手把手教你调试STL容器(上)

众所周知，调试(Debugging)是每个程序员所要必备的基本的技术素养，尤其是对C/C++的程序员来说。对于在linux下用C/C++开发的朋友，相信对GDB不会陌生，当程序有bug或者是出现core dump的时候，GDB是我们最好的朋友。STL是C++相较于C而言，增加的非常强有力的工具，它从某种程度上把C/C++程序员从繁琐的基本数据结构中解放了出来。不过，STL虽然用起来十分方便，但是，用GDB调试过C/C++程序的朋友都有这样痛苦的经历，在GDB状态下，要知道某个STL对象（比如容器）中的数据内容，并不是那么直接、简单。本文的主要内容就是介绍STL中大家比较常用到的容器的基本组成，帮助大家能够在调试的时候更好的驾驭它们。

1. string

string是STL中最为常用的类型，它是模板类basic_string用char类型特化后的结果，下面我们来看一下string类型的基本组成：

 

01<SPAN class=kwd>typedef</SPAN><SPANclass=pln> basic_string</SPAN><SPANclass=str><char></SPAN><SPANclass=pln>    </SPAN><SPANclass=kwd>string</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

02  

03</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Rep_base</SPAN><SPANclass=pln>

04</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

05    size_type     _M_length</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

06    size_type     _M_capacity</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

07    </SPAN><SPANclass=typ>_Atomic_word</SPAN><SPANclass=pln>  _M_refcount</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

08</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

09  

10</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Alloc_hider</SPAN><SPANclass=pln>

11</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

12    </SPAN><SPANclass=typ>_CharT</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_p</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln> 

13</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

14  

15</SPAN><SPAN class=kwd>mutable</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Alloc_hider</SPAN><SPANclass=pln>  _M_dataplus</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

16_M_data</SPAN><SPAN class=pun>()</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>const</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>{</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>return</SPAN><SPANclass=pln>  _M_dataplus</SPAN><SPANclass=pun>.</SPAN><SPANclass=pln>_M_p</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>}</SPAN><SPANclass=pln>

17</SPAN><SPAN class=typ>_Rep</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_rep</SPAN><SPANclass=pun>()</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>const</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>{</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>return</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>&((</SPAN><SPANclass=kwd>reinterpret_cast</SPAN><SPANclass=pun><</SPAN><SPANclass=pln>_rep </SPAN><SPANclass=pun>*></SPAN><SPANclass=pln> </SPAN><SPANclass=pun>(</SPAN><SPANclass=pln>_M_data</SPAN><SPANclass=pun>()))[-</SPAN><SPANclass=lit>1</SPAN><SPANclass=pun>]);</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>}</SPAN>

从上面来看，string只有一个成员_M_dataplus。但是这里需要注意的是，string类在实现的时候用了比较巧的方法，在_M_dataplus._M_p中保存了用户的数据，在_M_dataplus._M_p的第一个元素前面的位置，保存了string类本身所需要的一些信息rep。这样做的好处，一方面不增加string类的额外开销，另一方面可以保证用户在调试器(GDB)中用_M_dataplus._M_p查看数据内容的时候，不受干扰。用户可以通过reinterpret_cast<_rep *> (_M_data()))[-1])来查看rep相关的信息，也可以调用_M_rep()函数来查看。

2. vector

vector同样也是stl中最为常用类型，下面我们来看一下vector类型的基本组成:

 

1<SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Vector_impl</SPAN><SPANclass=pln>

2</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

3    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln>           _M_start</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

4    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln>           _M_finish</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

5    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln>           _M_end_of_storage</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

6</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

7  

8</SPAN><SPAN class=typ>_Vector_impl</SPAN><SPANclass=pln> _M_impl</SPAN><SPANclass=pun>;</SPAN>

vector本身很简单，就是一个动态数组。它只有一个数组成员_M_impl，用户可以通过_M_impl来查看vector内容的数据成员，具体包括动态数组的超始地址_M_impl._M_start，结束地址_M_impl._M_end_of_storage，数组内容的结束地址_M_impl._M_finish

3. list

list是STL中的双向链表结构，也是大家经常用来的，下面我们一起来看一下list的基本组成：

 

01<SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pln>

02</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

03    </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_next</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>   

04    </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_prev</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>   

05</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

06  

07</SPAN><SPAN class=kwd>template</SPAN><SPANclass=pun><</SPAN><SPANclass=kwd>typename</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>></SPAN><SPANclass=pln>

08</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_List_node</SPAN><SPANclass=pln> </SPAN><SPANclass=pun>:</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>public</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pln>

