Question

In which way it is possible to compile this code?

buf_right.el(j, k) = block.el(i, j, k);

Answer 1

Contrary to the other answers, it is not necessary for el to return a reference, consider this example:

#include <iostream>

struct A {
  A& operator=(float f) {
    std::cout << "hey: " << f << std::endl;
  }
};

struct BR {
  A el(int j, int k) {
    return A();
  }
  float el(int i, int j, int k) {
    return 4.2;
  }
};

int main() {
  BR buf_right, block;
  int i = 0, j = 0, k = 0;
  buf_right.el(j, k) = block.el(i, j, k);
  return 0;
}

Running this prints the following to stdout:

hey: 4.2