Golang-如何在结构内初始化地图字段？

Question

I'm confused about the best way to initialize a struct that contains a map. Running this code produces panic: runtime error: assignment to entry in nil map:

package main

type Vertex struct {
   label string
} 

type Graph struct {
  connections map[Vertex][]Vertex
} 

func main() {
  v1 := Vertex{"v1"}
  v2 := Vertex{"v2"}

  g := new(Graph)
  g.connections[v1] = append(g.coonections[v1], v2)
  g.connections[v2] = append(g.connections[v2], v1)
}

One idea is to create a constructor, as in this answer.

Another idea is to use an add_connection method that can initialize the map if it's empty:

func (g *Graph) add_connection(v1, v2 Vertex) {
  if g.connections == nil {
    g.connections = make(map[Vertex][]Vertex)
  }
  g.connections[v1] = append(g.connections[v1], v2)
  g.connections[v2] = append(g.connections[v2], v1)
}

Are there other options? Just wanted to see if there is a commonly-accepted way to do this.

Answer 1

I would probably use a constructor to do this:

func NewGraph() *Graph {
    var g Graph
    g.connections = make(map[Vertex][]Vertex)
    return &g
}

I've found this example in the standard image/jpeg package (not with a map though, but with a slice):

type Alpha struct {
    Pix []uint8
    Stride int
    Rect Rectangle
}

func NewAlpha(r Rectangle) *Alpha {
    w, h := r.Dx(), r.Dy()
    pix := make([]uint8, 1*w*h)
    return &Alpha{pix, 1 * w, r}
}