CODEWITHSUNDEEP
gohashmap
C. Sal
C. Sal

Reputation: 11

How large is the struct underlying a map in golang?

I know that map is a reference type in Go (it has a pointer to the map entries memory region in its underlying struct). However, I would like to know what is the size of the underlying struct of the map because I want to know if using a pointer to a map as a function argument would be faster than not using a pointer.

Looking at this blog post it seems that the maptype struct has a lot of fields and that it would take a long time to copy (relative to a pointer).

Looking through the golang standard libraries I have found almost no use of *map[x]x so I guess using just map[x]x should be efficient as a function argument. So this leads me to think that maybe the compiler actually replaces map[x]x by a pointer to the maptype struct. Is that the case? If not what actually is happening that may avoid the copying of the maptype struct with its many fields?

Upvotes: 0

Views: 938

Answers (1)

peterSO
peterSO

Reputation: 166569

The zero value for a Go map variable is a nil pointer.

var m map[string]int

make intializes a map and sets the map variable to point to a package runtime hmap struct.

m = make(map[string]int)

In Go, all arguments are passed by value. In the case of a map value, a reference type, a map value is a pointer. Therefore, passing a map value as a function or method argument is fast, you are passing a pointer.

The Go map runtime structs are currently located in the src/runtime/map.go Go source file. Since you only see a hmap pointer, their size is unlikely to be relevant.

See GopherCon 2016: Keith Randall - Inside the Map Implementation.

Upvotes: 1

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