Why does Go forbid taking the address of (&) map member, yet allows (&) slice element?

Question

Go doesn\'t allow taking the address of a map member:

// if I do this:
p := &mm[\"abc\"]
// Syntax Error - cannot take the address of mm[\"abc\"]
         


        

Answer 1

A fundamental difference between map and slice is that a map is a dynamic data structure that moves the values that it contains as it grows. The specific implementation of Go map may even grow incrementally, a little bit during insert and delete operations until all values are moved to a bigger memory structure. So you may delete a value and suddenly another value may move. A slice on the other hand is just an interface/pointer to a subarray. A slice never grows. The append function may copy a slice into another slice with more capacity, but it leaves the old slice intact and is also a function instead of just an indexing operator.

In the words of the map implementor himself:

https://www.youtube.com/watch?v=Tl7mi9QmLns&feature=youtu.be&t=21m45s "It interferes with this growing procedure, so if I take the address of some entry in the bucket, and then I keep that entry around for a long time and in the meantime the map grows, then all of a sudden that pointer points to an old bucket and not a new bucket and that pointer is now invalid, so it's hard to provide the ability to take the address of a value in a map, without constraining how grow works... C++ grows in a different way, so you can take the address of a bucket"

So, even though &m[x] could have been allowed and would be useful for short-lived operations (do a modification to the value and then not use that pointer again), and in fact the map internally does that, I think the language designers/implementors chose to be on the safe side with map, not allowing &m[x] in order to avoid subtle bugs with programs that might keep the pointer for a long time without realizing then it would point to different data than the programmer thought.

See also Why doesn't Go allow taking the address of map value? for related comments.