To define a map from int to struct vertex, should I define map[int]vertex
or map[int]*vertex
? Which one is preferred?
I extended Chickencha's code:
package main
type vertex struct {
x, y int
}
func main() {
a := make(map[int]vertex)
b := make(map[int]*vertex)
v := &vertex{0, 0}
a[0] = *v
b[0] = v
v.x, v.y = 4, 4
println(a[0].x, a[0].y, b[0].x, b[0].y)
//a[0].x = 3 // cannot assign to (a[0]).x
//a[0].y = 3 // cannot assign to (a[0]).y
b[0].x = 3
b[0].y = 3
println(a[0].x, a[0].y, b[0].x, b[0].y)
u1 := a[0]
u1.x = 2
u1.y = 2
u2 := b[0]
u2.x = 2
u2.y = 2
println(a[0].x, a[0].y, b[0].x, b[0].y)
}
The output:
0 0 4 4
0 0 3 3
0 0 2 2
From the output, my understanding is, if I want to change the struct member in place, I must use pointer to the struct. But I'm still not sure the underlying reasons. Especially, why I cannot assign to a[0].x?