Effective STL 阅读笔记： Item 3: Make copying cheap and correct for objects in containers
gentoo emerge unable to sync

Effective STL 阅读笔记： Item 4 ~ 5: Call empty instead of checking size() against zero.

tubo posted @ 2014年9月03日 00:10 in 未分类 , 658 阅读

 

1 Item 4: Call empty instead of checking size() against zero

简而言之，用 container.size() 来检查 container 是否为空，而不要用 /container.size() == 0/。

原因： empty is a constant-time operation for all standard containers, but for some list implementations, size takes linear time.

2 Item 5: Prefer range member functions to their single-element counterparts

如果 Container 提供了区间成员函数 (Range member functions)，尽量用它来替代自己写的循环和 algrithm 中的 copy 。

A range member function is a member function that, like STL algorithms, uses two iterator parameters to specify a range of elements over which something should be done.

Range member function 的好处在于：

  • 代码量更少，更清晰、直观。
  • 有可能会减少申请内存的次数, 减少数据的移动次数。

Range member function 适用的几种情况：

  1. 容器的构造： 所有标准容器都提供该方法： 
    container::container(InputIterator begin, // Beginning of range
                     InputIterator end);  // End of range
  2. 区间插入： 所有标准容器都提供该方法： 
    void container::insert(iterator      position, // where to insert the range
                       InputIterator begin,    // start of range to insert
                       InputIterator end);     // end of range to insert

    由于关联容器 (Associative Containers) 有自己的方法来计算插入的位置，insert 方法的第一个 参数经常可以省略：

    void container::insert(lnputIterator begin, InputIterator end);
  3. 区间删除 (Range Eraure):

    所有标准容器都提供了区间删除方法，但返回值有所不同：

    • Sequence Container: 
      iterator container::erase(iterator begin, iterator end);
    • Associative Container: 
      void container::erase(iterator begin, iterator end);
  4. 区间赋值 (Range Assignment)： 所有 Container 均提供： 
    void container::assign(lnputIterator begin, InputIterator end);
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