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How to generalize a tree structure with variant/visitor

由 天大地大妈咪最大 提交于 2020-01-01 12:09:08

问题


This is a part 2 of my question, originally posted here. Thanks to @sehe for clarifications and help. I ended up with the code that follows, but I can't figure out how can I reduce this thing to a generic solution with variant and visitor. Help/advise is greatly appreciated. Thanks.

#include "stdafx.h"
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <boost/format.hpp>
#include <boost/variant.hpp>


template <typename T> class A 
{
public:
    typename T L;
    typename std::shared_ptr<T> Lptr;
    using tlist = std::vector<std::shared_ptr<T>>;
    A(std::string n = "") : _n(n){}
    A(const A& another) : _n(another._n){};
    A(A&& a) : _n(a._n){ _lst = std::move(another._lst); }
    tlist &lst() { return _lst; }
    void emplace_back(std::shared_ptr<T> wp) {
        _lst.emplace_back(wp);
    }
    std::string n() const { return _n; }
private:
    tlist _lst;
    std::string _n;
};

/*
suppose I have following tree structure
    Store
        Shelves
            Products on the shelve
*/
using lA = A<boost::blank>; // product
using lB = A<lA>;           // shelf
using lC = A<lB>;           // store
using lAp = std::shared_ptr<lA>;
using lBp = std::shared_ptr<lB>;
using lCp = std::shared_ptr<lC>;


void printIt(lAp p, int indent){
    for (int i = 0; i < indent; ++i)
        std::cout << '\t';
    std::cout << p->n() << std::endl;
}


void printIt(lBp p, int indent){
    for (int i = 0; i < indent; ++i)
        std::cout << '\t';
    std::cout << p->n() << std::endl;;
    std::for_each(begin(p->lst()), end(p->lst()), [&](lAp i){
        printIt(i, indent + 1); }
    );
}


void printIt(lCp p, int indent){
    for (int i = 0; i < indent; ++i)
        std::cout << '\t';
    std::cout << p->n() << std::endl;
    std::for_each ( begin(p->lst()), end(p->lst()), [&](lBp i)
    {
        printIt(i, indent + 1);
    });
}


int main() {
    using storage = boost::variant<lAp, lBp, lCp>;
    std::vector<lCp> stores;
    for (int s = 0; s < 5; ++s) {
        lCp store(new lC((boost::format("store %1%") % s).str()));
        stores.emplace_back(store);
        for (int i = 0; i < 3; ++i) {// ten shelves in the store
            lBp shelf(new lB((boost::format("shelf %1%") % i).str()));
            store->emplace_back(shelf);
            for (int j = 0; j < 2; ++j) // twenty producs per shelf
                shelf->emplace_back(std::shared_ptr<lA>(new lA((boost::format("product %1%") % j).str())));
        }
    }
    std::for_each(begin(stores), end(stores), [](lCp p){printIt(p,0); });
    int i;
    std::cin >> i;
}

回答1:


  1. KISS first

    I'm not sure what the goal is with all the polymorphism, both static and dynamic. I'd say if your type structure is fixed like that, just use:

    Live On Coliru

    #include <iostream>
#include <algorithm>
#include <string>
#include <vector>

namespace SimpleDomain {

    struct Product {
        std::string name;
    };

    struct Shelf {
        std::string name;
        std::vector<Product> _products;
    };

    struct Store {
        std::string name;
        std::vector<Shelf> _shelves;
    };

    std::ostream& operator<<(std::ostream& os, Product const& p) {
        return os << "\t\t" << p.name << "\n";
    }
    std::ostream& operator<<(std::ostream& os, Shelf const& s) {
        os << "\t" << s.name << "\n";
        for (auto& p : s._products) os << p;
        return os;
    }
    std::ostream& operator<<(std::ostream& os, Store const& s) {
        os << s.name << "\n";
        for (auto& sh : s._shelves) os << sh;
        return os;
    }
}

int main() {
    std::vector<SimpleDomain::Store> stores = {
        { "store 1", {
                { "shelf 1", { { "product 1" }, { "product 2" }, { "product 3" }, } },
                { "shelf 2", { { "product 4" }, { "product 5" }, { "product 6" }, } },
             },
        },
        { "store 2", {
                { "shelf 1", { { "product 7" }, { "product 8" }, { "product 9" }, } },
                { "shelf 2", { { "product 10" }, { "product 11" }, { "product 12" }, } },
             },
        }
    };

    std::for_each(begin(stores), end(stores), 
            [](SimpleDomain::Store const& p){std::cout << p;});
}

