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A Map 's keys can be any value (including functions, objects, or any primitive). The keys of an Object must be either a String or a Symbol . The keys in Map are ordered in a simple, straightforward way: A Map object iterates entries, keys, and values in the order of entry insertion.
Maps, like channels, but unlike slices, are just pointers to runtime types. As you saw above, a map is just a pointer to a runtime. hmap structure. Maps have the same pointer semantics as any other pointer value in a Go program.
Go has pointers. A pointer holds the memory address of a value. The type *T is a pointer to a T value.
They are not reference types (though they are indirect types). If you assign to a map value, you will break its relationship with other variables. Go has no reference types, everything is a value.
I recently used a map in one of my golang projects, that had function pointers as keys like this:
map[*functiontype] somestructtype
One of my colleagues said this was a bad idea, so now I am unsure of this being feasible. I initially deemed it ok, because method pointers can be checked for equality and are immutable. Can someone provide some reasoning on that matter?
Complete example:
package main
import "fmt"
type s struct {
string
}
type f func() string
func func1() string { return "func 1" }
func func2() string { return "func 2" }
func main() {
// make two functions and two pointers to them
f1, f2 := func1, func2
p1, p2 := (*f)(&f1), (*f)(&f2)
// make a map of their function pointers
m := make(map[*f]s)
m[p1] = s{"struct 1"}
m[p2] = s{"struct 2"}
// print out the mapping
printmapping(m, p1, p2)
// reverse the pointers and have that printed
p1, p2 = (*f)(&f2), (*f)(&f1)
printmapping(m, p1, p2)
}
func printmapping(m map[*f]s, p1, p2 *f) {
fmt.Println("pointer 1:", m[(*f)(p1)])
fmt.Println("pointer 2:", m[(*f)(p2)])
}
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