Doomsday Engine

As the title states, `common` makes extreme use of the C/++ preprocessor in order to provide specific modifications for one game or another.

For example, in `hu_lib.h` the following is defined:

#if __JDOOM__ || __JHERETIC__
typedef struct {
    int slot;
    keytype_t keytypeA;
    patchid_t patchId;
#  if __JDOOM__
    keytype_t keytypeB;
    patchid_t patchId2;
#  endif
} guidata_keyslot_t;
#endif

If this such a type is only needed by two (three counting doom64) different plugins, then it should be defined at a plugin level, rather than putting it in the common plugin. On one hand, this will vastly reduce code fragmentation by moving single-plugin-only code in to the appropriate plugin, and on another it provides motivation to use some C++ features (as I understand, I fairly large portion of the code at hand was ported from C to C++, and as such was not written with classes, etc.. in mind).

Not knowing the nature of every usage off the top of my head, I would suggest redesigning this and similar situations as follows:

// in common/include/hu_lib.h
class guidata_keyslot_t {
    // common things here
}

// in doom/include/_pick a name_

class guidata_doom_keyslot : public guidata_keyslot_t {
   // common things here
}

Even furthermore, migrating static helper methods for these types in to such classes as instance methods would drastically reduce the number of symbols in the global namespace, which would help for (among other things) debugging, code clarity, and code sharing.