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Recursive inheritance with variable patterns

Consider the following code:

#include <iostream>

struct ActionOption {
    virtual void foo(int) const = 0;
};

template <int> struct ActionType;

template <> struct ActionType<0> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<0>::foo(int) called.\n";}
};

template <> struct ActionType<1> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<1>::foo(int) called.\n";} 
};

template <> struct ActionType<2> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<2>::foo(int) called.\n";}
};

template <> struct ActionType<3> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<3>::foo(int) called.\n";}
};

template <> struct ActionType<4> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<4>::foo(int) called.\n";}
};

template <int...> struct PossibleActions;

template <> struct PossibleActions<> { void operator()(int) const {} };

template <int First, int... Rest>
struct PossibleActions<First, Rest...> : ActionType<First>, PossibleActions<Rest...> {
    void operator()(int a) const {
        ActionType<First>::foo(a);
        PossibleActions<Rest...>::operator()(a);
    }
};

// Anything that can call ActionType<2>::foo(int) can also call ActionType<3>::foo(int).
struct Object : PossibleActions<1,  2,3,  4> {
    void foo(int a) {PossibleActions<1,2,3,4>()(a);}
};

struct Blob : PossibleActions<0,  2,3,  4> {
    void foo(int a) {PossibleActions<0,2,3,4>()(a);}
};

int main() {
    Object object;
    object.foo(12);  // ActionType<1>::foo(int) called  ActionType<2>::foo(int) called   ActionType<3>::foo(int) called  ActionType<4>::foo(int) called
    std::cout << std::endl;

    Blob blob;
    blob.foo(12);  // ActionType<0>::foo(int) called  ActionType<2>::foo(int) called   ActionType<3>::foo(int) called  ActionType<4>::foo(int) called
    std::cout << std::endl;
}

It is executed, except for the problem: everything that can cause ActionType<2>::foo(int)can also cause ActionType<3>::foo(int). That way, every time I define a new class, if I use 2 or 3, I have to use both in PossibleActions<I...>. This is problematic for maintaining the course (let's say I decide in the future that using 2 should also use 3, 7, and 20). Next solution:

using TwoAndThree = PossibleActions<2,3>;
struct Object : PossibleActions<1,4>, TwoAndThree {
    void foo(int a) {PossibleActions<1,4>()(a);  TwoAndThree()(a);}
};

struct Blob : PossibleActions<0,4>, TwoAndThree {
    void foo(int a) {PossibleActions<0,4>()(a);  TwoAndThree()(a);}
};

unacceptable because I need to ActionType<N>::foo(int)be called in numerical order . Sharing PossibleActions<1,4>()(a);is a very bad solution because it faces the same support problem (makes maintenance even worse, I think).

template <> struct ActionType<2> : ActionOption { virtual void foo(int) const override {std::cout << "ActionType<2>::foo(int) called.\n";} };
template <> struct ActionType<3> : ActionType<2> { virtual void foo(int) const override {std::cout << "ActionType<3>::foo(int) called.\n";} };

- ( ), . ?

, template <typename... Args> struct PossibleActions;? .

?

: Args... int, ( , ints, )?

PossibleActions<1, TwoAndThree, 4, EightAndTen, 20>()(a);

1,2,3,4,8,10,20, TwoAndThree = PossibleActions<2,3> EightAndTen = PossibleActions<8,10>??? , .

+4
1

. S ( , , - ). , , .

template <typename, typename...> struct Sort;

template <typename T, typename A, typename B>
struct Sort<T,A,B> {
    using type = typename Merge<T,A,B>::type;
};

template <typename T, typename First, typename Second, typename... Rest>
struct Sort<T, First, Second, Rest...> {
    using type = typename Sort<T, typename Sort<T, First, Second>::type, Rest...>::type;
};

.

struct Widget : Sort<PossibleActions<0,5>, OneAndFour, TwoAndThree>

. :

