Casting from one pointer to pointer type to another in Golang error

Question

Can anyone tell my why this wouldn\'t compile?

package main

type myint int
func set(a **myint) {
    i := myint(5)
    *a = &i 
}

func main() {
    var k *         


        

Answer 1

Here are two functionally equivalent working versions of your program.

package main

type mypint *int

func set(a *mypint) {
    i := int(5)
    *a = &i
}

func main() {
    var k *int
    set((*mypint)(&k))
    print(*k)
}

http://play.golang.org/p/l_b9LBElie

package main

type myint int

func set(a *myint) *myint {
    i := myint(5)
    a = &i
    return a
}

func main() {
    var k *int
    k = (*int)(set((*myint)(k)))
    print(*k)
}

http://play.golang.org/p/hyaPFUNlp8

Answer 2

Here's my analysis.

(**myint)(&k) -- cannot convert &k (type **int) to type **myint:

type **int and type **myint are unnamed pointer types and their pointer base types, type *int and type *myint, don't have identical underlying types.

If T (*int or *myint) is a pointer type literal, the corresponding underlying type is T itself.

(*myint)(k) -- can convert k (type *int) to type *myint:

type *int and type *myint are unnamed pointer types and their pointer base types, type int and type myint (type myint int), have identical underlying types.

If T (int) is a predeclared type, the corresponding underlying type is T itself. If T (myint) is neither a predeclared type or nor a type literal, T's underlying type is the underlying type of the type to which T refers in its type declaration (type myint int).

(myint)(*k) -- can convert *k (type int) to type myint:

type int and type myint have identical underlying types.

If T (int) is a predeclared type, the corresponding underlying type is T itself. If T (myint) is neither a predeclared type or nor a type literal, T's underlying type is the underlying type of the type to which T refers in its type declaration (type myint int).

Here's the underlying type example from the Types section revised to use integers and int pointers.

type T1 int
type T2 T1
type T3 *T1
type T4 T3

The underlying type of int, T1, and T2 is int. The underlying type of *T1, T3, and T4 is *T1.

References:

The Go Programming Language Specification

Conversions

Types

Properties of types and values

Type declarations

Predeclared identifiers

Pointer Type