Question

I’m working on implementing a very, very basic component system in C, but now I am at a point where I want to ‘dynamically’ call some functions. The set-up is very easy: the main program is simply an endless while loop, in which some conditions are checked and in which a “process” function is called for each enabled component.

For example, now it works like this:

while (1) {
  input_process();
  network_process();
  graphics_process();
}

But I want to separate it into separate components, and somehow define in a central place which parts are used. This could be done with simple defines, like so:

#define HAS_NETWORK
...
while (1) {
  input_process();
#ifdef HAS_NETWORK
  network_process();
#endif
  graphics_process();
}

As you can see this is alright for 1 or maybe only a few components, but if I want to do this for all of these (input, network and graphics) and additional components in the future, I would have to put separate #ifdefs in there for each of them, and that’s quite tedious.

In pseudo code, what I’m trying to accomplish is the following:

components = {'input', 'network', 'graphics'}
...
foreach component in components
  execute component_process()

This way components could easily be added in the future.
I don’t really mind if the checking is done compile time or run time (although I obviously prefer compile time, but I can imagine run time is easier to implement). I have no idea how to even start.

Answer 1

At compile time with an X macro :

component.x is a file containing :

COMPONENT( graphic , "3D graphics rendering" )
COMPONENT( network , "Network" )
COMPONENT( other , "usefull stuff" )
#undef COMPONENT

Use it with :

#define COMPONENT( what , description ) what ## _process();
while (1)
{
#include "components.x"
}

And in another place for instance :

std::cout << "We use :\n" ;
#define COMPONENT( what , description )\
std::cout << #what << " : " << description << "\n" ;
#include "components.x"

and with this you can place the HAS_ defines in a single place in component.x :

#ifdef HAS_GRAPHIC
COMPONENT( graphic , "3D graphics rendering" )
#endif
#ifdef HAS_NETWORK
COMPONENT( network , "Network" )
#endif
#ifdef HAS_OTHER
COMPONENT( other , "usefull stuff" )
#endif
#undef COMPONENT