std映射复合键 [英] std map composite key

问题描述

我对于std :: map所需的运算符<（）方法有问题。我使用一个struct作为复合键，如下所示：

I have a problem with the operator<() method which is required for a std::map. I'm using a struct as composite key that looks as follows:

struct MyKey {
  std::string string1;
  std::string string2;
  std::string string3;
  unsigned int uint1;

  friend bool operator<(const MyKey& mk1, const MyKey& mk2)
  {
    return mk1.string1 < mk2.string1 && mk1.string2 < mk2.string2 &&
           mk1.string3 < mk2.string3 && mk1.uint1 < mk2.uint1;
  }
}

值，但我不知道如何实现这个为运算符< 方法。我观察到每次只存储一个值！

As introduced I want to use a composite key with 4 values, but I don't know how to achieve this for the operator< method. I observed that only 1 value is stored at a time!

任何人都可以告诉我正确的条件是什么样子？

Can anybody tell me how the right condition looks like?

提前感谢！

推荐答案

标准库的关联容器，例如 std :: map ， std :: set ， std :: multiset std :: multimap ， std :: bitset 要求元素的排序必须遵循 严格弱订购 ，这意味着您实施运算符< 必须遵循严格弱排序。所以一个实现可以是这样：

The Standard library's associative containers such as std::map, std::set, std::multiset, std::multimap, std::bitset require that the ordering of elements must follow Strict Weak Ordering, which means your implementation of operator< must follow strict weak ordering. So one implementation could be this:

friend bool operator<(const MyKey& mk1, const MyKey& mk2)
{
  if (mk1.string1 != mk2.string1 )
       return mk1.string1 < mk2.string1;

  else if ( mk1.string2 != mk2.string2)
       return mk1.string2 < mk2.string2;

  else if (mk1.string3 != mk2.string3)
       return  mk1.string3 < mk2.string3;

  else
       return mk1.uint1 < mk2.uint1;
}

或者您可以实现：

friend bool operator<(const MyKey& mk1, const MyKey& mk2)
{
  auto const & t1 = std::tie(mk1.string1, mk1.string2, mk1.string3, mk1.uint1);
  auto const & t2 = std::tie(mk2.string1, mk2.string2, mk2.string3, mk2.uint1);
  return t1 < t2;
}

在此解决方案中， std :: tie 函数创建两个元组 t1 和 t1 的引用传递给它，然后比较 t1 和 t2 使用重载的运算符< for std :: tuple 代替。用于元组的运算符<将按字典顺序比较元素，严格的弱订单。

In this solution, std::tie function creates two tuples t1 and t1 of the references of the arguments passed to it, and then compare t1 and t2 using overloaded operator< for std::tuple instead. The operator< for tuple compares the elements lexicographically — strict-weak ordering is achieved..

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