在项目中，有时候对数据结构有这样的需求，既需要具备Map的高效读写，又要兼具插入数据成员的有序性，这时候你就需要使用Ordered Map了。
关于Ordered Map,相关资源比较多，实现思路比较简单，基本上都是通过list有序记录插入的数据的key值，重写遍历或者迭代的方法时，限定按照key的插入顺序遍历。
笔者只是在这里推荐一个现成的开源项目：《Qt Ordered Map》

#ifndef ORDEREDMAP_H
#define ORDEREDMAP_H

#include <QtGlobal>
#include <QHash>
#include <QLinkedList>
#include <QList>
#include <QPair>

#ifdef Q_COMPILER_INITIALIZER_LISTS
#include <initializer_list>
#endif

template <typename Key> inline bool oMHashEqualToKey(const Key &key1, const Key &key2)
{
    // Key type must provide '==' operator
    return key1 == key2;
}

template <typename Ptr> inline bool oMHashEqualToKey(Ptr *key1, Ptr *key2)
{
    Q_ASSERT(sizeof(quintptr) == sizeof(Ptr *));
    return quintptr(key1) == quintptr(key2);
}

template <typename Ptr> inline bool oMHashEqualToKey(const Ptr *key1, const Ptr *key2)
{
    Q_ASSERT(sizeof(quintptr) == sizeof(const Ptr *));
    return quintptr(key1) == quintptr(key2);
}

template <typename Key, typename Value>
class OrderedMap
{
    class OMHash;

    typedef typename QLinkedList<Key>::iterator QllIterator;
    typedef typename QLinkedList<Key>::const_iterator QllConstIterator;
    typedef QPair<Value, QllIterator> OMHashValue;

    typedef typename OMHash::iterator OMHashIterator;
    typedef typename OMHash::const_iterator OMHashConstIterator;

public:

    class iterator;
    class const_iterator;

    typedef typename OrderedMap<Key, Value>::iterator Iterator;
    typedef typename OrderedMap<Key, Value>::const_iterator ConstIterator;

    explicit OrderedMap();

#ifdef Q_COMPILER_INITIALIZER_LISTS
    OrderedMap(std::initializer_list<std::pair<Key,Value> > list);
#endif

    OrderedMap(const OrderedMap<Key, Value>& other);

#if (QT_VERSION >= 0x050200)
    OrderedMap(OrderedMap<Key, Value>&& other);
#endif

    void clear();

    bool contains(const Key &key) const;

    int count() const;

    bool empty() const;

    iterator insert(const Key &key, const Value &value);

    bool isEmpty() const;

    QList<Key> keys() const;

    int remove(const Key &key);

    int size() const;

    Value take(const Key &key);

    Value value(const Key &key) const;

    Value value(const Key &key, const Value &defaultValue) const;

    QList<Value> values() const;

    OrderedMap<Key, Value> & operator=(const OrderedMap<Key, Value>& other);

#if (QT_VERSION >= 0x050200)
    OrderedMap<Key, Value> & operator=(OrderedMap<Key, Value>&& other);
#endif

    bool operator==(const OrderedMap<Key, Value> &other) const;

    bool operator!=(const OrderedMap<Key, Value> &other) const;

    Value& operator[](const Key &key);

    const Value operator[](const Key &key) const;

    iterator begin();

    const_iterator begin() const;

    iterator end();

    const_iterator end() const;

    iterator erase(iterator pos);

    iterator find(const Key& key);

    const_iterator find(const Key& key) const;

    class const_iterator;

    class iterator
    {
        QllIterator qllIter;
        OMHash *data;
        friend class const_iterator;
        friend class OrderedMap;

    public:
        iterator() : data(NULL) {}

        iterator(const QllIterator &qllIter, OMHash *data) :
            qllIter(qllIter), data(data) {}

        const Key & key() const
        {
            return *qllIter;
        }

        Value & value() const
        {
            OMHashIterator hit = data->find(*qllIter);
            OMHashValue &pair = hit.value();
            return pair.first;
        }

        Value & operator*() const
        {
            return value();
        }

        iterator operator+(int i) const
        {
            QllIterator q = qllIter;
            q += i;

            return iterator(q, data);
        }

        iterator operator-(int i) const
        {
            return operator +(- i);
        }

        iterator& operator+=(int i)
        {
            qllIter += i;
            return *this;
        }

        iterator& operator-=(int i)
        {
            qllIter -= i;
            return *this;
        }

        iterator& operator++()
        {
            ++qllIter;
            return *this;
        }

        iterator operator++(int)
        {
            iterator it = *this;
            qllIter++;
            return it;
        }

        iterator operator--()
        {
            --qllIter;
            return *this;
        }

        iterator operator--(int)
        {
            iterator it = *this;
            qllIter--;
            return it;
        }

        bool operator ==(const iterator &other) const
        {
            return (qllIter == other.qllIter);
        }

        bool operator !=(const iterator &other) const
        {
            return (qllIter != other.qllIter);
        }
    };

    class const_iterator
    {

        QllConstIterator qllConstIter;
        const OMHash *data;

