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.

A constructor is the commonly accepted way (other than assuming the programer can do it unassisted) — JimB, Dec 18 '14 at 18:37
possible duplicate of [how to initialize members in go struct](http://stackoverflow.com/questions/4498998/how-to-initialize-members-in-go-struct) — JimB, Dec 18 '14 at 18:50

score 50 · Accepted Answer · answered Dec 18 '14 at 18:45

50

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}
}

answered Dec 18 '14 at 18:45

julienc

19,087
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4

great, thanks for finding the example in the standard lib, that definitely inspires confidence – Matt Dec 18 '14 at 20:58

score 19 · Answer 2 · answered Dec 18 '14 at 18:40

19

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),
}

answered Dec 18 '14 at 18:40

JimB

104,193
13
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5

But how do you initialize the connections value with some particular value? – lostboy_19 Jul 10 '15 at 23:14

score 4 · Answer 3 · answered Aug 24 '17 at 22:50

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

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

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