去地图查找返回元素的副本？

Question

It seems like Go's map lookup returns a copy of the element. Can't seem to find this described in the language spec. I wrote a program as below:

type T struct { n int }

m := make(map[string]T)
t := T{123}
m["123"] = t
t0 := m["123"]
t1 := m["123"]
t0.n = 456
t1.n = 789
fmt.Println(t, t0, t1)

I got the output as: {123} {456} {789}. Looks like every time a copy of the element is returned?

Answer 1

Go does not pass references, ever. It either passes values, making copies on assignment, or these values can be pointers, in which case the copy is of a pointer, which is practically a reference.

So let's say we have

type Foo struct {
   Bar string
}

if we make a map of values, i.e.

m := map[string]Foo{}

then map accesses return a copy of a Foo, or a zero valued Foo:

m["x"] = Foo{"hello"}

y := m["x"]

y is now a different object than what's in the map, so changing its Bar won't affect the object in the map.

But, if we make the map a map of pointers:

m := map[string]*Foo{}

and then access it:

m["x"] = &Foo{"bar"}
y := m["x"]

y is now a pointer to the same object as in the map. We can change its Bar and it will affect the map:

y.Bar = "wat"
fmt.Println(m["x"].Bar)

// Will print "wat"

Playground example.