【C++ Primer】【学习笔记】【第十章】关联容器之：pair类型

偏执的太偏执、 2022-08-06 09:18 181阅读 0赞

pair类型和容器一样，也是一种模板类型。

**一、pair类型对象的创建和初始化**

创建pair对象时，必须提供两个类型名。如下：

pair<string, string> anon; // 调用默认构造函数对其成员采用值初始化

pair<string, int> word\_count; // 调用默认构造函数对其成员采用值初始化

pair<string, vector<int> > line; // 调用默认构造函数对其成员采用值初始化

pair <string, string> author("James", "Joyce"); // 定义时为每个成员提供初始化式

由于pair类型的声明比较繁琐，一般使用typedef简化其声明：

typedef pair<string, string> Author;

Author author("James", "Joyce");

**二、pair对象的操作**

pair类型对象提供的操作定义在头文件utility当中，其成员都是公有的，分别命名为first和second。提供的操作如下：

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     <span style="background-color:inherit">操作</span> 
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      pair&lt;T1, T2&gt; p1; 
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      创建一个空的pair对象，两个元素的类型分别是T1和T2，采用值初始化。 
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     <span style="font-family:微软雅黑; font-size:14px">pair&lt;T1, T2&gt; p1(v1, v2);</span> 
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    <div style="min-width:2px; background-color:inherit"> 
     <span style="font-family:微软雅黑; font-size:14px">创建一个pair对象，两个元素的类型分别是T1和T2，其中first成员初始化为v1，second成员初始化为v2。</span> 
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     <span style="font-family:微软雅黑; font-size:14px">pair&lt;T1, T2&gt;(v1, v2);</span> 
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    <div style="min-width:2px; background-color:inherit"> 
     <span style="font-family:微软雅黑; font-size:14px">创建一个pair对象，两个元素的类型分别是T1和T2，其中first成员初始化为v1，second成员初始化为v2。</span> 
     <span style="font-family:微软雅黑; font-size:14px"><span style="color:#ff0000; background-color:inherit">该对象仅作为右值。</span></span> 
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      make_pair(v1, v2); 
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      以v1和v2值创建一个新的pair对象，其元素类型分别是v1和v2的类型。 
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      p1 &lt; p2 
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      对象p1如果小于p2，则返回true。 
     <span style="font-family:微软雅黑; font-size:14px">比较遵循字典次序，即先比较first成员，若first相等则比较second成员。</span> 
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   <td style="word-break:break-all; border:1px solid rgb(153,153,153); padding:5px 10px; min-height:25px; min-width:25px; height:69px; background-color:inherit; width:279px"> p1 == p2</td> 
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      对象p1如果等于p2，则返回true。 
     <span style="font-family:微软雅黑; font-size:14px">比较遵循字典次序，即first成员相等，同时second成员也相等。</span> 
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      p.first 
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      返回p中名为first的数据成员。 
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      p.second 
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     <span style="font-family:微软雅黑; font-size:14px">返回p中名为second的数据成员。</span> 
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**习题10.1：使用容器存储pair对象**

#include <iostream>
    #include <string>
    #include <utility>
    #include <vector>
    using namespace std;
    
    int main()
    {
        string str;
        int ival;
        pair<string, int> sipair;
        vector<pair<string, int> > pvec;
    
        cout << "The size of vector is: " << pvec.size() << endl;
        cout << "Enter a string and an integer(Ctrl + Z or Ctrl + D to end):" << endl;
        while (cin >> str >> ival)
        {
            sipair = make_pair(str, ival);
            pvec.push_back(sipair);
        }
        cout << "The size of vector is: " << pvec.size() << endl;
    
        return 0;
    }

[chapter10]$ ./a.out 
    The size of vector is: 0 
    Enter a string and an integer(Ctrl + Z or Ctrl + D to end): 
    aaa 1 bbb 2 ccc 3 
    The size of vector is: 3

**习题10.2：使用容器存储pair对象（其他方法创建pair对象）**

方法二：

pair<string, int> sipair; 
        while (cin >> str >> ival)
        {
            sipair.first = str;
            sipair.second = ival;
            pvec.push_back(sipair);
        }

方法三：

while (cin >> str >> ival)
        {
            pair<string, int> sipair(str, ival);
            pvec.push_back(sipair);
        }

方法四：

while (cin >> str >> ival)
        {
            pvec.push_back(pair<string, int>(str, ival));
        }