09</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

10    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pln> _M_data</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>               

11</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

12  

13</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_List_impl</SPAN><SPANclass=pln>

14</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

15    </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pln> _M_node</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

16</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

17  

18</SPAN><SPAN class=typ>_List_impl</SPAN><SPANclass=pln> _M_impl</SPAN><SPANclass=pun>;</SPAN>

通过上面的代码，我们可以看出，list本身只保留了一个空结点_M_impl._M_node，用来标示list的header，新加入的第一个结点，是直接链到_M_imp._M_node._M_next上的，后面的依次类推。我们可以把_M_impl._M_node.m_next强制转型成_List_node类型，然后通过_M_data来查看具体的数据内容，即((_List_node<T> *)(_M_impl._M_node->M_next))->M_data (T是数据的类型)。
我们知道,string和vector都是线型结构，只要知道数据的起始地址和容器大小，便可获取到所以的元素内容。但是list是非线型结构，这时候就需要iterator来帮忙了，下面我们来看下list的iterator的基本组成：

 

1<SPAN class=kwd>template</SPAN><SPANclass=pun><</SPAN><SPANclass=kwd>typename</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>></SPAN><SPANclass=pln>

2</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_List_iterator</SPAN><SPANclass=pln>

3</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

4     </SPAN><SPANclass=typ>_List_node_base</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_node</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

5</SPAN><SPAN class=pun>};</SPAN>

从上面的代码，我们可以看出，list的iterator只是保存了一个list结点的指针，有此足矣，通过它我们便可以获取到iter里面的数据内容：((_List_node<T> *)(iter->_M_node)->M_data (T是数据的类型)

4. deque

deque是STL中提供一个队列结构，它同时兼有vector和list的基本特点，因此实现要复杂一些，下面我们来看一下它的基本组成：

 

01<SPAN class=kwd>template</SPAN><SPANclass=pun><</SPAN><SPANclass=kwd>typename</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>></SPAN><SPANclass=pln>

02</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Deque_iterator</SPAN><SPANclass=pln>

03</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

04    </SPAN><SPANclass=kwd>typedef</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>**</SPAN><SPANclass=pln>   </SPAN><SPANclass=typ>_Map_pointer</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

05  

06    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_cur</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

07    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_first</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

08    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*</SPAN><SPANclass=pln> _M_last</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

09    </SPAN><SPANclass=typ>_Map_pointer</SPAN><SPANclass=pln> _M_node</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

10</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

11  

12</SPAN><SPAN class=kwd>typedef</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Deque_iterator</SPAN><SPANclass=pun><</SPAN><SPANclass=pln>_tp </SPAN><SPANclass=pun>,</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>&,</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>*></SPAN><SPANclass=pln>  iterator</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

13</SPAN><SPAN class=kwd>struct</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Deque_impl</SPAN><SPANclass=pln>

14    </SPAN><SPANclass=pun>:</SPAN><SPANclass=pln> </SPAN><SPANclass=kwd>public</SPAN><SPANclass=pln> </SPAN><SPANclass=typ>_Tp_alloc_type</SPAN><SPANclass=pln>

15</SPAN><SPAN class=pun>{</SPAN><SPANclass=pln>

16    </SPAN><SPANclass=typ>_Tp</SPAN><SPANclass=pun>**</SPAN><SPANclass=pln>    _M_map</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

17    </SPAN><SPANclass=typ>size_t</SPAN><SPANclass=pln>   _M_map_size</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

18    iterator _M_start</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

19    iterator _M_finish</SPAN><SPANclass=pun>;</SPAN><SPANclass=pln>

20</SPAN><SPAN class=pun>};</SPAN><SPANclass=pln>

21</SPAN><SPAN class=typ>_Deque_impl</SPAN><SPANclass=pln> _M_impl</SPAN><SPANclass=pun>;</SPAN>

从上面的代码，及STL的源码，我们可以了解到，deque是由多块连续buffer，通过一个中控的数组_M_map来链在一起，实现的。我们可以通过_M_map，获取到每一个块的起始地址，在每一块内的数据，地址都是连续的，可以像数组一起取出对应的数据内容。
另外，我们还可以通过iterator来访问deque的元素，上面的代码中，iterator的_M_cur是指向当前元素的指针，_M_first是当前块的起始地址，_M_last当前块的结束地址。
本篇是〈手把手教你调试STL容器〉的上篇，主要介绍了string, vector, list和deque这些基本的容器，下篇将介绍map/set/multimap/multiset, hash_map/hash_set/hash_multimap/hash_multiset，敬请期待！

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