Golang地图是按值还是按引用查找？

Question

When I retrieve a struct from a map in Go, do I get a copy of the value? Or do I get the actual value that's in the map?

For example, let's say I have a map from strings to structs:

type quality struct {
    goodness    int
    crunchiness int 
}
cookies := make(map[string]quality)
cookies["nutrageous"] = quality{goodness: 3, crunchiness: 10}

and I want to modify an entry.

Can I count on the returned value being the same as what's in the map?

c := cookies["nutrageous"]
c.goodness += 5

Or do I also have to go back and modify what's in the map?

c := cookies["nutrageous"]
c.goodness += 5
cookies["nutrageous"] = c

Answer 1

Indexing a map returns a copy of the map value. If the map value is a struct, then modifications to the returned struct value will not change the struct value in the map. You will need to assign the modified value back to the map.

If the value is a pointer to a struct, then modifications to the struct will be accessible through the map.