C++ STL--映射--添加字符串作为键，添加类对象作为值

代码解释了如何插入pair to map，其中键类型是string，值是对类对象的引用。但是，有3个重载函数，例如重载=运算符、重载<运算符和重载==运算符。。谁能解释一下为什么这里超载

#include <iostream>
    #include <map>
    using namespace std;

    class AAA
    {
       friend ostream &operator<<(ostream &, const AAA &);

       public:
          int x;
          int y;
          float z;

          AAA();
          AAA(const AAA &);
          ~AAA(){};
          AAA &operator=(const AAA &rhs);
          int operator==(const AAA &rhs) const;
          int operator<(const AAA &rhs) const;
    };

    AAA::AAA()   // Constructor
    {
       x = 0;
       y = 0;
       z = 0;
    }

    AAA::AAA(const AAA &copyin)   // Copy constructor to handle pass by value.
    {                             
       x = copyin.x;
       y = copyin.y;
       z = copyin.z;
    }

    ostream &operator<<(ostream &output, const AAA &aaa)
    {
       output << aaa.x << ' ' << aaa.y << ' ' << aaa.z << endl;
       return output;
    }

    AAA& AAA::operator=(const AAA &rhs)
    {
       this->x = rhs.x;
       this->y = rhs.y;
       this->z = rhs.z;
       return *this;
    }

    int AAA::operator==(const AAA &rhs) const
    {
       if( this->x != rhs.x) return 0;
       if( this->y != rhs.y) return 0;
       if( this->z != rhs.z) return 0;
       return 1;
    }

    int AAA::operator<(const AAA &rhs) const
    {
       if( this->x == rhs.x && this->y == rhs.y && this->z < rhs.z) return 1;
       if( this->x == rhs.x && this->y < rhs.y) return 1;
       if( this->x < rhs.x ) return 1;
       return 0;
    }

    main()
    {
       map<string, AAA> M;
       AAA Ablob ;

       Ablob.x=7;
       Ablob.y=2;
       Ablob.z=4.2355;
       M["A"] = Ablob;

       Ablob.x=5;
       M["B"] = Ablob;

       Ablob.z=3.2355;
       M["C"] = Ablob;

       Ablob.x=3;
       Ablob.y=7;
       Ablob.z=7.2355;
       M["D"] = Ablob;

       for( map<string, AAA>::iterator ii=M.begin(); ii!=M.end(); ++ii)
       {
           cout << (*ii).first << ": " << (*ii).second << endl;
       }

       return 0;
    }

#包括
#包括
使用名称空间std；
AAA级
{
friend ostream&operatorxcout它们不是重载，它们是运算符的定义。在这种特殊情况下，复制构造函数和赋值运算符执行默认的操作，因此应该忽略它们。比较运算符应该返回
bool
，而不是
int
，如果AAA是那么，<比较运算符将如何工作？请详细说明它的工作原理？
for (auto ii = M.begin(); ii != M.end(); ++i)