FilterFunc)(_pixels[i]);
}
}
class cBitmap
{
private:
cPixelBuffer* _pixels;
public:
inline void cBitmap::Filter(FilterFunc_t FilterFunc)
{
_pixels->FilterAll( FilterFunc);
}
};

Answer 2

The Boost libraries can make your life easier here. see boost function.
For example here is a function that takes a call back function that takes two ints and returns an int:
void do_something( boost::function<int (int, int)> callback_fn );
Then it can be used like a normal function:
int result = callback_fn(1,2);
Pass it to do_something like this:
boost::function<int (int, int)> myfn = &the_actual_fn;
do_something(myfn);
With boost function you can also pass class member functions easily (see boost bind).
Good luck with your program.

Answer 3

You could make things clearer by using a typedef for your function pointer type:
typedef sColour (*FilterFunc_t)(sColour)
void FilterAll(FilterFunc_t FilterFunc);
Passing a variable containing a function pointer to a different function works the same as passing any other variable:
inline void cBitmap::Filter(FilterFunc_t FilterFunc) {
FilterAll(FilterFunc);
}