Map的键作为struct的指针？ （用例）

Question

Given a struct:

type Foo struct {
    Bar int
}  

option 1, Map's key: struct value

map[Foo]bool 

option 2, Map's key: pointer to struct

map[*Foo]bool

---

Would there be situations that I must pick option 2? I.e:

1. memory efficiency?
2. huge struct value?

In general, what are the use-cases where its plausible of having map's key as pointer to struct?

Answer 1

For that specific case, a pointer wouldn't really work, because for example:

func main() {
    m1, m2 := map[Vertex]int{}, map[*Vertex]int{}
    a, b := Vertex{10}, Vertex{10}
    m1[a], m2[&a] = 10, 10

    fmt.Println(m1[a], m1[b], m1[a] == m1[b])
    fmt.Println(m2[&a], m2[&b], m2[&a] == m2[&b])
    fmt.Printf("%p %p", &a, &b)
}

<kbd>playground</kbd>

// edit after the question got edited

Only reasons to use a pointer is if the struct's contents aren't comparable (aka has a slice or a map, etc) or if it's too large that using it by value is causing performance issues.

IMHO, your current case should be using a value not a pointer.