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

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