golang - how to initialize a map field within a struct?

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} } 

Answer 2

It's very common for code (especially code fully under your control) to assume you initialize the data structure correctly. A struct literal is usually used in this case

g := &Graph{
    connections: make(map[Vertex][]Vertex),
}

Answer 3

Composite literals work just fine inside a constructor. Contriving an example using the initial question (and naively storing copies of Vertices in the map):

func NewGraph(v1 Vertex, v2 Vertex) *Graph {
    return &Graph{ map[Vertex][]Vertex{ v1: []Vertex{v2}, v2: []Vertex{v1} }}
}

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

  g := NewGraph(v1, v2)
  fmt.Println(g)
}

https://play.golang.org/p/Lf4Gomp4tJ