Question

I currently have a class like this:

struct Rgb
{
  static const int NUM_CHANNELS = 3;
  unsigned char channel[NUM_CHANNELS];
};

which is never instantiated but is used to determine the stride of a pointer increment.

I wanted to allow this stride to change based on some runtime information. Will something like this work?

#include <iostream>

class Rgb
{
public:
  static int* Channel;
  static void SetChannelSize(unsigned int channelSize)
  {
    Channel = new int(channelSize);
  }
};

int main()
{

  int numbers[1000];
  for(unsigned int i = 0; i < 1000; i++)
    {
    numbers[i] = i;
    }

  Rgb::SetChannelSize(3);

  Rgb* rgbNnumbers = reinterpret_cast<Rgb*>(numbers);

  for(unsigned int i = 0; i < 5; i++)
    {
    std::cout << rgbNnumbers[i] << std::endl; //error: no match for ‘operator<<’ in ‘std::cout << *(rgbNnumbers + ((unsigned int)i))’

    }

  return 0;
}

(I wasn’t sure how to step over the demo array I setup in this example to test it out).

Any comments?

Answer 1

As [i] is a compile-time operation, it will simply not work for element sizes that are not known at compile-time. You must increment the pointer yourself with arithmetic.

But that being said, most likely you actually know the size of the element at compile-time. You are probably writing specific code for 24-bit and 32-bit (et al) pixel depths, so you should still be able to use brackets to dereference. Just organize your code to use the right class (Rgb32, Rgb565, Rgb24, etc).

Regarding your compiler error, iostream doesn’t know how to convert a Rgb to a string, so you naturally get this error. I would suggest creating a Rgb::ToString() function.