STL之deque双向队列

deque双向队列是一种双向开口的连续线性空间，可以高效的在头尾两端插入和删除元素，提供随机访问，deque在接口上和vector非常相似，下面列出deque的常用成员函数：

Table 6.9. Constructors and Destructor of Deques

Operation	Effect
deque<Elem> c	Creates an empty deque without any elements
deque<Elem> c1(c2)	Creates a copy of another deque of the same type (all elements are copied)
deque<Elem> c(n)	Creates a deque with n elements that are created by the default constructor
deque<Elem> c(n,elem)	Creates a deque initialized with n copies of element elem
deque<Elem> c(beg,end)	Creates a deque initialized with the elements of the range [beg,end)
c.~deque<Elem>()	Destroys all elements and frees the memory

Table 6.10. Nonmodifying Operations of Deques

Operation	Effect
c.size()	Returns the actual number of elements
c.empty ()	Returns whether the container is empty (equivalent to size()==0, but might be faster)
c.max_size()	Returns the maximum number of elements possible
c1 == c2	Returns whether c1 is equal to c2
c1 != c2	Returns whether c1 is not equal to c2 (equivalent to ! (c1==c2))
c1 < c2	Returns whether c1 is less than c2
c1 > c2	Returns whether c1 is greater than c2 (equivalent to c2<c1)
c1 <= c2	Returns whether c1 is less than or equal to c2 (equivalent to ! (c2<c1) )
c1 >= c2	Returns whether c1 is greater than or equal to c2 (equivalent to ! (c1<c2) )
c.at(idx)	Returns the element with index idx (throws range error exception if idx is out of range)
c[idx]	Returns the element with index idx (no range checking)
c.front()	Returns the first element (no check whether a first element exists)
c.back()	Returns the last element (no check whether a last element exists)
c.begin()	Returns a random access iterator for the first element
c.end()	Returns a random access iterator for the position after the last element
c.rbegin()	Returns a reverse iterator for the first element of a reverse iteration
c.rend()	Returns a reverse iterator for the position after the last element of a reverse iteration

Table 6.11. Modifying Operations of Deques

Operation	Effect
c1 = c2	Assigns all elements of c2 to c1
c.assign (n,elem)	Assigns n copies of element elem
c.assign (beg,end)	Assigns the elements of the range [beg,end)
c1.swap(c2)	Swaps the data of c1 and c2
swap(c1,c2)	Same (as global function)
c.insert (pos,elem)	Inserts at iterator position pos a copy of elem and returns the position of the new element
c. insert (pos,n,elem)	Inserts at iterator position pos n copies of elem (returns nothing)
c.insert (pos,beg,end)	Inserts at iterator position pos a copy of all elements of the range [beg,end) (returns nothing)
c.push_back (elem)	Appends a copy of elem at the end
c.pop_back()	Removes the last element (does not return it)
c.push_front (elem)	Inserts a copy of elem at the beginning
c.pop_front()	Removes the first element (does not return it)
c.erase(pos)	Removes the element at iterator position pos and returns the position of the next element
c.erase (beg,end)	Removes all elements of the range [beg,end) and returns the position of the next element
c. resize (num)	Changes the number of elements to num (if size () grows, new elements are created by their default constructor)
c.resize (num, elem)	Changes the number of elements to num (if size () grows, new elements are copies of elem)
c.clear()	Removes all elements (makes the container empty)

　　deque的实现比较复杂，内部会维护一个map（注意！不是STL中的map容器）即一小块连续的空间，该空间中每个元素都是指针，指向另一段（较大的）区域，这个区域称为缓冲区，缓冲区用来保存deque中的数据。因此deque在随机访问和遍历数据会比vector慢。具体的deque实现可以参考《STL源码剖析》，当然此书中使用的SGI STL与VS2008所使用的PJ STL的实现方法还是有区别的。下面给出了deque的结构图：

由于篇幅问题，deque的实现细节就不再深入了，下面给出deque的使用范例：

// cont/deque1. cpp

   #include <iostream>
   #include <deque>
   #include <string>
   #include <algorithm>
   using namespace std;

   int main()
   {

       //create empty deque of strings
       deque<string> coll;

       //insert several elements
       coll.assign (, string("string"));
       coll.push_back ("last string");
       coll.push_front ("first string");

       //print elements separated by newlines
       copy (coll.begin(), coll.end(),
             ostream_iterator<string>(cout,"\n"));
       cout << endl;

       //remove first and last element
       coll.pop_front();
       coll.pop_back();

       //insert ''another'' into every element but the first
       for (int i=; i<coll.size(); ++i) {
           coll[i] = "another " + coll [i];

       }

       //change size to four elements
       coll.resize (, "resized string");

       //print elements separated by newlines
       copy (coll.begin(), coll.end(),
             ostream_iterator<string>(cout,"\n"));

   }

The program has the following output:

   first string
   string
   string
   string
   last string

   string
   another string
   another string
   resized string