如何创建 std::set 结构?

By far the easiest method to accomplish this is to provide a member operator < for your Point class. In doing so pt1 < pt2 becomes valid and std::less<Point> is then happy. Assuming your class is a traditional x,y point, it would look like this:

struct Point
{
    int x,y;

    // compare for order.     
    bool operator <(const Point& pt) const
    {
        return (x < pt.x) || ((!(pt.x < x)) && (y < pt.y));
    }
};

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提供自定义比较器类型

另一种方法是提供自定义比较器类型，而不是依赖于 std::less.这方面的最大优势是能够定义几个可能意味着不同事物的东西，并根据需要在容器或算法中使用它们.

Another method would be to provide a custom comparator type rather than relying on std::less<Point>. The biggest advantage in this is the ability to define several that can mean different things, and use them in containers or algorithms as appropriately needed.

struct CmpPoint
{
    bool operator()(const Point& lhs, const Point& rhs) const
    {
        return (lhs.x < rhs.x) || ((!(rhs.x < lhs.x)) && (lhs.y < rhs.y));
    }
};

有了它，您现在可以像这样声明您的 std::set:

With that, you can now declare your std::set like this:

std::set<Point,CmpPoint> mySet;

使用这种方法需要考虑的一些事情:类型不是 Point 的一部分，因此任何对私有成员变量或函数的访问都必须通过增加能力来考虑.

Something to consider with this approach: The type is not part of Point, so any access to private member variables or functions has to be accounted for via friending in come capacity.

提供一个自由函数operator <

另一种不太常见的机制是简单地提供一个全局自由函数，该函数提供 operator <.这不是成员函数.再次执行此操作时，默认的 std::less 将生成有效代码.

Another less common mechanism is simply provide a global free-function that provides operator <. This is NOT a member function. In doing this, once again, the default std::less<Point> will result in valid code.

bool operator <(const Point& lhs, const Point& rhs)
{
    return (lhs.x < rhs.x) || ((!(rhs.x < lhs.x)) && (lhs.y < rhs.y));
}

这似乎是自定义比较器和成员运算符的混合体，实际上它们各有优缺点.例如:就像成员operator <，你可以使用默认的std::less.与自定义比较器一样，这是一个非类函数，因此必须通过友元或访问器提供对私有成员的访问.

This may seem a mix of both the custom comparator and the member operator, and indeed many of the pros and cons of each come along. Ex: like the member operator <, you can just use the default std::less<Point>. Like the custom comparator, this is a non-class function, so access to private members must be provided via friending or accessors.

总结

根据您的需要，我会采用简单的方法；只需创建一个成员operator <.您可能总是希望以这种方式订购您的 Point.如果没有，请使用自定义比较器.在任一情况下，请确保遵守严格的弱排序.

For your needs, I'd go with the simple approach; just make a member operator <. Chances are you'll always want to order your Points in that fashion. If not, go with the custom comparator. In either case make sure you honor strict weak ordering.

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