背景

这纯粹是为了教育目的。如果您不想阅读整个背景，可以跳到底部的问题。

我已经编写了一个 Queue 接口(interface)(抽象类)，以及 2 个基于调整大小的数组和链表的派生实现。

template <typename T>
class IQueue {
public:
  virtual void enqueue(T item) = 0;
  virtual T dequeue() = 0;
  virtual bool isEmpty() = 0;
  virtual int size() = 0;
}

template <typename T>
class LinkedListQueue : public IQueue<T> {...}

template <typename T>
class ResizingArrayQueue : public IQueue<T> {...}

我希望能够使用符合 STL 的迭代器遍历队列的元素(我知道队列不应该是可迭代的)，所以我可以使用 for (auto e: c) 或 queue.begin()/queue.end()。

因为我使用运行时多态性，所以我必须向 IQueue 添加一个客户端迭代器类，并使用 Pimpl 惯用法在派生队列类中实例化实际实现特定的迭代器，以避免对象切片问题. 所以增强代码看起来像:

template <typename T>
class IQueue {
public:
    virtual void enqueue(T item) = 0;
    virtual T dequeue() = 0;
    virtual bool isEmpty() = 0;
    virtual int size() = 0;

public:
    class IteratorImpl {
    public:
        virtual void increment () = 0;
        virtual bool operator== (const IteratorImpl& other) const = 0;
        virtual bool operator!= (const IteratorImpl& other) const = 0;
        virtual T& operator* () const = 0;
        virtual T& operator-> () const = 0;
        virtual void swap (IteratorImpl& other) = 0;
        virtual IteratorImpl* clone() = 0;
    };

public:
    class ClientIterator : public std::iterator<std::forward_iterator_tag, T> {
        std::unique_ptr<IteratorImpl> impl;

    public:
        ClientIterator(const ClientIterator& other) : impl(other.impl->clone()) {}
        ClientIterator(std::unique_ptr<IteratorImpl> it) : impl(std::move(it)) {}
        void swap(ClientIterator& other) noexcept {
            impl->swap(*(other.impl));
        }

        ClientIterator& operator++ () {
            impl->increment();
            return *this;
        }

        ClientIterator operator++ (int) {
            ClientIterator tmp(*this);
            impl->increment();
            return tmp;
        }

        bool operator== (const ClientIterator& other) const {
            return *impl == *other.impl;
        }

        bool operator!= (const ClientIterator& other) const {
            return *impl != *other.impl;
        }

        T& operator* () const {
            return **impl;
        }

        T& operator-> () const {
            return **impl;
        }
    };

    typedef ClientIterator iterator;

    virtual iterator begin() = 0;
    virtual iterator end() = 0;
};

其中一个派生类实现了 begin()/end() 方法和派生的 Iterator 实现:

template <typename T>
class LinkedListQueue : public IQueue<T> {
// ... queue implementation details.
public:
    class LinkedListForwardIterator : public IQueue<T>::IteratorImpl {
    // ... implementation that goes through linked list.
    };

    typename IQueue<T>::ClientIterator begin() {
        std::unique_ptr<LinkedListForwardIterator> impl(new LinkedListForwardIterator(head));
        return typename IQueue<T>::iterator(std::move(impl));
    }

    typename IQueue<T>::ClientIterator end() {
        std::unique_ptr<LinkedListForwardIterator> impl(new LinkedListForwardIterator(nullptr));
        return typename IQueue<T>::iterator(std::move(impl));
    }
};

现在为了测试迭代器是否工作，我有以下两个函数:

template <typename T>
void testQueueImpl(std::shared_ptr<IQueue<T> > queue) {
    queue->enqueue(1);
    queue->enqueue(2);
    queue->enqueue(3);
    queue->enqueue(4);
    queue->enqueue(5);
    queue->enqueue(6);

    std::cout << "Iterator behavior check 1st: ";
    for (auto e: *queue) {
        std::cout << e << " ";
    }
    std::cout << std::endl;

    std::cout << "Iterator behavior check 2nd: ";
    for (auto it = queue->begin(); it != queue->end(); it++) {
        std::cout << *it << " ";
    }
}

void testQueue() {
    auto queue = std::make_shared<LinkedListQueue<int> >();
    testQueueImpl<int>(queue);

    auto queue2 = std::make_shared<ResizingArrayQueue<int> >();
    testQueueImpl<int>(queue2);
}

问题

如何摆脱运行时多态性(删除 IQueue，删除迭代器 Pimpl 实现)，并重写 testQueue()/testQueueImpl() 函数这样:

