先做地图

Question

I am following the go tour and something bothered me.

Maps must be created with make (not new) before use

Fair enough:

map = make(map[int]Cats)

However the very next slide shows something different:

   var m = map[string]Vertex{
    "Bell Labs": Vertex{
        40.68433, -74.39967,
    },
    "Google": Vertex{
        37.42202, -122.08408,
    },
}

This slide shows how you can ignore make when creating maps

Why did the tour say maps have to be created with make before they can be used? Am I missing something here?

Answer 1

Actually the only reason to use make to create a map is to preallocate a specific number of values, just like with slices (except you can't set a cap on a map)

m := map[int]Cats{}
s := []Cats{}
//is the same as
m := make(map[int]Cats)
s := make([]Cats, 0, 0)

However if you know you will have a minimum of X amount of items in a map you can do something like:

m := make(map[int]Cats, 100)// this will speed things up initially

Also check http://dave.cheney.net/2014/08/17/go-has-both-make-and-new-functions-what-gives