Question

Here is the relevant code from the full listing:

#include "CImg.h"
using namespace cimg_library;

int main() {
  CImg<unsigned char> src("Tulips.jpg");
  int width = src.width();
  int height = src.height();
  int depth = src.depth();

  //New grayscale images.
  CImg<unsigned char> gray1(width,height,depth,1);
  CImg<unsigned char> gray2(width,height,depth,1);

  // ... 

  (src,gray1,gray2).display("RGB to Grayscale");
}

How does the line (src,gray1,gray2).display("RGB to Grayscale"); work? How is the display member function applied to each of the objects in the comma-separated list?

Answer 1

CImg overloads operator, which returns a CImgList object which is a list containing the two CImg objects given as operands. That object also overloads operator, to allow CImg objects to be added to the list.

The expression (src,gray1,gray2) is equivalent to ((src,gray1),gray2). The inner set of parentheses, (src,gray1), create the CImgList, and then (...,gray2) appends gray2 to that list, returning a reference to the same list. CImgList has the member function display.