C++，比较器，函数指针

Question

我想根据 x 或 y 坐标创建排序的点集。

typedef std::set <Point2D, sorter> Points;

对于“点”的类型，我想使用不同的比较器。第一个根据 x 坐标对点进行排序，第二个根据 y 坐标对点进行排序:

typedef std::set <Point2D, sortPointsByX()> Points;
typedef std::set <Point2D, sortPointsByy()> Points;

在哪里

class sortPoints2DByX
{
    public:

            bool operator() ( const Point2D &p1, const Point2D &p2 );

};

class sortPoints2DByY
{
    public:

            bool operator() ( const Point2D &p1, const Point2D &p2 );

};

是否可以在 Points 声明中创建指向构造函数 sortPoints2DByX/sortPoints2DByY 类的指针

typedef std::set <Point2D, pointer_to_somparator_class> Points;

以及是否需要使用它们中的任何一个？

我需要一种数据类型可以用两种方式排序。

如果这个想法是错误的，有没有更合适的解决方案？

我需要计算 x 和 y 坐标的中值...

感谢您的帮助...

Answer 1

Is it possible to create a pointer to the constructor sortPoints2DByX/sortPoints2DByY classes in Points declaration?

不，您不能将地址作为类的构造函数。

I need to have one data type can be sorted in two ways.

您可以使用指向函数的指针。示例实现:

#include <algorithm>
#include <set>
#include <iostream>

    // Your implementation may differ.
struct Point
{
    int x; int y;
    Point(int x_, int y_)
        : x(x_), y(y_) {}
};

    // For display purposes.
void print(const Point& point)
{
    std::cout << '(' << point.x
        << ',' << point.y << ')' << std::endl;
}

bool OrderByX ( const Point& lhs, const Point& rhs )
{
    return (lhs.x < rhs.x);
}

bool OrderByY ( const Point& lhs, const Point& rhs )
{
    return (lhs.y < rhs.y);
}

    // Type of comparison operator.
typedef bool(*Comparator)
    (const Point&lhs,const Point&rhs);

    // Set used to store points in sorted order.
typedef std::set<Point, Comparator> Points;

int main ( int, char ** )
{
        // Each set ordered with it's own criteria.
    Points by_x(&OrderByX);
    Points by_y(&OrderByY);

        // Insert each point in both sets.
    by_x.insert(Point(1,2)); by_y.insert(Point(1,2));
    by_x.insert(Point(3,1)); by_y.insert(Point(3,1));
    by_x.insert(Point(4,3)); by_y.insert(Point(4,3));
    by_x.insert(Point(2,4)); by_y.insert(Point(2,4));

        // Show that 1st set is in proper order.
    std::cout << "Sorted by X:" << std::endl;
    std::for_each(by_x.begin(), by_x.end(), &print);
    std::cout << std::endl;

        // Show that 2nd set is in proper order.
    std::cout << "Sorted by Y:" << std::endl;
    std::for_each(by_y.begin(), by_y.end(), &print);
    std::cout << std::endl;
}

它生成以下输出:

Sorted by X:
(1,2)
(2,4)
(3,1)
(4,3)

Sorted by Y:
(3,1)
(1,2)
(4,3)
(2,4)