Using A Struct As Key For std::unordered_map

Question

I am trying to use a struct as a key for an unordered_map. I added the 'spaceship' operator to the structure, which solved errors I was getting with normal comparisons, such as "is struct 1 greater than struct 2?", etc. However, I am getting attempting to reference a deleted function when using it as a key for my map. From what understood, adding the spaceship operator should have allowed me to use the struct as the map key. What is wrong?

struct test
{
    uint32_t a;
    uint32_t b;

    auto operator<=>(const test&) const = default;
};

std::unordered_map<test, uint32_t> x; // Causes error

This question is similar to: C++ unordered_map using a custom class type as the key. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — idmean
– idmean, Commented Mar 26 at 9:04

Marshall Clow · Accepted Answer · 2022-07-21 01:01:15Z

11

To use a structure as a key in an unordered_map, you need two things:

A "hasher", something that will take a const test & and compute a hash, which defaults to std:hash<test>, and
A comparison predicate, which defaults to std::equal_to<test>.

You've got the second one covered with your spaceship operator, but not the first. There is no std::hash<test>.

See C++ unordered_map using a custom class type as the key for an example as how to define your own hasher.

answered Jul 21, 2022 at 1:01

Marshall Clow

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2 Comments

Yksisarvinen Over a year ago

Don't you think this question is a duplicate of the one linked in your answer?

Marshall Clow Over a year ago

Yes, mostly. But not the bit about the comparator. (especially using <=>)

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