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I try to loop through a map, that I pass as a pointer to a function, but I can't find a way to access the elements. This is the code:
func refreshSession(sessions *map[string]Session) { now := time.Now() for sid := range *sessions { if now.After(*sessions[sid].timestamp.Add(sessionRefresh)) { delete( *sessions, sid ) } } } Line 4 in this example return following compile error:
./controller.go:120: invalid operation: sessions[sid] (type *map[string]Session does not support indexing) I tried brackets, but it had no effect. If I take away all reference operators (* &) then it compiles fine.
How must I write this?
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If you assign to a map value, you will break its relationship with other variables. Go has no reference types, everything is a value. You can demonstrate this with func mapToAnotherFunction(m map[string]int) { m = nil } , it doesn't empty the map, just overwrites its value in a local way.
Maps, like channels, but unlike slices, are just pointers to runtime types. As you saw above, a map is just a pointer to a runtime.
If the key is not present in the map, get() returns null. The get() method returns the value almost instantly, even if the map contains 100 million key/value pairs.
You don't need to use a pointer with a map.
Map types are reference types, like pointers or slices
If you needed to change the Session you could use a pointer:
map[string]*Session
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De-reference the map first and then access it (Example on play):
(*sessions)[sid] It's also noteworthy that maps are actually reference types and therefore there is a very limited use-case of using pointers. Just passing a map value to a function will not copy the content. Example on play.
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