"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, Mar 28 '20 at 22:48
Fix by declaring the field as `TagService tagservice.TagService`. See [Why can't I assign a *Struct to an *Interface?](https://stackoverflow.com/questions/13511203/why-cant-i-assign-a-struct-to-an-interface) for an explanation. — Charlie Tumahai, Mar 28 '20 at 22:48
Change the `Handler` to use a `tagservice.TagService` instead of `*tagservice.TagService`. That's a pointer to an interface. — Burak Serdar, Mar 28 '20 at 22:48
@OleksiiMiroshnyk, can you convert your comment into an answer so it can be accepted? — Cameron Little, Apr 02 '20 at 06:05

score 2 · Accepted Answer · answered Apr 02 '20 at 06:10

2

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

type Handler struct { 
    TagService tagservice.TagService 
}

answered Apr 02 '20 at 06:10

Oleksii Miroshnyk

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

1 Answers1