Go with Generics: type *T is pointer to type parameter, not type parameter

Probably a golang beginner's question

I'm facing following compiler error when trying to compile the code below.

I want to implement an object store for different types (here A and B) sharing a common ID field. Following the DRY idea, I want to implement the store using generics.

When adding an object, I want to set its ID field using the GS interface (the actual code is a bit more complex of course), but the compiler does not want me to do that.

> ./prog.go:29:7: item.SetId undefined (type *T is pointer to type parameter, not type parameter)
>
> ./prog.go:34:24: A does not implement GS (SetId method has pointer receiver)

Is there a recommended way to solve this? Thanks in advance!!

package main

import "fmt"

type A struct {
	ID      string
	AMember string
}
type B struct {
	ID      string
	BMember string
}

type GS interface {
	Id() string
	SetId(string)
}

func (s A) Id() string      { return s.ID }
func (s *A) SetId(i string) { s.ID = i }
func (s B) Id() string      { return s.ID }
func (s *B) SetId(i string) { s.ID = i }

type MyStore[T GS] struct {
	values map[string]*T
}

func (s *MyStore[T]) add(item *T) {
	item.SetId("aa")
	s.values["aa"] = item
}

func main() {
	var storeA = &MyStore[A]{}
	storeA.values = make(map[string]*A)
	a := &A{}

	storeA.add(a)

	fmt.Println(a.Id())
}

About using *T

In short, a type parameter is not its constraint. The constraint only determines what operations are available on T, it doesn't imply anything about *T, which is now just an unnamed pointer type. This is the meaning of:

> type *T is pointer to type parameter, not type parameter

As a consequence, as in your case, the method set of *T does not automatically include pointer receiver methods declared on the T's concrete type A, and it does not implement interfaces that would be implemented by *A.

You'd have to make that explicit to the compiler by setting additional constraints. In a simplified form it would be something like:

func Foo[T any, PT interface { SetId(string); *T}](v T) {}

You can find more extensive examples and variations on this use case here:

• https://stackoverflow.com/questions/71440697/go-1-18-generics-how-to-define-a-new-able-type-parameter-with-interface
• https://stackoverflow.com/questions/69573113/how-can-i-instantiate-a-new-pointer-of-type-argument-with-generic-go/69575720#69575720
• https://stackoverflow.com/questions/72090387/what-is-the-generic-type-for-a-pointer-that-implements-an-interface/72091526#72091526

About implementing constraints

The reason this instantiation &MyStore[A]{} fails is clearly reported by the error message:

> A does not implement GS (SetId method has pointer receiver)

In other words SetId() is declared on *A, and not A. Therefore you should instantiate MyStore with *A:

var storeA = &MyStore[*A]{}

Then change the occurrences of *T in the struct/method definition to T:

type MyStore[T GS] struct {
    values map[string]T // just T instead of *T
}

func (s *MyStore[T]) add(item T) {
}

Upon instantiation with *A the type of the field would become equivalent to map[string]*A thus making the assignment storeA.values = make(map[string]*A) valid, and the method signature to add(item *A) thus allowing storeA.add(&A{}).

Fixed playground: https://gotipplay.golang.org/p/dcUVJ5YQK_b