1. 这些函数可以成功测试 Stack 实现和 Stack 迭代器，而无需基类指针。
2. LinkedListQueue 和 ResizingArrayQueue 都遵循某种编译时接口(interface)(存在 enqueue、dequeue、isEmpty、size 方法，存在 begin/end 方法，两个类都包含有效的迭代器类)？

可能的解决方案

对于 1) 似乎我可以简单地将模板参数更改为整个容器，程序成功编译并运行。但这不会检查 begin()/end()/enqueue() 方法是否存在。

对于 2)，从我在互联网上找到的内容来看，相关解决方案似乎涉及类型特征/SFINAE/或概念(容器概念、前向迭代器概念)。似乎 Boost Concepts 库允许注释类以符合容器概念，但我对用于教育目的的自包含解决方案(除 STL 外没有外部库)感兴趣。

template <typename Container>
void testQueueImpl(Container queue) {
    queue->enqueue(1);
    queue->enqueue(2);
    queue->enqueue(3);
    queue->enqueue(4);
    queue->enqueue(5);
    queue->enqueue(6);

    std::cout << "Size: " << queue->size() << std::endl;

    std::cout << "Iterator behavior check 1st: ";
    for (auto e: *queue) {
        std::cout << e << " ";
    }
    std::cout << std::endl;

    std::cout << "Iterator behavior check 2nd: ";
    for (auto it = queue->begin(); it != queue->end(); it++) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;
}

void testQueue() {
    auto queue = std::make_shared<LinkedListQueue<int> >();
    testQueueImpl<std::shared_ptr<LinkedListQueue<int> > >(queue);

    auto queue2 = std::make_shared<ResizingArrayQueue<int> >();
    testQueueImpl<std::shared_ptr<ResizingArrayQueue<int> > >(queue2);
}

最佳答案

这是一个最小的可编译示例，说明您可能希望如何做到这一点。

请注意，目前此示例仅支持 const begin() 和 const end()。

添加更多方法和可变迭代器是对读者的练习

编辑:提供了共享相同策略类的编译时和运行时多态队列的工作示例。

#include <iostream>
#include <list>
#include <vector>
#include <memory>
#include <typeinfo>
#include <typeindex>

/// COMPILE TIME Polymorphic queue of objects of type Element

template<typename Element, class Policy>
struct queue_concept
{
    // Define interface
    struct const_iterator;
    void push_back(Element e);
    const_iterator begin() const;
    const_iterator end() const;


    // Implementation
private:
    Policy _policy;
};

// implement class methods an inner classes

template<typename Element, class Policy>
struct queue_concept<Element, Policy>::const_iterator
{
    using iterator_type = typename Policy::container_type::const_iterator;

    const_iterator(iterator_type iter = iterator_type {})
    : _iter { std::move(iter) }
    {}

    const Element& operator*() const {
        return *_iter;
    }

    const_iterator& operator++() {
        std::advance(_iter, 1);
    }

    bool operator!=(const const_iterator& other) const {
        return _iter != other._iter;
    }

    iterator_type _iter;
};

template<typename Element, class Policy>
void queue_concept<Element, Policy>::push_back(Element e)
{
    _policy._data.push_back(std::move(e));
}

template<typename Element, class Policy>
typename queue_concept<Element, Policy>::const_iterator queue_concept<Element, Policy>::begin() const
{
    return const_iterator { _policy._data.begin() };
}

template<typename Element, class Policy>
typename queue_concept<Element, Policy>::const_iterator queue_concept<Element, Policy>::end() const
{
    return const_iterator { _policy._data.end() };
}

/// RUNTIME Polymorphic queue of objects of type Element
template<typename Element>
struct IQueue
{
    struct const_iterator
    {
        struct Concept {
            // virtual base class so make destructor virtual...
            virtual ~Concept() = default;
            virtual const Element& get_element() const = 0;
            virtual void increment(std::size_t distance) = 0;
            bool equal_to(const Concept& rhs)
            {
                if (this->get_type() == rhs.get_type()) {
                    return unsafe_is_equal(rhs);
                }
                return false;
            }

            virtual bool unsafe_is_equal(const Concept& rhs) const = 0;
            virtual std::type_index get_type() const = 0;

            // provide copy support
            virtual std::unique_ptr<Concept> clone() const = 0;

        };

        template<class Iter>
        struct Model : public Concept {
            Model(Iter iter) : _iter { std::move(iter) }
            {}

            const Element& get_element() const override {
                return *_iter;
            }

            void increment(std::size_t distance) override {
                std::advance(_iter, distance);
            }

            bool unsafe_is_equal(const Concept& rhs) const override {
                auto _rhs = static_cast<const Model&>(rhs);
                return _iter == _rhs._iter;
            }

            std::type_index get_type() const override {
                return std::type_index(typeid(*this));
            }

            std::unique_ptr<Concept> clone() const override {
                return std::unique_ptr<Concept> { new Model(*this) };
            }

        private:
            Iter _iter;    
        };

