容器适配器（stack、 queue 、priority_queue）源码浅析与使用示例

2022-10-18 08:12:35

一、容器适配器
stack
queue
priority_queue

stack、queue、priority_queue 都不支持任一种迭代器，它们都是容器适配器类型，stack是用vector/deque/list对象创建了一个先进后出容器；queue是用deque或list对象创建了一个先进先出容器；priority_queue是用vector/deque创建了一个排序队列，内部用二叉堆实现。

前面或多或少谈到过list/vector的实现，而没提到过deque的实现，可以用以下一句话概括，具体可以看看《stl源码剖析》：

Storing contents in multiple smaller arrays, allocating additional arrays at the beginning or end as needed.
Indexing is implemented by keeping a dynamic array containing pointers to each of the smaller arrays.

（一）、stack

首先来看示例代码：

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#include <iostream>
#include <vector>
#include <list>
#include <stack>

using namespace std;

int main(void)
{
    stack<int, list<int> > s;
    for (int i = 0; i < 5; i++)
    {
        s.push(i);
    }

//for (size_t i=0; i<s.size(); i++)
    //{
    //  cout<<s.top()<<' ';   Error:size()一直在变化
    //  s.pop();
    //}

while (!s.empty())
    {
        cout << s.top() << ' ';
        s.pop();
    }
    cout << endl;
    return 0;
}

再看stack 的源码：

C++ Code

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// TEMPLATE CLASS stack
template < class _Ty,
         class _Container = deque<_Ty> >
class stack
{
    // LIFO queue implemented with a container
public:
    typedef _Container container_type;
    typedef typename _Container::value_type value_type;
    typedef typename _Container::size_type size_type;
    typedef typename _Container::reference reference;
    typedef typename _Container::const_reference const_reference;

stack()
        : c()
    {
        // construct with empty container
    }

explicit stack(const _Container &_Cont)
        : c(_Cont)
    {
        // construct by copying specified container
    }

bool empty() const
    {
        // test if stack is empty
        return (c.empty());
    }

size_type size() const
    {
        // test length of stack
        return (c.size());
    }

reference top()
    {
        // return last element of mutable stack
        return (c.back());
    }

const_reference top() const
    {
        // return last element of nonmutable stack
        return (c.back());
    }

void push(const value_type &_Val)
    {
        // insert element at end
        c.push_back(_Val);
    }

void pop()
    {
        // erase last element
        c.pop_back();
    }

const _Container &_Get_container() const
    {
        // get reference to container
        return (c);
    }

protected:
_Container c; // the underlying container
};

即有一个_Container 成员，默认是deque<_Ty> ，当然也可以传递vector, list 进去，只要支持push_back，pop_back 等接口。内部的函数实现

都借助了容器的函数，跟以前实现过的Stack 很像。

（二）、queue

先来看示例代码：

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#include <iostream>
#include <vector>
#include <list>
#include <stack>
#include <queue>

using namespace std;

int main(void)
{
//int a[] = {1, 2, 3, 4, 5};
//vector<int> v(a, a+5);

queue<int, list<int> > q;
    for (int i = 0; i < 5; i++)
    {
        q.push(i);
    }

while (!q.empty())
    {
        cout << q.front() << ' ';
        q.pop();
    }

cout << endl;

return 0;
}

再来看queue 源码：

C++ Code

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// TEMPLATE CLASS queue
template < class _Ty,
         class _Container = deque<_Ty> >
class queue
{
    // FIFO queue implemented with a container
public:
    typedef _Container container_type;
    typedef typename _Container::value_type value_type;
    typedef typename _Container::size_type size_type;
    typedef typename _Container::reference reference;
    typedef typename _Container::const_reference const_reference;

queue()
        : c()
    {
        // construct with empty container
    }

explicit queue(const _Container &_Cont)
        : c(_Cont)
    {
        // construct by copying specified container
    }

bool empty() const
    {
        // test if queue is empty
        return (c.empty());
    }

size_type size() const
    {
        // return length of queue
        return (c.size());
    }

reference front()
    {
        // return first element of mutable queue
        return (c.front());
    }

const_reference front() const
    {
        // return first element of nonmutable queue
        return (c.front());
    }

reference back()
    {
        // return last element of mutable queue
        return (c.back());
    }

const_reference back() const
    {
        // return last element of nonmutable queue
        return (c.back());
    }

void push(const value_type &_Val)
    {
        // insert element at beginning
        c.push_back(_Val);
    }

void pop()
    {
        // erase element at end
        c.pop_front();
    }

const _Container &_Get_container() const
    {
        // get reference to container
        return (c);
    }

protected:
_Container c; // the underlying container
};