#include <iostream>

namespace Detail {
    template <typename T, typename, typename, T...> struct Merge;

    template <typename T, template <T...> class S, T... Ks>
    struct Merge<T, S<>, S<>, Ks...> {
        using type = S<Ks...>;
    };

    template <typename T, template <T...> class S, T... Is, T... Ks>
    struct Merge<T, S<Is...>, S<>, Ks...> {
        using type = S<Ks..., Is...>;
    };

    template <typename T, template <T...> class S, T... Js, T... Ks>
    struct Merge<T, S<>, S<Js...>, Ks...> {
        using type = S<Ks..., Js...>;
    };

    template <typename T, bool, typename, typename, T...> struct Strip;

    template <typename T, template <T...> class S, T I, T... Is, T J, T... Js, T... Ks>
    struct Strip<T, true, S<I, Is...>, S<J, Js...>, Ks...> {
        using type = Merge<T, S<I, Is...>, S<Js...>, Ks..., J>;
    };

    template <typename T, template <T...> class S, T I, T... Is, T J, T... Js, T... Ks>
    struct Strip<T, false, S<I, Is...>, S<J, Js...>, Ks...> {
        using type = Merge<T, S<Is...>, S<J, Js...>, Ks..., I>;
    };

    template <typename T, template <T...> class S, T I, T... Is, T J, T... Js, T... Ks>
    struct Merge<T, S<I, Is...>, S<J, Js...>, Ks...> : Strip<T, (I > J), S<I, Is...>, S<J, Js...>, Ks...>::type {};

    template <typename, typename...> struct Sort;

    template <typename T, typename A, typename B>
    struct Sort<T,A,B> {
        using type = typename Merge<T,A,B>::type;
    };

    // Piotr S. Sort generalized to accept any number of template arguments.
    template <typename T, typename First, typename Second, typename... Rest>
    struct Sort<T, First, Second, Rest...> {
        using type = typename Sort<T, typename Sort<T, First, Second>::type, Rest...>::type;
    };
}

template <typename... P>
using Sort = typename Detail::Sort<int, P...>::type;

struct ActionOption {
    virtual void foo(int) const = 0;
};

template <int> struct ActionType;

template <> struct ActionType<0> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<0>::foo(int) called.\n";}
};

template <> struct ActionType<1> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<1>::foo(int) called.\n";} 
};

template <> struct ActionType<2> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<2>::foo(int) called.\n";}
};

template <> struct ActionType<3> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<3>::foo(int) called.\n";}
};

template <> struct ActionType<4> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<4>::foo(int) called.\n";}
};

template <> struct ActionType<5> : ActionOption {
    virtual void foo(int) const override {std::cout << "ActionType<5>::foo(int) called.\n";}
};

template <int...> struct PossibleActions;

template <> struct PossibleActions<> { void operator()(int) const {} };

template <int First, int... Rest>
struct PossibleActions<First, Rest...> : ActionType<First>, PossibleActions<Rest...> {
    void operator()(int a) const {
        ActionType<First>::foo(a);
        PossibleActions<Rest...>::operator()(a);
    }
};

using OneAndFour = PossibleActions<1,4>;
using TwoAndThree = PossibleActions<2,3>;

struct Thing : PossibleActions<0,1,2,3,4> {
    void foo(int a) {PossibleActions<0,1,2,3,4>::operator()(a);}
};

struct Object : Sort<PossibleActions<1,4>, TwoAndThree> {
    void foo(int a) {Sort<PossibleActions<1,4>, TwoAndThree>()(a);}
};

struct Blob : Sort<PossibleActions<0,4>, TwoAndThree> {
    void foo(int a) {Sort<PossibleActions<0,4>, TwoAndThree>()(a);}
};

struct Widget : Sort<PossibleActions<0,5>, OneAndFour, TwoAndThree> {
    void foo(int a) {Sort<PossibleActions<0,5>, OneAndFour, TwoAndThree>()(a);} 
};

int main() {
    Thing thing;
    thing.foo(12);  // ActionType<0>::foo(int)  ActionType<1>::foo(int) called  ActionType<2>::foo(int) called   ActionType<3>::foo(int) called  ActionType<4>::foo(int) called
    std::cout << std::endl;

    Object object;
    object.foo(12);  // ActionType<1>::foo(int) called  ActionType<2>::foo(int) called   ActionType<3>::foo(int) called   ActionType<4>::foo(int) called
    std::cout << std::endl;

    Blob blob;
    blob.foo(12);  // ActionType<0>::foo(int) called  ActionType<2>::foo(int) called   ActionType<3>::foo(int) called   ActionType<4>::foo(int) called
    std::cout << std::endl;

    Widget widget;
    widget.foo(12);  // ActionType<0>::foo(int) called  ActionType<1>::foo(int) called   ActionType<2>::foo(int) called   ActionType<3>::foo(int) called   ActionType<4>::foo(int) called   ActionType<5>::foo(int) called
}

, , , . , , , .

0

Source: https://habr.com/ru/post/1569828/

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