    public:
        const_iterator() : data(NULL) {}

        const_iterator(const iterator &i) :
            qllConstIter(i.qllIter), data(i.data) {}

        const_iterator(const QllConstIterator &qllConstIter, const OMHash* data) :
            qllConstIter(qllConstIter), data(data) {}

        const Key & key() const
        {
            return *qllConstIter;
        }

        const Value & value() const
        {
            OMHashConstIterator hit = data->find(*qllConstIter);
            const OMHashValue &pair = hit.value();
            return pair.first;
        }

        const Value & operator*() const
        {
            return value();
        }

        const_iterator operator+(int i) const
        {
            QllConstIterator q = qllConstIter;
            q += i;

            return const_iterator(q, data);
        }

        const_iterator operator-(int i) const
        {
            return operator +(- i);
        }

        const_iterator& operator+=(int i)
        {
            qllConstIter += i;
            return *this;
        }

        const_iterator& operator-=(int i)
        {
            qllConstIter -= i;
            return *this;
        }

        const_iterator& operator++()
        {
            ++qllConstIter;
            return *this;
        }

        const_iterator operator++(int)
        {
            const_iterator it = *this;
            qllConstIter++;
            return it;
        }

        const_iterator operator--()
        {
            --qllConstIter;
            return *this;
        }

        const_iterator operator--(int)
        {
            const_iterator it = *this;
            qllConstIter--;
            return it;
        }

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

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

private:

    class OMHash : public QHash<Key, OMHashValue >
    {
    public:
        bool operator == (const OMHash &other) const
        {
            if (size() != other.size()) {
                return false;
            }

            if (QHash<Key, OMHashValue >::operator ==(other)) {
                return true;
            }

            typename QHash<Key, OMHashValue >::const_iterator it1 = this->constBegin();
            typename QHash<Key, OMHashValue >::const_iterator it2 = other.constBegin();

            while(it1 != this->end()) {
                OMHashValue v1 = it1.value();
                OMHashValue v2 = it2.value();

                if ((v1.first != v2.first) || !oMHashEqualToKey<Key>(it1.key(), it2.key())) {
                    return false;
                }
                ++it1;
                ++it2;
            }
            return true;
        }
    };

private:
    void copy(const OrderedMap<Key, Value> &other);

    OMHash data;
    QLinkedList<Key> insertOrder;
};

template <typename Key, typename Value>
OrderedMap<Key, Value>::OrderedMap() {}

#ifdef Q_COMPILER_INITIALIZER_LISTS
template<typename Key, typename Value>
OrderedMap<Key, Value>::OrderedMap(std::initializer_list<std::pair<Key, Value> > list)
{
    typedef typename std::initializer_list<std::pair<Key,Value> >::const_iterator const_initlist_iter;
    for (const_initlist_iter it = list.begin(); it != list.end(); ++it)
        insert(it->first, it->second);
}
#endif


template <typename Key, typename Value>
OrderedMap<Key, Value>::OrderedMap(const OrderedMap<Key, Value>& other)
{
    copy(other);
}

#if (QT_VERSION >= 0x050200)
template <typename Key, typename Value>
OrderedMap<Key, Value>::OrderedMap(OrderedMap<Key, Value>&& other)
{
    data = std::move(other.data);
    insertOrder = std::move(other.insertOrder);
}
#endif

template <typename Key, typename Value>
void OrderedMap<Key, Value>::clear()
{
    data.clear();
    insertOrder.clear();
}

template <typename Key, typename Value>
bool OrderedMap<Key, Value>::contains(const Key &key) const
{
    return data.contains(key);
}

template <typename Key, typename Value>
int OrderedMap<Key, Value>::count() const
{
    return data.count();
}

template <typename Key, typename Value>
bool OrderedMap<Key, Value>::empty() const
{
    return data.empty();
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::iterator OrderedMap<Key, Value>::insert(const Key &key, const Value &value)
{
    OMHashIterator it = data.find(key);

    if (it == data.end()) {
        // New key
        QllIterator ioIter = insertOrder.insert(insertOrder.end(), key);
        OMHashValue pair(value, ioIter);
        data.insert(key, pair);
        return iterator(ioIter, &data);
    }

    OMHashValue pair = it.value();
    // remove old reference
    insertOrder.erase(pair.second);
    // Add new reference
    QllIterator ioIter = insertOrder.insert(insertOrder.end(), key);
    pair.first = value;
    pair.second = ioIter;
    data.insert(key, pair);
    return iterator(ioIter, &data);
}

template <typename Key, typename Value>
bool OrderedMap<Key, Value>::isEmpty() const
{
    return data.isEmpty();
}

template<typename Key, typename Value>
QList<Key> OrderedMap<Key, Value>::keys() const
{
    return QList<Key>::fromStdList(insertOrder.toStdList());
}

template<typename Key, typename Value>
int OrderedMap<Key, Value>::remove(const Key &key)
{
    OMHashIterator it = data.find(key);
    if (it == data.end()) {
        return 0;
    }
    OMHashValue pair = it.value();
    insertOrder.erase(pair.second);
    data.erase(it);
    return 1;
}

