[ALACPP] abstract factories in C

[Josh Dybnis <jdybnis@xxxxxxxxx>]

This is how I would do an abstract factory in C:

    Soldier *MakeSillySoldier(void);
    Monster *MakeSillyMonster(void);
    SuperMonster *MakeSillySuperMonster(void);

    Soldier *MakeBadSoldier(void);
    Monster *MakeBadMonster(void);
    SuperMonster *MakeBadSuperMonster(void);

    typedef struct EnemyFactory
    {
        Soldier *(*MakeSoldier)(void);
        Monster *(*MakeMonster)(void);
        SuperMonster *(*MakeSuperMonster)(void);
    } EnemyFactory;

    EnemyFactory easyLevelEnemyFactory = 
    { MakeSillySoldier, MakeSillyMonster, MakeSillySuperMonster };

    EnemyFactory dieHardLevelEnemyFactory =
    { MakeBadSoldier, MakeBadMonster, MakeBadSuperMonster };

Note that is easy to define, and it is granular, because you can pass
in indivual elements of the struct to different modules.

Unfortunately this does not work as well in C++, because it breaks with
inheritance. The problem is that covariant return types only apply to
method overloading, not type conversion.

So this is fine:

    struct A     { A *foo(); };
    struct B : A { B *foo(); }; // ok, overriding foo with a derived
return type

But this is not:

    A *(*a)() = 0;
    B *(*b)() = 0;

    a = b; // invalid conversion, even though b's return type is
dirived from a's

-Josh Dybnis


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