"<type> is pointer to interface, not interface" confusion

Issue

I have this problem which seems a bit weird to me. Take a look at this snippet of code:

package coreinterfaces

type FilterInterface interface {
    Filter(s *string) bool
}

type FieldFilter struct {
    Key string
    Val string
}

func (ff *FieldFilter) Filter(s *string) bool {
    // Some code
}

type FilterMapInterface interface {
    AddFilter(f *FilterInterface) uuid.UUID     
    RemoveFilter(i uuid.UUID)                   
    GetFilterByID(i uuid.UUID) *FilterInterface
}

type FilterMap struct {
    mutex   sync.Mutex
    Filters map[uuid.UUID]FilterInterface
}

func (fp *FilterMap) AddFilter(f *FilterInterface) uuid.UUID {
    // Some code
}

func (fp *FilterMap) RemoveFilter(i uuid.UUID) {
    // Some code
}

func (fp *FilterMap) GetFilterByID(i uuid.UUID) *FilterInterface {
    // Some code
}

On some other package, I have the following code:

func DoFilter() {
    fieldfilter := &coreinterfaces.FieldFilter{Key: "app", Val: "152511"}
    filtermap := &coreinterfaces.FilterMap{}
    _ = filtermap.AddFilter(fieldfilter) // <--- Exception is raised here
}

The run-time won’t accept the line mentioned because

"cannot use fieldfilter (type *coreinterfaces.FieldFilter) as type
*coreinterfaces.FilterInterface in argument to fieldint.AddFilter:
*coreinterfaces.FilterInterface is pointer to interface, not interface"

However, when changing the code to:

func DoBid() error {
    bs := string(b)
    var ifilterfield coreinterfaces.FilterInterface
    fieldfilter := &coreinterfaces.FieldFilter{Key: "app", Val: "152511"}
    ifilterfield = fieldfilter
    filtermap := &coreinterfaces.FilterMap{}
    _ = filtermap.AddFilter(&ifilterfield)
}

Everything is alright and when debugging the application it really seems to include

I’m a bit confused on this topic. When looking at other blog posts and stack overflow threads discussing this exact same issue (for example – This, or
This) the first snippet which raises this exception should work, because both fieldfilter and fieldmap are initialized as pointers to interfaces, rather than value of interfaces. I haven’t been able to wrap my head around what actually happens here that I need to change in order for me not to declare a FieldInterface and assign the implementation for that interface. There must be an elegant way to do this.

Solution

So you’re confusing two concepts here. A pointer to a struct and a pointer to an interface are not the same. An interface can store either a struct directly or a pointer to a struct. In the latter case, you still just use the interface directly, not a pointer to the interface. For example:

type Fooer interface {
    Dummy()
}

type Foo struct{}

func (f Foo) Dummy() {}

func main() {
    var f1 Foo
    var f2 *Foo = &Foo{}

    DoFoo(f1)
    DoFoo(f2)
}

func DoFoo(f Fooer) {
    fmt.Printf("[%T] %+v\n", f, f)
}

Output:

[main.Foo] {}
[*main.Foo] &{}

https://play.golang.org/p/I7H_pv5H3Xl

In both cases, the f variable in DoFoo is just an interface, not a pointer to an interface. However, when storing f2, the interface holds a pointer to a Foo structure.

Pointers to interfaces are almost never useful. In fact, the Go runtime was specifically changed a few versions back to no longer automatically dereference interface pointers (like it does for structure pointers), to discourage their use. In the overwhelming majority of cases, a pointer to an interface reflects a misunderstanding of how interfaces are supposed to work.

However, there is a limitation on interfaces. If you pass a structure directly into an interface, only value methods of that type (ie. func (f Foo) Dummy(), not func (f *Foo) Dummy()) can be used to fulfill the interface. This is because you’re storing a copy of the original structure in the interface, so pointer methods would have unexpected effects (ie. unable to alter the original structure). Thus the default rule of thumb is to store pointers to structures in interfaces, unless there’s a compelling reason not to.

Specifically with your code, if you change the AddFilter function signature to:

func (fp *FilterMap) AddFilter(f FilterInterface) uuid.UUID

And the GetFilterByID signature to:

func (fp *FilterMap) GetFilterByID(i uuid.UUID) FilterInterface

Your code will work as expected. fieldfilter is of type *FieldFilter, which fullfills the FilterInterface interface type, and thus AddFilter will accept it.

Here’s a couple of good references for understanding how methods, types, and interfaces work and integrate with each other in Go:

Answered By – Kaedys

Answer Checked By – Jay B. (GoLangFix Admin)

Leave a Reply

Your email address will not be published.