STL之针对自定义类型的操作

对于四种关联式容器而言，它们的模板参数中都有一个 Compare ，默认采用的是std:: less ，所以如果 Key 是自定义类型，需要自己传递 Compare 类型的参数才能满足条件，否则无法通过编译。下面以自定义类型Point 为例，以点到原点的距离为标准进行比较。

改写的方式有三种：模板的特化、运算符的重载（小于符号的重载）、函数对象的写法。

#include <math.h>
#include <iostream>
#include <ostream>
#include <set>

using std::cout;
using std::endl;
using std::set;

template <typename Container>
void display(const Container &con)
{
    for(auto &elem : con)
    {
        cout << elem << "  ";
    }
    cout << endl;
}

class Point
{
public:
    Point(int ix = 0, int iy = 0)
    : _ix(ix)
    , _iy(iy)
    {
    }

    double getDistance() const
    {
        /* sqrt(pow(_ix, 2) + pow(_iy, 2)); */
        return hypot(_ix, _iy);
    }

    int getX() const
    {
        return _ix;
    }

    int getY() const
    {
        return _iy;
    }

    ~Point()
    {

    }

    friend std::ostream &operator<<(std::ostream &os, const Point &rhs);
    friend bool operator<(const Point &lhs, const Point &rhs);
    friend struct ComparePoint;
private:
    int _ix;
    int _iy;
};

std::ostream &operator<<(std::ostream &os, const Point &rhs)
{
    os << "(" << rhs._ix
       << ", " << rhs._iy
       << ")"; 

    return os;
}

//小于符号的重载
bool operator<(const Point &lhs, const Point &rhs)
{
    cout << "bool operator<" << endl;
    if(lhs.getDistance() < rhs.getDistance())
    {
        return true;
    }
    else if(lhs.getDistance() == rhs.getDistance())
    {
        if(lhs._ix < rhs._ix)
        {
            return true;
        }
        else if(lhs._ix == rhs._ix)
        {
            if(lhs._iy < rhs._iy)
            {
                return true;
            }
            else
            {
                return false;
            }
        }
        else
        {
            return false;
        }
    }
    else
    {
        return false;
    }
}

//y + n(目标行号) + G  复制
//d + n(目标行号) + G  删除
//函数对象的写法
struct ComparePoint
{
    bool operator()( const Point& lhs, const Point& rhs ) const
    {
        cout << "bool ComparePoint" << endl;
        if(lhs.getDistance() < rhs.getDistance())
        {
            return true;
        }
        else if(lhs.getDistance() == rhs.getDistance())
        {
            if(lhs._ix < rhs._ix)
            {
                return true;
            }
            else if(lhs._ix == rhs._ix)
            {
                if(lhs._iy < rhs._iy)
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
            else
            {
                return false;
            }
        }
        else
        {
            return false;
        }
    }
};


//命名空间的扩展
namespace std
{
//模板的特化
template <>
struct less<Point>
{
    bool operator()(const Point &lhs, const Point &rhs) const
    {
        cout << "template <> struct less" << endl;
        if(lhs.getDistance() < rhs.getDistance())
        {
            return true;
        }
        else if(lhs.getDistance() == rhs.getDistance())
        {
            if(lhs.getX() < rhs.getX())
            {
                return true;
            }
            else if(lhs.getX() == rhs.getX())
            {
                if(lhs.getY() < rhs.getY())
                {
                    return true;
                }
                else
                {
                    return false;
                }
            }
            else
            {
                return false;
            }
        }
        else
        {
            return false;
        }
    }
};

}//end of namespace std

void test()
{
    set<Point> number = {
    /* set<Point, ComparePoint> number = { */
        Point(1, 2),
        Point(1, -2),
        Point(-1, 2),
        Point(1, 2),
        Point(3, 2),
    };
    display(number);
}

int main(int argc, char *argv[])
{
    test();
    return 0;
}