        // constructor
        template<class Iter>
        const_iterator(Iter iter)
        : _impl { new Model<Iter> { std::move(iter) } }
        {}

        // default constructor - constructs an invalid iterator
        const_iterator()
        {}

        // provide copy support since impl is a unique_ptr
        const_iterator(const const_iterator& other)
        : _impl { other._impl ? other._impl->clone() : std::unique_ptr<Concept>{} }
        {}

        const_iterator& operator=(const_iterator& other)
        {
            auto p = other._impl ? other._impl->clone() : std::unique_ptr<Concept>{};
            std::swap(_impl, p);
        }

        // since we provided copy support we must provide move support
        const_iterator(const_iterator&& rhs) = default;
        const_iterator& operator=(const_iterator&& rhs) = default;

        const Element& operator*() const {
            return _impl->get_element();
        }
        const_iterator& operator++() {
            _impl->increment(1);
            return *this;
        }
        bool operator!=(const const_iterator& rhs) const
        {
            return !(_impl->equal_to(*(rhs._impl)));
        }

    private:
        std::unique_ptr<Concept> _impl;
    };


    virtual void push_back(Element e) = 0;
    virtual const_iterator begin() const = 0;
    virtual const_iterator end() const = 0;
};


template<class Element, class Policy>
struct QueueImpl : public IQueue<Element>
{
    void push_back(Element e) override {
        _policy._data.push_back(std::move(e));
    }

    typename IQueue<Element>::const_iterator begin() const override {
        return typename IQueue<Element>::const_iterator { std::begin(_policy._data) };
    }

    typename IQueue<Element>::const_iterator end() const override {
        return typename IQueue<Element>::const_iterator { std::end(_policy._data) };
    }


    Policy _policy;
};

template<class Element>
struct ResizingArrayPolicy
{
    using container_type = std::vector<Element>;
    container_type _data;
};

template<class Element>
struct LinkedListPolicy
{
    using container_type = std::list<Element>;
    container_type _data;
};

template<class Element>
std::unique_ptr<IQueue<Element>> make_poly_resizing_array_queue()
{
    return std::unique_ptr<IQueue<Element>> { new QueueImpl<Element, ResizingArrayPolicy<Element>> };
}

template<class Element>
std::unique_ptr<IQueue<Element>> make_poly_linked_list_queue()
{
    return std::unique_ptr<IQueue<Element>> { new QueueImpl<Element, LinkedListPolicy<Element>>{} };
}

template<class Element>
queue_concept<Element, ResizingArrayPolicy<Element>> make_static_resizing_array_queue()
{
    return queue_concept<Element, ResizingArrayPolicy<Element>>{};
}

template<class Element>
queue_concept<Element, LinkedListPolicy<Element>> make_static_linked_list_queue()
{
    return queue_concept<Element, LinkedListPolicy<Element>>{};
}

using namespace std;

int main()
{
    // create the queues
    auto pq1 = make_poly_resizing_array_queue<int>();
    auto pq2 = make_poly_linked_list_queue<int>();

    // put data in them    
    pq1->push_back(10);
    pq1->push_back(20);

    pq2->push_back(30);
    pq2->push_back(40);

    // prove that iterators are assignable and moveable
    IQueue<int>::const_iterator it;
    it = pq1->begin();
    cout << *it << endl; // should print 10
    auto i2 = pq2->begin();
    it = move(i2);
    cout << *it << endl; // should print 30

    // prove that queues are polymorphic

    auto queues = vector<unique_ptr<IQueue<int>>>{};
    queues.push_back(move(pq1));
    queues.push_back(move(pq2));

    // print the vector of queues
    for(const auto& queue_ptr : queues) {
        for(const auto& item : *queue_ptr) {
            cout << item << endl;
        }
        cout << endl;
    }

    // now the static versions
    auto q1 = make_static_resizing_array_queue<int>();
    auto q2 = make_static_linked_list_queue<int>();

    q1.push_back(10);
    q1.push_back(20);

    q2.push_back(30);
    q2.push_back(40);

    cout << "static queues\n";
    for(const auto& item : q1) {
        cout << item << endl;
    }
    cout << endl;    
    for(const auto& item : q2) {
        cout << item << endl;
    }

    return 0;
}

关于C++:多态容器/迭代器与编译时概念/特征，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30101567/