    Prints

    store 1
    shelf 1
        product 1
        product 2
        product 3
    shelf 2
        product 4
        product 5
        product 6
store 2
    shelf 1
        product 7
        product 8
        product 9
    shelf 2
        product 10
        product 11
        product 12

  2. Full Genericity, No Dynamic Polymorphism:

    Here you could use a recursive variant to be more generic:

    Live On Coliru

    #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <boost/variant.hpp>

namespace GenericDomain {

    namespace Tag {
        struct Store{};
        struct Shelf{};
        struct Product{};
    }

    template <typename Kind> struct Node;

    using Store   = Node<Tag::Store>;
    using Shelf   = Node<Tag::Shelf>;
    using Product = Node<Tag::Product>;

    using Tree = boost::variant<
        boost::recursive_wrapper<Product>,
        boost::recursive_wrapper<Store>,
        boost::recursive_wrapper<Shelf>
    >;

    template <typename Kind> struct Node {
        std::string name;
        std::vector<Tree> children;
    };

    template <> struct Node<Tag::Product> {
        std::string name;
    };

    std::ostream& operator<<(std::ostream& os, Tag::Store)   { return os << "Store";   }
    std::ostream& operator<<(std::ostream& os, Tag::Shelf)   { return os << "\tShelf";   }
    std::ostream& operator<<(std::ostream& os, Tag::Product) { return os << "\t\tProduct"; }

    template <typename Kind> std::ostream& operator<<(std::ostream& os, Node<Kind> const& n) {
        os << Kind{} << ": " << n.name << "\n";
        for (auto& child : n.children) os << child;
        return os;
    }
    std::ostream& operator<<(std::ostream& os, Product const& p) {
        return os << Tag::Product{} << ": " << p.name << "\n";
    }
}

int main() {
    using namespace GenericDomain;
    std::vector<Store> stores = {
        Store { "store 1", {
                Shelf { "shelf 1", { Product { "product 1" },  Product { "product 2" },  Product { "product 3" }, } },
                Shelf { "shelf 2", { Product { "product 4" },  Product { "product 5" },  Product { "product 6" }, } },
             },
        },
        Store { "store 2", {
                Shelf { "shelf 1", { Product { "product 7" },  Product { "product 8" },  Product { "product 9" }, } },
                Shelf { "shelf 2", { Product { "product 10" }, Product { "product 11" }, Product { "product 12" }, } },
             },
        }
    };

    std::for_each(begin(stores), end(stores), 
            [](GenericDomain::Store const& p){std::cout << p;});
}

    Prints

    Store: store 1
    Shelf: shelf 1
        Product: product 1
        Product: product 2
        Product: product 3
    Shelf: shelf 2
        Product: product 4
        Product: product 5
        Product: product 6
Store: store 2
    Shelf: shelf 1
        Product: product 7
        Product: product 8
        Product: product 9
    Shelf: shelf 2
        Product: product 10
        Product: product 11
        Product: product 12

    You can see that we can detect the type of node. Of course, nothing prevents us from making bizarre hierarchies:

    std::vector<Store> stores = {
    Store { "store 1", {
        Shelf { "shelf 1", { 
            Product { "product 1" },
            Store { "store 2", {
                Shelf { "shelf 1", { Product { "product 7" },  Product { "product 8" },  Product { "product 9" }, } },
                Shelf { "shelf 2", { Product { "product 10" }, Product { "product 11" }, Product { "product 12" }, } },
            }, },
            Product { "product 3" },
        } },
        Shelf { "shelf 2", { Product { "product 4" },  Product { "product 5" },  Product { "product 6" }, } },
    }, },
};

