Replace switch-case's with polymorphism

switch-poly.cpp

// === Before ===

typedef enum {
    Access_Vanilla,
    Access_Special,
    Access_Cache,
    Access_Cache_L2,
    Access_Invalid
} access_t;

string get_name(access_t a) {
    switch (a) {
    case Access_Vanilla:
        return "vanilla";
    case Access_Special:
        return "special";
    case Access_Cache:
        return "cache";
    case Access_Cache_L2:
        return "cache L2";
    case Access_Invalid:
        return "invalid";
    }
}

bool is_cached(access_t a) {
    switch (a) {
    case Access_Vanilla:
    case Access_Invalid:
    case Access_Special:
        return false;
    case Access_Cache:
    case Access_Cache_L2:
        return true;       
    }
}

void trigger_notification(access_t a, void *data) {
    switch (a) {
    case Access_Vanilla:
        notify_vanilla(data);
        break;
    case Access_Cache:
    case Access_Cache_L2:
        notify_cache(data);
        break;
    default:
        throw NotificationException();
    }
}

// =========================================================================
// === After ===

class Access { /* interface */
public:
    virtual string name() = 0; /* These are not functions, but rather pointers to functions! */
    virtual bool is_cached() = 0;
    virtual notify(void *data) = 0;
};
class VanillaAccess: public Access {
    virtual string name() {
        return "vanilla"; // note how `switch` has disappeared
    }
    ...
};

class SpecialAccess: public Access {...};
...
class InvalidAccess: public Access {...};

 // the only remaining switch is hidden here
construct_access_by_tag(type) {
    switch (type) {
        case Access_Vanilla:
            return VanillaAccess();
        case Access_Special:
            return AccessSpecial();
        ...
    }
}

// Anywhere where we want to discriminate on access_t
access_t type = ...;
Access *access = construct_access_by_tag(type);

// The original functions after the refactoring
string get_name(Access *access) {
    return access->name();
}

bool is_cached(Access *a) {
    return a->is_cached();
}

void trigger_notification(Access *access, void *data) {
    access->notify(data);
}