无法将std :: unorded_set与自定义KeyEqual进行比较 [英] Can not compare std::unorded_set with custom KeyEqual

问题描述

以下程序无法编译.但是，如果我不注释 operator == ，它就会编译.当我已经提供了 FooEqual

The following program does not compile. But If I do not comment out operator==, it compiles. Why operator== is still needed when I already provide FooEqual

#include <cstddef>
#include <unordered_set>

struct Foo {
};

struct FooHasher {
  size_t operator()(const Foo&) const {
    return 1;
  }
};

struct FooEqual {
  bool operator()(const Foo& lhs, const Foo& rhs) const {
    return true;
  }
};

// bool operator==(const Foo& lhs, const Foo& rhs) {
//   return true;
// }

int main() {
  std::unordered_set<Foo, FooHasher, FooEqual> s1;
  std::unordered_set<Foo, FooHasher, FooEqual> s2;
  (void)(s1 == s2);
  return 0;
}

推荐答案

"23.2.5无序关联容器"状态:

"23.2.5 Unordered associative containers" states:

如果a.size()== b.size()，则两个无序容器a和b比较相等并且，对于从中获得的每个等效键组[Ea1，Ea2)a.equal_range(Ea1)，存在一个等效密钥组[Eb1，Eb2)从b.equal_range(Ea1)获得，从而距离(Ea1，Ea2)==distance(Eb1，Eb2)和is_permutation(Ea1，Ea2，Eb1)返回true.

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent=key group [Ea1,Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1,Eb2) obtained from b.equal_range(Ea1), such that distance(Ea1, Ea2) == distance(Eb1, Eb2) and is_permutation(Ea1, Ea2, Eb1) returns true.

将其剥离，归结为根据 std :: is_permutation()定义的无序容器的相等性.

Stripping this down, it all comes down to the equality of unordered containers being defined in terms of std::is_permutation().

重要的是，它引用了 std :: is_permutation ()的三个参数形式，而不是四个参数形式！

The important part is that this references the three argument form of std::is_permutation(), and not the four argument form!

换句话说，对于无序容器的内容，而不是容器的官方比较功能，整个纸牌屋最终被缩减为默认的 operator == .

In other words, the whole house of cards ends up being reduced to the default operator==, for the contents of the unordered container, rather than the container's official comparison function.

这是我的读物.

这篇关于无法将std :: unorded_set与自定义KeyEqual进行比较的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！