容器配接器之queue · STL源码剖析

### 前言 queue是一种“先进先出”的数据结构，可以对两端进行操作，但是只能在队列头部进行移除元素，只能在队列尾部新增元素，可以访问队列尾部和头部的元素，但是不能遍历容器，所以queue不需要设计自己的容器。在SGI STL的源码<stl_queue.h>的class queue设计中，它是基于某种容器作为底部结构的，默认容器是deque容器，用户也可以自己指定容器的类型。 ### queue容器配接器源码剖析如下： ~~~ #ifndef __SGI_STL_INTERNAL_QUEUE_H #define __SGI_STL_INTERNAL_QUEUE_H #include <sequence_concepts.h> __STL_BEGIN_NAMESPACE // Forward declarations of operators < and ==, needed for friend declaration. //默认底层容器为deque容器 template <class _Tp, class _Sequence __STL_DEPENDENT_DEFAULT_TMPL(deque<_Tp>) > class queue; template <class _Tp, class _Seq> inline bool operator==(const queue<_Tp, _Seq>&, const queue<_Tp, _Seq>&); template <class _Tp, class _Seq> inline bool operator<(const queue<_Tp, _Seq>&, const queue<_Tp, _Seq>&); template <class _Tp, class _Sequence> class queue { // requirements: __STL_CLASS_REQUIRES(_Tp, _Assignable); __STL_CLASS_REQUIRES(_Sequence, _FrontInsertionSequence); __STL_CLASS_REQUIRES(_Sequence, _BackInsertionSequence); typedef typename _Sequence::value_type _Sequence_value_type; __STL_CLASS_REQUIRES_SAME_TYPE(_Tp, _Sequence_value_type); #ifdef __STL_MEMBER_TEMPLATES template <class _Tp1, class _Seq1> friend bool operator== (const queue<_Tp1, _Seq1>&, const queue<_Tp1, _Seq1>&); template <class _Tp1, class _Seq1> friend bool operator< (const queue<_Tp1, _Seq1>&, const queue<_Tp1, _Seq1>&); #else /* __STL_MEMBER_TEMPLATES */ friend bool __STD_QUALIFIER operator== __STL_NULL_TMPL_ARGS (const queue&, const queue&); friend bool __STD_QUALIFIER operator< __STL_NULL_TMPL_ARGS (const queue&, const queue&); #endif /* __STL_MEMBER_TEMPLATES */ public: // queue仅支持对头部和尾部的操作, 所以不定义STL要求的 // pointer, iterator, difference_type typedef typename _Sequence::value_type value_type; typedef typename _Sequence::size_type size_type; typedef _Sequence container_type; typedef typename _Sequence::reference reference; typedef typename _Sequence::const_reference const_reference; protected: _Sequence c;//底层容器，默认为deque容器，用户可自行指定容器类型 public: //下面对queue的维护完全依赖于底层容器的操作 queue() : c() {} explicit queue(const _Sequence& __c) : c(__c) {} //判断容器是否为空 bool empty() const { return c.empty(); } //返回容器中元素的个数 size_type size() const { return c.size(); } //返回队头元素的引用 reference front() { return c.front(); } const_reference front() const { return c.front(); } //返回队尾元素的引用 reference back() { return c.back(); } const_reference back() const { return c.back(); } //只能在队尾新增元素 void push(const value_type& __x) { c.push_back(__x); } //只能在队头移除元素 void pop() { c.pop_front(); } }; //下面是依赖于底层容器的操作运算符 template <class _Tp, class _Sequence> bool operator==(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return __x.c == __y.c; } template <class _Tp, class _Sequence> bool operator<(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return __x.c < __y.c; } #ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER template <class _Tp, class _Sequence> bool operator!=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return !(__x == __y); } template <class _Tp, class _Sequence> bool operator>(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return __y < __x; } template <class _Tp, class _Sequence> bool operator<=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return !(__y < __x); } template <class _Tp, class _Sequence> bool operator>=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y) { return !(__x < __y); } #endif /* __STL_FUNCTION_TMPL_PARTIAL_ORDER */ ~~~ 下面举例说明： ~~~ // constructing queues #include <iostream> // std::cout #include <deque> // std::deque #include <list> // std::list #include <queue> // std::queue int main () { std::deque<int> mydeck (3,100); // deque with 3 elements std::list<int> mylist (2,200); // list with 2 elements std::queue<int> first; // empty queue std::queue<int> second (mydeck); // queue initialized to copy of deque std::queue<int,std::list<int> > third; // empty queue with list as underlying container std::queue<int,std::list<int> > fourth (mylist); std::cout << "size of first: " << first.size() << '\n'; std::cout << "size of second: " << second.size() << '\n'; std::cout << "size of third: " << third.size() << '\n'; std::cout << "size of fourth: " << fourth.size() << '\n'; std::cout << "The element at the front of queue second is: " << second.front( ) << std::endl; second.push(10); std::cout << "The element at the back of queue second is: " << second.back( ) << std::endl; return 0; } Output: size of first: 0 size of second: 3 size of third: 0 size of fourth: 2 The element at the front of queue second is: 100 The element at the back of queue second is: 10 ~~~ 参考资料：《STL源码剖析》侯捷