std :: map，只对一个变量排序的自定义键类型 [英] std::map, custom key type with sorting only on one variable

问题描述

我有一个键类型：

struct KeyT {
    uint32_t timestamp;

    // example!
    uint32_t a;
    uint32_t b;
    uint32_t c;
    uint32_t d;
    uint32_t e;
    // ...

    bool operator== (const KeyT& key) const
    {
        if(timestamp == key.timestamp && a == key.a && b == key.b && d == key.d && c == key.c && e == key.e)
            return true;
        return false;

    }
    bool operator< (const KeyT& key) const
    {
        if(timestamp < key.timestamp)
            return true;
        else if(timestamp == key.timestamp && a < key.a && b < key.b && c < key.c && d < key.d && e < key.e)
            return true;
        else if(timestamp == key.timestamp && a == key.a && b < key.b && c < key.c && d < key.d && e < key.e)
            return true;
        else if(timestamp == key.timestamp && a == key.a && b == key.b && c < key.c && d < key.d && e < key.e)
            return true;
        else if(timestamp == key.timestamp && a == key.a && b == key.b && c == key.c && d < key.d && e < key.e)
            return true;
        else if(timestamp == key.timestamp && a == key.a && b == key.b && c == key.c && d == key.d && e < key.e)
            return true;
        // ..
        return false;
    }
};

现在，我真的不在乎成员vars a，b，c，d ，e，我唯一要确保的是地图是在时间戳上排序的。我也只是意识到，如果我有两个实例KeyT一，二，其中一切都是相同的，除了d，然后，两个，一个是假的。解决这个问题的唯一方法是对所有成员变量的所有可能的组合进行比较。我相当确定我缺少一些明显的东西，那么这种情况下最好的解决方案是什么？

Now, I don't really care about the sorting on member vars a, b, c, d, e, the only thing I want to ensure is that the map is sorted on timestamp. I also just realized that if I have two instances of KeyT one, two, where everything is the same except for "d", then both one < two, and two < one would be false. The only way to fix that is to write comparisons for all possible combinations of all member variables.. I'm fairly certain I'm missing something obvious, so what's the best solution in this case?

谢谢！

推荐答案

我认为这是你需要的：

bool operator< (const KeyT& key) const
{
    if(timestamp != key.timestamp) return timestamp < key.timestamp;
    else if ( a != key.a ) return a < key.a;
    else if ( b != key.b ) return b < key.b;
    else if ( c != key.c ) return c < key.c;
    else if ( d != key.d ) return d < key.d;
    else return e < key.e;
}

这是一个明智的丑陋模式，您想在可比类中排序的变量。

This is a sensible if ugly pattern to use whenever you have a prioritised list of variables you want to sort on within a comparable class.

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