c++ - 获取此对象的原始类型

标签 c++ parent-child metaprogramming variadic-templates

我正在尝试使用元编程来防止父子结构中的重复代码。我让它工作到一定程度。

底部编译器中显示的代码正确运行，但一些关系(/*Tree_tag,*/ 和 /*Parasite_tag*/)被注释掉了。如果未注释，MSVS2017 显示

error C2664: 'void Obj<std::tuple<Human_tag>,std::tuple<>>::removeParent(const Obj<std::tuple<>,std::tuple<Tree_tag,Dog_tag>> *const )': cannot convert argument 1 from 'Obj<std::tuple<>,std::tuple<Dog_tag>> *' to 'const Obj<std::tuple<>,std::tuple<Tree_tag,Dog_tag>> *const '
note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

和 G++ 显示

In instantiation of ‘void Obj<std::tuple<>, std::tuple<_El0, _El ...> ::removeAllChildren() [with TChildTag = Dog_tag; TChildTags = {}]’:
Main.cpp:126:1:   required from here
Main.cpp:73:43: error: invalid conversion from ‘Obj<std::tuple<>, std::tuple<Dog_tag> >*’ to ‘const TParent* {aka const Obj<std::tuple<>, std::tuple<Tree_tag, Dog_tag> >*}’ [-fpermissive]
    for (auto&& child : childrenPtrs) child->removeParent(this);

问题出在 this类型限定符。因为我用例如迭代地剥离模板参数

class Obj<std::tuple<>, std::tuple<TChildTag, TChildTags...>>
: public Obj<std::tuple<>, std::tuple<TChildTags...>>

结果this基类型与原始类型不匹配。如报错所示:Human的原始类型= Obj<std::tuple<>,std::tuple<Tree_tag,Dog_tag>> .但是，由于迭代剥离，this 的类型在基地是Obj<std::tuple<>,std::tuple<Dog_tag>> .

我尝试使用 reinterpret_cast按照建议:

template<typename T>
void addParent(T* const parentPtr) {
    parentsPtrs.push_back(reinterpret_cast<TParent* const>(parentPtr));
}
template<typename T>
void removeParent(T const* const parentPtr) {
    auto it = std::find(std::cbegin(parentsPtrs), std::cend(parentsPtrs),
        reinterpret_cast<TParent const* const>(parentPtr));
    if (it != std::cend(parentsPtrs)) parentsPtrs.erase(it);
}

但问题是所有内容 都被强制转换为允许的参数。 IE。此代码有效:

int main() {
    Human h1;
    Parasite p1;
    addRelation(&h1, &p1);
}

...这应该是不可能的，因为Human和 Parasite没有直接关系。

那么我怎样才能正确地保留 this顶级(最派生)类的类型限定符，符合 Human , Dog等类型？

工作代码(带注释):

#include <tuple>
#include <vector>

template<class T>
using prtVector = std::vector<T*>;

class BaseObject {
public:
    virtual prtVector<BaseObject> getAllParents() const = 0;
    virtual prtVector<BaseObject> getAllChildren() const = 0;
    virtual void removeAllParents() = 0;
    virtual void removeAllChildren() = 0;
};

template<typename TParentTuple, typename TChilderenTuple>
class Obj;

template<typename TParentTag, typename... TParentTags, typename... TChildTags>
class Obj<std::tuple<TParentTag, TParentTags...>, std::tuple<TChildTags...>>
    : public Obj<std::tuple<TParentTags...>, std::tuple<TChildTags...>>
{
    using TParent = typename TParentTag::obj_type;
    prtVector<TParent> parentsPtrs;
public:
    void addParent(TParent* const parentPtr) { parentsPtrs.push_back(parentPtr); }
    void removeParent(TParent const* const parentPtr) {
        auto it = std::find(std::cbegin(parentsPtrs), std::cend(parentsPtrs), parentPtr);
        if (it != std::cend(parentsPtrs)) parentsPtrs.erase(it);
    }

    virtual prtVector<BaseObject> getAllParents() const override {
        auto result = Obj<std::tuple<TParentTags...>, std::tuple<TChildTags...>>::getAllParents();
        result.insert(std::begin(result), std::cbegin(parentsPtrs), std::cend(parentsPtrs));
        return result;
    }
    virtual prtVector<BaseObject> getAllChildren() const override {
        return Obj<std::tuple<TParentTags...>, std::tuple<TChildTags...>>::getAllChildren();
    }
    virtual void removeAllParents() override {
        Obj<std::tuple<TParentTags...>, std::tuple<TChildTags...>>::removeAllParents();
        for (auto&& parent : parentsPtrs) parent->removeChild(this);
    }
    virtual void removeAllChildren() override {
        Obj<std::tuple<TParentTags...>, std::tuple<TChildTags...>>::removeAllChildren();
    }
};

template<typename TChildTag, typename... TChildTags>
class Obj<std::tuple<>, std::tuple<TChildTag, TChildTags...>>
    : public Obj<std::tuple<>, std::tuple<TChildTags...>>
{
    using TChild = typename TChildTag::obj_type;
    prtVector<TChild> childrenPtrs;
public:
    void addChild(TChild* const childPtr) { childrenPtrs.push_back(childPtr); }
    void removeChild(TChild const* const childPtr) {
        auto it = std::find(std::cbegin(childrenPtrs), std::cend(childrenPtrs), childPtr);
        if (it != std::cend(childrenPtrs)) childrenPtrs.erase(it);
    }