    To generically handle the indentation, make a stateful visitor:

    std::ostream& operator<<(std::ostream& os, Tag::Store)   { return os << "Store";   }
std::ostream& operator<<(std::ostream& os, Tag::Shelf)   { return os << "Shelf";   }
std::ostream& operator<<(std::ostream& os, Tag::Product) { return os << "Product"; }

struct print_vis {
    size_t indent = 0;
    std::ostream& _os;

    using result_type = void;

    template <typename Kind> void operator()(Node<Kind> const& n) const {
        _os << std::string(indent, ' ') << Kind{} << ": " << n.name << "\n";
        print_vis sub { indent+4, _os };
        for (auto& child : n.children) sub(child);
    }

    void operator()(Product const& p) const {
        _os << std::string(indent, ' ') << Tag::Product{} << ": " << p.name << "\n";
    }

    void operator()(Tree const& tree) const {
        boost::apply_visitor(*this, tree);
    }

    Prints: Live On Coliru

    Store: store 1
    Shelf: shelf 1
        Product: product 1
        Store: store 2
            Shelf: shelf 1
                Product: product 7
                Product: product 8
                Product: product 9
            Shelf: shelf 2
                Product: product 10
                Product: product 11
                Product: product 12
        Product: product 3
    Shelf: shelf 2
        Product: product 4
        Product: product 5
        Product: product 6

  3. No Variants, Dynamic Polymorphism Only

    With the same "weird" tree as just above with the GenericDomain tree:

    Live On Coliru 

    #include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <memory>

namespace DynamicDomain {

    struct Node;
    using Tree = std::shared_ptr<Node>;

    struct Node {
        virtual std::string type() const = 0;
        std::string name;
        std::vector<Tree> children;

        template <typename... Child>
        Node(std::string name, Child&&... children) : 
            name(std::move(name)), children { std::forward<Child>(children)... }
        { }
    };

    struct Product : Node { using Node::Node; virtual std::string type() const { return "Product"; } };
    struct Shelf   : Node { using Node::Node; virtual std::string type() const { return "Shelf"; } };
    struct Store   : Node { using Node::Node; virtual std::string type() const { return "Store"; } };

    struct print_vis {
        size_t indent;
        std::ostream* _os;

        using result_type = void;

        void operator()(Tree const& tree) const {
            if (tree) (*this) (*tree); else *_os << "[null]";
        }
        void operator()(Node const& node) const {
            *_os << std::string(indent, ' ') << node.type() << ": " << node.name << "\n";
            print_vis sub { indent+4, _os };
            for (auto const& child : node.children) sub(child);
        }
    };

    std::ostream& operator<<(std::ostream& os, Tree const& n) {
        print_vis{0, &os} (n);
        return os;
    }
}

int main() {
    using namespace DynamicDomain;
    std::vector<Tree> stores = {
        std::make_shared<Store> ("store 1", 
                std::make_shared<Shelf> ("shelf 1",
                        std::make_shared<Product> ("product 1"),
                        std::make_shared<Store> ("store 2",
                                std::make_shared<Shelf> ("shelf 1",  std::make_shared<Product> ("product 7"),  std::make_shared<Product> ("product 8"),  std::make_shared<Product> ("product 9") ),
                                std::make_shared<Shelf> ("shelf 2",  std::make_shared<Product> ("product 10"), std::make_shared<Product> ("product 11"), std::make_shared<Product> ("product 12") )
                                ),
                        std::make_shared<Product> ("product 3")
                        ),
                std::make_shared<Shelf> ("shelf 2",  std::make_shared<Product> ("product 4"),  std::make_shared<Product> ("product 5"),  std::make_shared<Product> ("product 6") )
                ),
    };

    std::for_each(begin(stores), end(stores), 
            [](DynamicDomain::Tree const& p){ std::cout << p; });
}

    Not my idea of "neat" and potentially much less efficient - although it does allow for nullable nodes and sharing of subtrees.



来源：https://stackoverflow.com/questions/37197445/how-to-generalize-a-tree-structure-with-variant-visitor

标签
c++
boost
boost-variant
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