实现跟stack 是很类似的，只是queue不能用vector 实现，因为没有pop_front 接口。

（三）、priority_queue

先来看示例代码：

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#include <iostream>
#include <functional>
#include <vector>
#include <list>
#include <stack>
#include <queue>

using namespace std;

int main(void)
{
int a[] = {5, 1, 2, 4, 3};
priority_queue<int, vector<int>, greater<int> > q(a, a + 5);

while (!q.empty())
    {
        cout << q.top() << ' ';
        q.pop();
    }

cout << endl;

return 0;
}

再来看priority_queue 的源码：

C++ Code

// TEMPLATE CLASS priority_queue
template < class _Ty,
         class _Container = vector<_Ty>,
         class _Pr = less<typename _Container::value_type> >
class priority_queue
{
    // priority queue implemented with a _Container
public:
    typedef _Container container_type;
    typedef typename _Container::value_type value_type;
    typedef typename _Container::size_type size_type;
    typedef typename _Container::reference reference;
    typedef typename _Container::const_reference const_reference;

priority_queue()
        : c(), comp()
    {
        // construct with empty container, default comparator
    }

explicit priority_queue(const _Pr &_Pred)
        : c(), comp(_Pred)
    {
        // construct with empty container, specified comparator
    }

priority_queue(const _Pr &_Pred, const _Container &_Cont)
        : c(_Cont), comp(_Pred)
    {
        // construct by copying specified container, comparator
        make_heap(c.begin(), c.end(), comp);
    }

template<class _Iter>
    priority_queue(_Iter _First, _Iter _Last)
        : c(_First, _Last), comp()
    {
        // construct by copying [_First, _Last), default comparator
        make_heap(c.begin(), c.end(), comp);
    }

template<class _Iter>
    priority_queue(_Iter _First, _Iter _Last, const _Pr &_Pred)
        : c(_First, _Last), comp(_Pred)
    {
        // construct by copying [_First, _Last), specified comparator
        make_heap(c.begin(), c.end(), comp);
    }

template<class _Iter>
    priority_queue(_Iter _First, _Iter _Last, const _Pr &_Pred,
                   const _Container &_Cont)
        : c(_Cont), comp(_Pred)
    {
        // construct by copying [_First, _Last), container, and comparator
        c.insert(c.end(), _First, _Last);
        make_heap(c.begin(), c.end(), comp);
    }

bool empty() const
    {
        // test if queue is empty
        return (c.empty());
    }

size_type size() const
    {
        // return length of queue
        return (c.size());
    }

const_reference top() const
    {
        // return highest-priority element
        return (c.front());
    }

reference top()
    {
        // return mutable highest-priority element (retained)
        return (c.front());
    }

void push(const value_type &_Pred)
    {
        // insert value in priority order
        c.push_back(_Pred);
        push_heap(c.begin(), c.end(), comp);
    }

void pop()
    {
        // erase highest-priority element
        pop_heap(c.begin(), c.end(), comp);
        c.pop_back();
    }

protected:
_Container c; // the underlying container
_Pr comp; // the comparator functor
};

priority_queue 的实现稍微复杂一点，可以传递3个参数，而且有两个成员，comp 即自定义比较逻辑，默认是less<value_type>，在构造函数中

调用make_heap函数构造二叉堆，comp 主要是用于构造二叉堆时的判别，如果是less
则构造大堆，如果传递greater 则构造小堆.

注意，priority_queue 不能用list 实现，因为list 只支持双向迭代器，而不支持随机迭代器。

下面举个例子说明make_heap 函数的用法：

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#include <iostream>
#include <functional>
#include <vector>
#include <list>
#include <stack>
#include <queue>

using namespace std;

int main(void)
{
int a[] = {5, 1, 2, 4, 3};
make_heap(a, a + 5, less<int>());

copy(a, a + 5, ostream_iterator<int>(cout, " "));
cout << endl;

sort(a, a + 5);
    //  sort_heap(a, a+5, less<int>());
    copy(a, a + 5, ostream_iterator<int>(cout, " "));
    cout << endl;

return 0;
}

输出：

5 4 2 1 3

1 2 3 4 5

make_heap() 将容器的元素构造成二叉堆，传递的是less，即构造的是大堆，把大堆层序遍历的结果存入数组，再调用sort()
进行排序，内部调用

的实际算法不一定，可以是堆排序、插入排序、选择排序等等，跟踪进去发现调用的是插入排序；当然也可以直接指定使用堆排序
sort_heap（调用

者必须已经是堆了，也就是前面已经先调用了make_heap，而且大小堆类型得匹配），与make_heap
一样，第三个参数传递的都是函数对象的用

法。sort 和 sort_heap 默认都是从小到大排序，除非重载的版本传递了第三个参数，如下，第三个参数可以是函数指针，也可以是函数对象：

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// order heap by repeatedly popping, using operator<
template<class _RanIt> inline
void sort_heap(_RanIt _First, _RanIt _Last);

// order heap by repeatedly popping, using _Pred
template < class _RanIt,
class _Pr > inline
void sort_heap(_RanIt _First, _RanIt _Last, _Pr _Pred);

传递greater 构造的是小堆，如下图所示：

参考：

C++ primer 第四版
Effective C++ 3rd
C++编程规范

码农公寓

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