STL源码剖析读书笔记第4章

第4章　　序列式容器

容器是个大东西，还是带着问题分析好。

序列式容器里，我用的最多的还是vector。

1.我知道vector中有内存有预先分配，这种机制是好的，但有时候会浪费内存。

2.重点分析vector初始化大小，resize，erase等

vector容器

4.1.vector(n,v),使用这个构造函数可以指定含有n个v值的元素

看下源码：

/**
        *  @brief  Creates a %vector with copies of an exemplar element.
        *  @param  n  The number of elements to initially create.
        *  @param  value  An element to copy.
        *  @param  a  An allocator.
        *
        *  This constructor fills the %vector with @a n copies of @a value.
        */
       explicit
       vector(size_type __n, const value_type& __value = value_type(),
          const allocator_type& __a = allocator_type())
       : _Base(__n, __a)
       { _M_fill_initialize(__n, __value); }
 
       // Called by the first initialize_dispatch above and by the
       // vector(n,value,a) constructor.
       void
       _M_fill_initialize(size_type __n, const value_type& __value)
       {
     std::__uninitialized_fill_n_a(this->_M_impl._M_start, __n, __value, 
                       _M_get_Tp_allocator());
     this->_M_impl._M_finish = this->_M_impl._M_end_of_storage;
       }

最后一句this->_M_impl._M_finish = this->_M_impl._M_end_of_storage;

说明内存申请不会多不会少，刚刚好。

4.2.那么resize呢

看源码：

/**
       *  @brief  Resizes the %vector to the specified number of elements.
       *  @param  new_size  Number of elements the %vector should contain.
       *  @param  x  Data with which new elements should be populated.
       *
       *  This function will %resize the %vector to the specified
       *  number of elements.  If the number is smaller than the
       *  %vector's current size the %vector is truncated, otherwise
       *  the %vector is extended and new elements are populated with
       *  given data.
       */
      void
      resize(size_type __new_size, value_type __x = value_type())
      {
    if (__new_size < size())
      _M_erase_at_end(this->_M_impl._M_start + __new_size);
    else
      insert(end(), __new_size - size(), __x);
      }
//你懂的
void
      clear()
      { _M_erase_at_end(this->_M_impl._M_start); }
//insert有好多，别找错了
 /**
       *  @brief  Inserts a number of copies of given data into the %vector.
       *  @param  position  An iterator into the %vector.
       *  @param  n  Number of elements to be inserted.
       *  @param  x  Data to be inserted.
       *
       *  This function will insert a specified number of copies of
       *  the given data before the location specified by @a position.
       *
       *  Note that this kind of operation could be expensive for a
       *  %vector and if it is frequently used the user should
       *  consider using std::list.
       */
      void
      insert(iterator __position, size_type __n, const value_type& __x)
      { _M_fill_insert(__position, __n, __x); }

clear没有改变capacity这个大家都知道。所以resize小了，capacity也不会小掉。

里面的 Note that this kind of operation could be expensive for a
       *  %vector and if it is frequently used the user should
       *  consider using std::list.

值得玩味，如果能看到invert后start和finsh的指针位置发生改变，就可以证明_M_fill_insert有开辟新的空间。

这个_M_fill_insert(iterator __pos, size_type __n, const value_type& __x);

实现在vector.tcc里，比较复杂，就不深究了

4.3.push_back新增元素，如果vector中size==capacity，则会开辟一个双倍大小的连续新空间，将原来的数据拷贝过去，插入新元素。

给个小例子：

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int n = 16;
    vector<int> temp;
    for (int i = 1; i <= n; i ++)
    {
        cout << "before push, n: " << i << " capacity: " << temp.capacity() << endl;
        temp.push_back(i);
        cout << "after  push, n: " << i << " capacity: " << temp.capacity() << endl;
    }
    return 0;

 源码：

/**
       *  @brief  Add data to the end of the %vector.
       *  @param  x  Data to be added.
       *
       *  This is a typical stack operation.  The function creates an
       *  element at the end of the %vector and assigns the given data
       *  to it.  Due to the nature of a %vector this operation can be
       *  done in constant time if the %vector has preallocated space
       *  available.
       */
      void
      push_back(const value_type& __x)
      {
    if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
      {
        this->_M_impl.construct(this->_M_impl._M_finish, __x);
        ++this->_M_impl._M_finish;
      }
    else
      _M_insert_aux(end(), __x);
      }

_M_insert_aux没找到实现，不过书上有。总之push_back慎用，还是resize在赋值好。

4.4.erase函数

template<typename _Tp, typename _Alloc>
    typename vector<_Tp, _Alloc>::iterator
    vector<_Tp, _Alloc>::
    erase(iterator __position)
    {
      if (__position + 1 != end())
    _GLIBCXX_MOVE3(__position + 1, end(), __position);
      --this->_M_impl._M_finish;
      this->_M_impl.destroy(this->_M_impl._M_finish);
      return __position;
    }

源码有点怪，反正要注意的是erase会返回一个iterator，指向被删除的下一个。

list容器

 list容器erase时只有被删除的那个iterator无效，其它的都有效，这点也在情理之中。其它的就不讲了。

deque容器

这个容器还真没用过，书上说deque由一段一段的定量连续空间构成。

一旦有必要在deque的前端或尾端增加新空间，便配置一段定量连续空间，

串接在整个deque的头端或尾端。deque的最大任务，

便是在这些分段的定量连续空间上，维护其整体连续的假象。

并提供随机存取的结果。避开了像Vector那样“重新配置，复制，释放”的轮回，

代价则是复杂的迭代器架构。具体细节以后用到再看，先略过。

stack容器

 stack是容器适配器，就是基于其它容器封装出来的。

默认stack用的是deque，我们可以用vectror,list指定。

比如 stack<int, vector<int> >，除此之外，stack不提供iterator。

queue容器

queue也不允许遍历，即不提供iterator。

queue也是容器适配器，默认基于deque构造。同样我们可以指定vector或list