template<typename Key, typename Value>
int OrderedMap<Key, Value>::size() const
{
    return data.size();
}

template<typename Key, typename Value>
void OrderedMap<Key, Value>::copy(const OrderedMap<Key, Value> &other)
{
    /* Since I'm storing iterators of QLinkedList, I simply cannot make
     * a trivial copy of the linked list. This is a limitation due to implicit
     * sharing used in Qt containers, due to which iterator active on one
     * QLL can change the data of another QLL even after creating a copy.
     *
     * Because of this, the old iterators have to be invalidated and new ones
     * have to be generated.
     */
    insertOrder.clear();
    // Copy hash
    data = other.data;

    QllConstIterator cit = other.insertOrder.begin();
    for (; cit != other.insertOrder.end(); ++cit) {
        Key key = *cit;
        QllIterator ioIter = insertOrder.insert(insertOrder.end(), key);
        OMHashIterator it = data.find(key);
        (*it).second = ioIter;
    }
}

template<typename Key, typename Value>
Value OrderedMap<Key, Value>::take(const Key &key)
{
    OMHashIterator it = data.find(key);
    if (it == data.end()) {
        return Value();
    }
    OMHashValue pair = it.value();
    insertOrder.erase(pair.second);
    data.erase(it);
    return pair.first;
}

template <typename Key, typename Value>
Value OrderedMap<Key, Value>::value(const Key &key) const
{
    return data.value(key).first;
}

template <typename Key, typename Value>
Value OrderedMap<Key, Value>::value(const Key &key, const Value &defaultValue) const
{
    OMHashConstIterator it = data.constFind(key);
    if (it == data.end()) {
        return defaultValue;
    }
    OMHashValue pair = it.value();
    return pair.first;
}

template <typename Key, typename Value>
QList<Value> OrderedMap<Key, Value>::values() const
{
    QList<Value> values;
    foreach (const Key &key, insertOrder.toStdList()) {
        OMHashValue v = data.value(key);
        values.append(v.first);
    }
    return values;
}

template <typename Key, typename Value>
OrderedMap<Key, Value> & OrderedMap<Key, Value>::operator=(const OrderedMap<Key, Value>& other)
{
    if (this != &other) {
        copy(other);
    }
    return *this;
}

#if (QT_VERSION >= 0x050200)
template <typename Key, typename Value>
OrderedMap<Key, Value> & OrderedMap<Key, Value>::operator=(OrderedMap<Key, Value>&& other)
{
    if (this != &other) {
        data = other.data;
        insertOrder = other.insertOrder;
    }
    return *this;
}
#endif

template <typename Key, typename Value>
bool OrderedMap<Key, Value>::operator==(const OrderedMap<Key, Value> &other) const
{
    // 2 Ordered maps are equal if they have the same contents in the same order
    return ((data == other.data) && (insertOrder == other.insertOrder));
}

template <typename Key, typename Value>
bool OrderedMap<Key, Value>::operator!=(const OrderedMap<Key, Value> &other) const
{
    return ((data != other.data) || (insertOrder != other.insertOrder));
}

template <typename Key, typename Value>
Value& OrderedMap<Key, Value>::operator[](const Key &key)
{
    OMHashIterator it = data.find(key);
    if (it == data.end()) {
        insert(key, Value());
        it = data.find(key);
    }
    OMHashValue &pair = it.value();
    return pair.first;
}

template <typename Key, typename Value>
const Value OrderedMap<Key, Value>::operator[](const Key &key) const
{
    return value(key);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::iterator OrderedMap<Key, Value>::begin()
{
    return iterator(insertOrder.begin(), &data);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::const_iterator OrderedMap<Key, Value>::begin() const
{
    return const_iterator(insertOrder.begin(), &data);
}


template <typename Key, typename Value>
typename OrderedMap<Key, Value>::iterator OrderedMap<Key, Value>::end()
{
    return iterator(insertOrder.end(), &data);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::const_iterator OrderedMap<Key, Value>::end() const
{
    return const_iterator(insertOrder.end(), &data);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::iterator OrderedMap<Key, Value>::erase(iterator pos)
{
    OMHashIterator hit = data.find(*(pos.qllIter));
    if (hit == data.end()) {
        return pos;
    }
    data.erase(hit);
    QllIterator ioIter = insertOrder.erase(pos.qllIter);

    return iterator(ioIter, &data);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::iterator OrderedMap<Key, Value>::find(const Key& key)
{
    OMHashIterator hit = data.find(key);
    if (hit == data.end()) {
        return end();
    }

    return iterator(hit.value().second, &data);
}

template <typename Key, typename Value>
typename OrderedMap<Key, Value>::const_iterator OrderedMap<Key, Value>::find(const Key& key) const
{
    OMHashConstIterator hit = data.find(key);
    if (hit == data.end()) {
        return end();
    }

    return const_iterator(hit.value().second, &data);
}
#endif // ORDEREDMAP_H