    virtual prtVector<BaseObject> getAllParents() const override {
        return Obj<std::tuple<>, std::tuple<TChildTags...>>::getAllChildren();
    }
    virtual prtVector<BaseObject> getAllChildren() const override {
        auto result = Obj<std::tuple<>, std::tuple<TChildTags...>>::getAllChildren();
        result.insert(std::begin(result), std::cbegin(childrenPtrs), std::cend(childrenPtrs));
        return result;
    }
    virtual void removeAllParents() override {}
    virtual void removeAllChildren() override {
        Obj<std::tuple<>, std::tuple<TChildTags...>>::removeAllChildren();
        for (auto&& child : childrenPtrs) child->removeParent(this);
    }
};

template<>
class Obj<std::tuple<>, std::tuple<>> : public BaseObject {
public:
    virtual prtVector<BaseObject> getAllParents() const override {
        return prtVector<BaseObject>();
    }
    virtual prtVector<BaseObject> getAllChildren() const override {
        return prtVector<BaseObject>();
    }
    virtual void removeAllParents() override {}
    virtual void removeAllChildren() override {}
};

struct Human_tag;
struct Tree_tag;
struct Dog_tag;
struct Parasite_tag;

using Human = Obj<std::tuple<>, std::tuple</*Tree_tag,*/ Dog_tag>>;
using Tree = Obj<std::tuple<Human_tag>, std::tuple<>>;
using Dog = Obj<std::tuple<Human_tag>, std::tuple</*Parasite_tag*/>>;
using Parasite = Obj<std::tuple<Dog_tag>, std::tuple<>>;

struct Human_tag { using obj_type = Human; };
struct Tree_tag { using obj_type = Tree; };
struct Dog_tag { using obj_type = Dog; };
struct Parasite_tag { using obj_type = Parasite; };

template<class A, class B>
void addRelation(A* a, B* b)
{
    a->addChild(b);
    b->addParent(a);
}

#include <iostream>
int main() {
    Human h1;
    Dog d1, d2;

    addRelation(&h1, &d1);
    addRelation(&h1, &d2);
    auto result = h1.getAllChildren();
    std::cout << result.size() << "\n"; //print 2
    d1.removeAllParents();
    result = h1.getAllChildren();
    std::cout << result.size() << "\n"; //print 1

    std::cin.ignore();
}

最佳答案

使用 C++17，你可以这样做(不强制转换):

template<typename TParentTuple, typename TChilderenTuple>
class Obj;

template<typename... ParentTags,
         typename... ChildTags>
class Obj<std::tuple<ParentTags...>, std::tuple<ChildTags...>> : public BaseObject
{
    std::tuple<std::vector<typename ParentTags::obj_type*>...> parents;
    std::tuple<std::vector<typename ChildTags::obj_type*>...> children;

public:

    template <typename T>
    void addParent(T* parent) { std::get<std::vector<T*>>(parents).push_back(parent); }

    template <typename T>
    void removeParent(const T* parent) {
        auto& v = std::get<std::vector<T*>>(parents);
        auto it = std::find(std::cbegin(v), std::cend(v), parent);
        if (it != std::cend(v)) { v.erase(it); }
    }

    template <typename T>
    void addChild(T* child) { std::get<std::vector<T*>>(children).push_back(child); }

    template <typename T>
    void removeChild(const T* child) {
        auto& v = std::get<std::vector<T*>>(children);
        auto it = std::find(std::cbegin(v), std::cend(v), child);
        if (it != std::cend(v)) { v.erase(it); }
    }

    std::vector<BaseObject*> getAllParents() const override {
        std::vector<BaseObject*> res;

        std::apply([&](auto&... v){ (res.insert(res.end(), v.begin(), v.end()), ...); },
                   parents);
        return res;
    }
    std::vector<BaseObject*> getAllChildren() const override {
        std::vector<BaseObject*> res;

        std::apply([&](auto&... v){ (res.insert(res.end(), v.begin(), v.end()), ...); },
                   children);
        return res;
    }

    void removeAllParents() override {
        std::apply(
            [this](auto&... v)
            {
                [[maybe_unused]] auto clean = [this](auto& v) {
                    for (auto* parent : v) {
                        parent->removeChild(this);
                    }
                    v.clear();
                };
                (clean(v), ...);
            },
            parents);
    }

    void removeAllChildren() override {
        std::apply(
            [this](auto&... v)
            {
                [[maybe_unused]] auto clean = [this](auto& v) {
                    for (auto* child : v) {
                        child->removeParent(this);
                    }
                    v.clear();
                };
                ( clean(v), ...);
            },
            children);
    }
};

Demo

在 C++14 中，用 std::apply 替换“for_each_tuple”会更冗长和折叠表达式。

在 C++11 中，更多的是 std::get<T>(tuple) .

关于c++ - 获取此对象的原始类型，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/51173345/

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