为什么地图值即使不是指针也会发生变化-Golang

Question

I'm trying to test a method. Its signature is func (gv *myType) Update(all map[string][]SomeType) error

Although the test fails (the expected value doesn't match the received value) I've found that also the map it receives(allSub) is "updated" after the method is evaluated. Here is the code:

t.Logf("allsub %v", allSub)
if err := gv.Update(allSub); err != nil {
    t.Error(err)
    return
}
if !reflect.DeepEqual(egv, gv) {
    t.Errorf("allSub %v", allSub)
    return
}

The output

mth_test.go:265: allsub map[b:[{1 3 3}] a:[{1 0 0} {1 2 0}]]
mth_test.go:271: allSub map[a:[{1 2 0}]]

As you can see on t.Logf("allsub %v", allSub) we have map[b:[{1 3 3}] a:[{1 0 0} {1 2 0}]] and then after the evaluation if !reflect.DeepEqual(egv, gv) allSub becomes allSub map[a:[{1 2 0}]]. This drives me crazy and I'm wondering how is this possible? allSub is not a pointer.

Answer 1

Map, slice, channels are reference types. The spec is in-process to become clearer about this but basically the implementation of these builtin types entails a reference to an underlying data structure.

So you can consider that an object of type map is actually a reference to an associative array.

Effective Go has a little paragraph on it : https://golang.org/doc/effective_go.html#maps

I quote :

Like slices, maps hold references to an underlying data structure. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller.