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In golang, I defined a custom struct

type Station struct {
    Name           string
    StationLine    []string
}

Station struct is not comparable because StationLine is []string which is not comparable. Hence, I cannot create map[Station]distance (error: invalid map key type) and I cannot use slices.Contains(stations, stn1). This is very inconvenient.

Actually, for comparing purposes, Station struct can just compare Station.Name and ignore StationLine. Station.Name will be unique.

I cannot use Station.Name as replacement of Station in map key, because Station contains metadata like StationLine, which will be used in function. And I don't want to create a reverse lookup function from string to Station, because it looks clumsy.

I remember in C++, we can define custom operation in class, so I am wondering if I can define comparition function for my custom struct so that it can be used as map[key]?

In golang, I defined a custom struct

type Station struct {
    Name           string
    StationLine    []string
}

Station struct is not comparable because StationLine is []string which is not comparable. Hence, I cannot create map[Station]distance (error: invalid map key type) and I cannot use slices.Contains(stations, stn1). This is very inconvenient.

Actually, for comparing purposes, Station struct can just compare Station.Name and ignore StationLine. Station.Name will be unique.

I cannot use Station.Name as replacement of Station in map key, because Station contains metadata like StationLine, which will be used in function. And I don't want to create a reverse lookup function from string to Station, because it looks clumsy.

I remember in C++, we can define custom operation in class, so I am wondering if I can define comparition function for my custom struct so that it can be used as map[key]?

• dictionary
• go
• struct
• compare

Share Improve this question Follow asked Mar 22 at 14:29 naive programmernaive programmer 911 silver badge55 bronze badges 5

• In Go the type itself must be comparable. Just use whatever form you would use in a custom comparator as the map key directly. In almost every case, a string is sufficient. – Mr_Pink Commented Mar 22 at 14:49
• 1 "How to make custom struct comparable and be used as key to map" This is simply impossible, you cannot and must redesign. – Volker Commented Mar 22 at 14:49
• @Mr_Pink the reason to use string map key is that I need to access Station. StationLine. For example, in a function getDistance(Station1, Station2, PreviousStation) // I would need to access distances which is a map[station]distance and then check Station1. StationLine. If I have to use string, then I need to implement reverse lookup from string to Station. It is working, but I am exploring a better solution – naive programmer Commented Mar 23 at 2:45
• You can serialize your struct to a comparable type (such as string) and use the serialized value as key for your map. For example, JSON-encode your struct or create a hash function that converts your struct to an integer (which is also a comparable type). – Victor Ruiz Commented Mar 28 at 8:42
• Got it. Many thanks for the comments. I find it a bit weird and hard. In c++, as long as you define comparsion, then c++ allows you to use custom struct as map key. But I am now aware of language limitation – naive programmer Commented Mar 29 at 13:12

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Sorted by: Reset to default 0

Basically you can't unless all the types in the struct are comparable.

One way is to implement some kind of hash func and use that as keys for the map.

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