作为用户定义数据类型的具有密钥的C++映射-yiteyi-C++库

C++映射以有序的形式存储密钥（请注意，它在内部使用自平衡二进制搜索树）。排序是使用运算符“

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让我们考虑一下地图以键数据类型为结构，映射值为整数。

// key's structure
struct key
{
    int a;
};

                     // CPP program to demonstrate how a map can                   
                     // be used to have a user defined data type                   
                     // as key.                   
                     #include <bits/stdc++.h>                   
                     using                               namespace                               std;                   
                     struct                               Test {                   
                                         int                               id;                   
                     };                   
                             
                     // We compare Test objects by their ids.                   
                     bool                               operator<(                               const                               Test& t1,                               const                               Test& t2)                   
                     {                   
                                         return                               (t1.id < t2.id);                   
                     }                   
                             
                     // Driver code                   
                     int                               main()                   
                     {                   
                                         Test t1 = { 110 }, t2 = { 102 }, t3 = { 101 }, t4 = { 115 };                   
                             
                                         // Inserting above four objects in an empty map                   
                                         map<Test,                               int                               > mp;                   
                                         mp[t1] = 1;                   
                                         mp[t2] = 2;                   
                                         mp[t3] = 3;                   
                                         mp[t4] = 4;                   
                             
                                         // Printing Test objects in sorted order                   
                                         for                               (                               auto                               x : mp)                   
                                         cout << x.first.id <<                               " "                               << x.second << endl;                   
                                         return                               0;                   
                     }                   

输出：

我们还可以使

                     // With < operator defined as member method.                   
                     #include <bits/stdc++.h>                   
                     using                               namespace                               std;                   
                     struct                               Test {                   
                                         int                               id;                   
                             
                                         // We compare Test objects by their ids.                   
                                         bool                               operator<(                               const                               Test& t)                               const                   
                                         {                   
                                         return                               (                               this                               ->id < t.id);                   
                                         }                   
                     };                   
                             
                     // Driver code                   
                     int                               main()                   
                     {                   
                                         Test t1 = { 110 }, t2 = { 102 }, t3 = { 101 }, t4 = { 115 };                   
                             
                                         // Inserting above four objects in an empty map                   
                                         map<Test,                               int                               > mp;                   
                                         mp[t1] = 1;                   
                                         mp[t2] = 2;                   
                                         mp[t3] = 3;                   
                                         mp[t4] = 4;                   
                             
                                         // Printing Test objects in sorted order                   
                                         for                               (                               auto                               x : mp)                   
                                         cout << x.first.id <<                               " "                               << x.second << endl;                   
                                         return                               0;                   
                     }                   

输出：

如果我们不让操作员超负荷，会发生什么？ 如果我们试图在映射中插入任何内容，就会出现编译器错误。

                     #include <bits/stdc++.h>                   
                     using                               namespace                               std;                   
                     struct                               Test {                   
                                         int                               id;                   
                     };                   
                             
                     // Driver code                   
                     int                               main()                   
                     {                   
                                         map<Test,                               int                               > mp;                   
                                         Test t1 = {10};                   
                                         mp[t1] = 10;                   
                                         return                               0;                   
                     }                   

输出：

/usr/include/c++/5/bits/stl_function.h:387:20: error: no match for 'operator<' (operand types are 'const Test' and 'const Test')
       { return __x < __y; }

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