使用联合作为 std::unordered_map 的键

Question

I want to use union with a structure{int, int, string} and integer as a key for unordered_map. What are the best practices to do this? How does the hashing work, is there need for custom hashing?

struct nodeinfo {
int a;
int b;
string c;
};

union key{
nodeinfo structKey;
int intKey; 
};

unordered_map<key, int> Map;

I want to stick to C++11 so cannot use variant.

Answer 1

What are the best practices to do this?

Best practice is probably to not do this.

is there need for custom hashing?

Yes.

How does the hashing work

The hasher somehow needs to know which union member is active, and then apply a hash function appropriate for that type. Since it is not possible to find out the active member by inspecting the union object, this quite tricky.

Instead of union, in practice you need to use some sort of "tagged union" type such as std::variant . Despite the name, tagged union is typically implemented without using union internally in C++. If you cannot use C++17, then you need to use some other implementation or write one yourself.