MatthiasWM left a comment (fltk/fltk#1597)

@MatthiasWM Please, explain how to use the new code to create a timeout callback with several arguments transmitted as captures of a lambda, if that is indeed possible.

I created PR #1600 to test this out. It's not fully implemented, but good enough for testing. So here is a silly code change for test/button.cxx that shows a timeout lambda inside a callback lambda with captures:

  Fl_Button *b2 = new Fl_Button(120,20, 80, 25, "&no op");
  b2->callback(
    [b1, b2]() {
      Fl::add_timeout(1.0, [b1, b2]() {
        fl_message("The label of b1 is currently '%s'", b1->label());
        b2->deactivate();
      });
    }
  );

I think the code is very readable. Better than spreading it across multiple callback functions and global variables.

An alternative syntax that can capture as well and avoids creating a lambda is std::bind(), (here we capture b1) for example:

      Fl::add_timeout(
        1.0,
        std::bind(
          fl_message,
          "The label of b1 is currently '%s'",
          b1->label())
      );

This can also bind C++ member functions if needed.

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