"cannot use x (variable of type [struct]) as [interface] value in assignment" when struct implements interface

Question

I have an interface TagService with a struct implementation. They reside in two different packages due to SoC. I want to instantiate the implementation, and add it to a another struct Handler, which has a field of the interface type.

This takes place in the main package:

package main

...

ts := &postgres.TagService{DB: db} 
var h http.Handler
h.TagService = ts

However, with this setup, i receive the following error in VS code:

cannot use ts (variable of type *postgres.TagService) as *tagservice.TagService value in assignment

The Handler struct:

type Handler struct {
    TagService *tagservice.TagService
}

The interface:

package tagservice

type Tag struct {
    ID          int64
    Name        string
    Description string
}

type TagService interface {
    CreateTag(tag *Tag) (*Tag, error)
    GetTag(id int64) (*Tag, error)
    GetAllTags() ([]*Tag, error)
    UpdateTag(tag *Tag) (*Tag, error)
    DeleteTag(id int64) error
}

And the implementation (Omitted func bodies)

package postgres

type TagService struct {
    DB *sql.DB
}

func (s *TagService) CreateTag(tag *tagservice.Tag) (*tagservice.Tag, error) {
...
}

func (s *TagService) GetTag(id int64) (*tagservice.Tag, error) {
...
}

func (s *TagService) GetAllTags() ([]*tagservice.Tag, error) {
...
}

func (s *TagService) UpdateTag(tag *tagservice.Tag) (*tagservice.Tag, error) {
...
}

func (s *TagService) DeleteTag(id int64) error {
...
}

What am I doing wrong? Thanks in advance

Because you are trying to put pointer to the interface instead of just interface. Try type Handler struct { TagService tagservice.TagService } — Oleksii Miroshnyk
– Oleksii Miroshnyk, Commented Mar 28, 2020 at 22:48
Fix by declaring the field as TagService tagservice.TagService. See Why can't I assign a *Struct to an *Interface? for an explanation. — user5728991
– user5728991, Commented Mar 28, 2020 at 22:48
Change the Handler to use a tagservice.TagService instead of *tagservice.TagService. That's a pointer to an interface. — Burak Serdar
– Burak Serdar, Commented Mar 28, 2020 at 22:48
@OleksiiMiroshnyk, can you convert your comment into an answer so it can be accepted? — Cameron Little
– Cameron Little, Commented Apr 2, 2020 at 6:05

Oleksii Miroshnyk · Accepted Answer · 2020-04-02 06:10:18Z

3

You are trying to put pointer to the interface instead of just interface. Try

type Handler struct { 
    TagService tagservice.TagService 
}

answered Apr 2, 2020 at 6:10

Oleksii Miroshnyk

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