STL 之 queue、priority_queue 源码剖析

/*
* Copyright (c) 1994
* Hewlett-Packard Company
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation. Hewlett-Packard Company makes no
* representations about the suitability of this software for any
* purpose. It is provided "as is" without express or implied warranty.
*
*
* Copyright (c) 1996,1997
* Silicon Graphics Computer Systems, Inc.
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation. Silicon Graphics makes no
* representations about the suitability of this software for any
* purpose. It is provided "as is" without express or implied warranty.
*/
/* NOTE: This is an internal header file, included by other STL headers.
* You should not attempt to use it directly.
*/
#ifndef __SGI_STL_INTERNAL_QUEUE_H
#define __SGI_STL_INTERNAL_QUEUE_H
__STL_BEGIN_NAMESPACE
#ifndef __STL_LIMITED_DEFAULT_TEMPLATES
template <class T, class Sequence = deque<T> >
#else
template <class T, class Sequence>
#endif
class queue {
friend bool operator== __STL_NULL_TMPL_ARGS (const queue& x, const queue& y);
friend bool operator< __STL_NULL_TMPL_ARGS (const queue& x, const queue& y);
public:
typedef typename Sequence::value_type value_type;
typedef typename Sequence::size_type size_type;
typedef typename Sequence::reference reference;
typedef typename Sequence::const_reference const_reference;
protected:
Sequence c; // 底層容器
public:
// 以下完全利用 Sequence c 的操作，完成 queue 的操作。
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
reference front() { return c.front(); }
const_reference front() const { return c.front(); }
reference back() { return c.back(); }
const_reference back() const { return c.back(); }
// deque 是兩頭可進出，queue 是末端進，前端出（所以先進者先出）。
void push(const value_type& x) { c.push_back(x); }
void pop() { c.pop_front(); }
};
template <class T, class Sequence>
bool operator==(const queue<T, Sequence>& x, const queue<T, Sequence>& y)
{
return x.c == y.c;
}
template <class T, class Sequence>
bool operator<(const queue<T, Sequence>& x, const queue<T, Sequence>& y)
{
return x.c < y.c;
}
#ifndef __STL_LIMITED_DEFAULT_TEMPLATES
/*
預設情況下 priority_queue 係利用 vector 完成一個 max-heap，後者乃為一個以
array（或 vector）表現的二元樹，其條件是，必須為完全樹（complete tree，此為
結構特性），且每個節點值都大於或等於其任一子節點值（此為次序特性）。因此根節點為
最大值。Max-heap 適用於 priority_queue 所需特性。
*/
template <class T, class Sequence = vector<T>,
class Compare = less<typename Sequence::value_type> >
#else
template <class T, class Sequence, class Compare>
#endif
class priority_queue {
public:
typedef typename Sequence::value_type value_type;
typedef typename Sequence::size_type size_type;
typedef typename Sequence::reference reference;
typedef typename Sequence::const_reference const_reference;
protected:
Sequence c; // 底層容器
Compare comp; // 元素大小比較標準
public:

priority_queue() : c() {}
explicit priority_queue(const Compare& x) : c(), comp(x) {}
// 以下用到的make_heap(), push_heap(), pop_heap()都是泛型演算法
// 注意，任一個建構式都立刻於底層容器內產生一個implicit representation heap。
#ifdef __STL_MEMBER_TEMPLATES
template <class InputIterator>
priority_queue(InputIterator first, InputIterator last, const Compare& x)
: c(first, last), comp(x) { make_heap(c.begin(), c.end(), comp); }
template <class InputIterator>
priority_queue(InputIterator first, InputIterator last)
: c(first, last) { make_heap(c.begin(), c.end(), comp); }
#else /* __STL_MEMBER_TEMPLATES */
priority_queue(const value_type* first, const value_type* last,
const Compare& x) : c(first, last), comp(x) {
make_heap(c.begin(), c.end(), comp);
}
priority_queue(const value_type* first, const value_type* last)
: c(first, last) { make_heap(c.begin(), c.end(), comp); }
#endif /* __STL_MEMBER_TEMPLATES */
bool empty() const { return c.empty(); }
size_type size() const { return c.size(); }
const_reference top() const { return c.front(); }
void push(const value_type& x) {
__STL_TRY {
// push_heap 是泛型演算法，先利用底層容器的 push_back() 將新元素
// 推入末端，再重排 heap。見C++ Primer p.1195。
c.push_back(x);
push_heap(c.begin(), c.end(), comp); // push_heap 是泛型演算法
}
__STL_UNWIND(c.clear());
}
void pop() {
__STL_TRY {
// pop_heap 是泛型演算法，從 heap 內取出一個元素。它並不是真正將元素
// 彈出，而是重排 heap，然後再以底層容器的 pop_back() 取得被彈出
// 的元素。見C++ Primer p.1195。
pop_heap(c.begin(), c.end(), comp);
c.pop_back();
}
__STL_UNWIND(c.clear());
}
};
// no equality is provided
__STL_END_NAMESPACE
#endif /* __SGI_STL_INTERNAL_QUEUE_H */
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// mode:C++
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