【C++ Primer】【学习笔记】【第三章】标准库类型之：vector类型

旧城等待， 2022-08-06 03:27 158阅读 0赞

标准库vector类型

vector称为容器，因为它可以包含其他对象，一个容器中的所有对象都必须是同一种类型的。

vector不是一种数据类型，而是一个类模板，可用来定义任意多种数据类型。如：vector<int>、vector<string>都是数据类型。

对于size\_type类型，写法vector::size\_type是错误的，必须写成vector<T>::size\_type。

1、几种初始化vector对象的方式如下：

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     <span style="background-color:inherit">方式</span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit">vector&lt;T&gt; v1;</span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit">vector保存类型为T的对象，默认构造函数v1为空。</span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit"><span style="font-size:14.399999618530273px">vector&lt;T&gt; v2(v1);</span></span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit">v2是v1的一个副本。</span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit"><span style="font-size:14.399999618530273px">vector&lt;T&gt; v3(n, i);</span></span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit">v3包含n个值为i的元素。</span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit"><span style="font-size:14.399999618530273px">vector&lt;T&gt; v4(n);</span></span> 
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     <span style="font-family:Microsoft Yahei; background-color:inherit">v4含有n个相同值的元素，元素的值根据T来确定。如果T为int则值为0；如果T为string则值为""。</span> 
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2、vector对象的操作

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     <span style="background-color:inherit">操作</span> 
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      v.empty() 
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      判断v是否为空，是则返回true，否则返回false 
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      v.size() 
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      返回v中元素的个数；该函数的返回类型为 
     <span style="background-color:inherit">vector&lt;T&gt;::size_type</span>，为一个无符号数，至于是unsigned int还是unsigned long由机器决定。 
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      v.push_back(t) 
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      在v的末尾增加一个值为t的元素。 
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      v[n] 
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      返回v中位置为n的字符，位置从0开始计数。这里的下标n的类型最好也使用 
     <span style="background-color:inherit">string&lt;T&gt;::size_type</span>。 
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      s1 + s2 
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      s1 + "string" 
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      将s1和s2连接成一个新的字符串，并返回新生成的字符串。 
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      字符串进行连接时，+操作符的左右操作数必须至少有一个是string类型的（即：两个字符串字面值相加是非法的），并且+操作返回一个新的string对象。 
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      s1 = s2 
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      把s1的内容替换成s2的副本 
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      s1 == s2 
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      比较s1和s2的内容，相等则返回true，不等则返回false。这里相等的含义是：长度相同，并且含有相同的字符。 
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      !=, &gt;, &gt;=, &lt;, &lt;= 
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      保持惯有含义，参考 == 
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迭代器简介

迭代器是一种检查容器内元素并遍历元素的数据类型。

标准库为每一种标准容器（包括vector）都定义了一种迭代器类型，迭代器类型提供了比下标操作更通用化的方法，即：所有的标准库容器都定义了相应的迭代器类型，而只有少数的容器支持下标操作。C++程序更倾向于推荐使用迭代器而不是下标操作。

1、容器的iterator类型

每种容器类型都定义了自己的迭代器类型，比如：vector迭代器类型表示为：

vector<int>::iterator iter;

其中，vector<int>::iterator为迭代器类型，iter为由迭代器类型定义的迭代器。

2、begin和end操作

每种容器都定义了begin函数和end函数，这两个函数均返回迭代器。如果容器为空，则begin返回的迭代器和end返回的迭代器相同；如果容器不为空，则begin返回的迭代器指向容器的第一个元素，end返回的迭代器指向容器的最后一个元素的下一个元素，此时，end返回的迭代器也被称为超出末端迭代器。

3、迭代器的算术操作

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     <span style="background-color:inherit">支持情况</span> 
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      iter + n 
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      支持 
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      iter - n 
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      支持 
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      iter1 - iter2 
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      支持，用于计算两个迭代器对象的距离，结果类型为difference_type，是signed类型的值 
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      iter1 + iter2 
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正确： vector<int>::iterator mid = ivec.begin() + ivec.size() / 2;

错误： vector<int>::iterator mid = (ivec.begin() + ivec.end()) / 2;

4、容器的const\_iterator类型

与容器的iterator类型对应，const\_iterator类型表示只能用于读取容器内元素，但是不能改变其值。即：当对const\_iterator类型解引用时，得到的是一个指向const对象的引用。

例如：对于 vector<int>::const\_iterator iter; 迭代器iter本身的值是可以改变的，但其指向的元素是不能改变的。对应C语言，就好像是定义：const int \*iter;

4、const\_iterator对象与const的iterator对象之间的区别

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     <span style="background-color:inherit">说明</span> 
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      const_iterator对象 
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      vector&lt;int&gt;::const_iterator iter; 
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      好比C语言中的： 
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      const int *iter; 或&nbsp; 
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      int const *iter; 
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      const的iterator对象 
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      vector&lt;int&gt; nums(10); 
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      const vector&lt;int&gt;::itreator cit = num.begin(); 
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      好比C语言中的： 
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      int nums[10]; 
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      int * const cit = &amp;num[0]; 
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5、容器本身可以是const的或者是非const的，对应写法如下：

vector<int> numbers(10, 9); // 非const的容器

const vector<int> numbers(10, 9); // const的容器

对于非const的容器，可以使用vector<int>::iterator定义的迭代器进行访问，也可以使用vector<int>::const\_iterator定义的迭代器进行访问。

对于const的容器，则只能使用 vector<int>::const\_iterator定义的迭代器进行访问。