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>
#endiftemplate <typename Key> inline bool oMHashEqualToKey(const Key &key1, const Key &key2)
{// Key type must provide '==' operatorreturn 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_LISTSOrderedMap(std::initializer_list<std::pair<Key,Value> > list);
#endifOrderedMap(const OrderedMap<Key, Value>& other);#if (QT_VERSION >= 0x050200)OrderedMap(OrderedMap<Key, Value>&& other);
#endifvoid 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);
#endifbool 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);
}
#endiftemplate <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);
}
#endiftemplate <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 keyQllIterator ioIter = insertOrder.insert(insertOrder.end(), key);OMHashValue pair(value, ioIter);data.insert(key, pair);return iterator(ioIter, &data);}OMHashValue pair = it.value();// remove old referenceinsertOrder.erase(pair.second);// Add new referenceQllIterator 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 hashdata = 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;
}
#endiftemplate <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 orderreturn ((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