It's really hard to tell without knowing every detail of your specification but you could do something like code below. Keep in mind that this is in C and it's just a mock-up;

struct Item {
    some_var_for_item_type; // enum ,enum class or something else
    void * data;
    ItemInfo * item_info;
};

struct ItemInfo {
    char * name; 
    unsigned int usable : 1;
    unsigned int breakable : 1;
    unsigned int stackable : 1;

};

struct GunData {
    GunInfo * gun_info;
    int cur_ammo;
    int attachment_list[max_attachments];
    ...
};

    struct GunInfo {
    int fire_rate;
    int max_ammo;
};

struct FoodData {
    FoodInfo * ...
    int stacks;
    int cur_nutrition_value;
};

Then you create an array for each of those infos you need ,and w/e else (vector ,array ,...) for all the actual data. eg. two ak-47s have the same pointer to item_info and the same data->gun_info but different cur_ammo and attachment_list