在Golang中，如果我将struct作为键，是否可以自定义键比较？

Question

How does map in Golang compare keys? For some reason, I need to have a struct as a key, which has 2 values inside. I want map to compare by only first value, not second. Second is for my usage. Like in java, I can customise equals method, so map will take only logically equal keys within. Is there any way to do that?

Edit: Looks like there is no way to do that. So I am now putting down my problem here. Please help me to think in 'Go-way'.

So, I want to implement a 'timed map', which tracks the key insertion time. In other words, there is a map which accepts and processes the values. Now, if the data in map is older than some specific time-interval, then I should clear it out.

So, I thought of having a key struct which has id and timestamp. When a new key comes, map takes it with id and currentTimeInMillis. After sometime, if a key comes which already exists, then map should preserve the first insertion time and only updates the value array.

To process, I will have a looping over map and check if any particular key is inside for more than threshold limit, then I clear it out. I can have this timestamp in value array, but that also has a timestamp of its own, so putting one more might confuse someone else.

Please suggest something.

Answer 1

Put the time on your value. Here's some example of how to structure your data.

type DataObj struct {
     Id int
     Updated time.Date
     // other fields
}

m := map[int]DataObj{}
m[d.Id] = d // assign using the id as your key

for k, v := range m {
    if time.Since(v.Updated) > duration {
         delete(m, k) // remove the stale item
    }
}

// some logic like this for adding/overwriting
v, ok := m[newObj.Id]
if ok { // an element with this id existed
     if time.Since(v.Updated) > duration {
          m[v.Id] = newObj // assign new value over old one
     }
}

I can't provide anything much more specific because you don't have any code with which to work. It seems like you'd probably like some of this (like the remove bits) to run on a timer. To do that, invoke the function as a goroutine and use a timer so every X seconds it unblocks and removes items from the map. If you're doing this you also need to use a mutex so the calling scope doesn't access the map while the remove function running the background is filtering out old items.

The overwrite bit is really straight forward, just test if the item is in the map, check it's time stamp, if it's beyond the threshold assign the new value, if not do nothing.

The main thing to take away here is to not use a struct for your key... There is no reason to do object equality, your object has an id, us it as your key. Everything else you care about can be held on the value (even the key itself is). As someone pointed out this isn't Java and even if it were, equality overrides in C# and Java are literally a fucking nightmare.