Checking of variable implements interface without compiling

Question

I want to know whether a concrete type implements a specefic interface and print it out. I have written an example [0] with a self defined struct (MyPoint) beeing not an interface-type. MyPoint has the function Read as defined in the interface of io.Reader:

type MyPoint struct {
   X, Y int
}

func (pnt *MyPoint) Read(p []byte) (n int, err error) {
    return 42, nil
}

The aim is to get the information that the concrete type p implements the interface io.Writer. Therefore I have written a short main trieng to get a true for the check.

func main() {
     p := MyPoint{1, 2}
}

The first idea was to check it with the help of reflection and a type-switch and adding check(p) to to the main-function.

func checkType(tst interface{}) {
    switch tst.(type) {
    case nil:
        fmt.Printf("nil")
    case *io.Reader:
        fmt.Printf("p is of type io.Reader\n")
    case MyPoint:
        fmt.Printf("p is of type MyPoint\n")
    default:
        fmt.Println("p is unknown.")
    }
}

The output is: p is of type MyPoint. After a bit of research I know that I should have expected that because Go's types are static and therefore the type of p is MyPoint and not io.Reader. In addition to that io.Reader is an interface-type which is different to type MyPoint.

I found one solution e.g. at [1] which checks whether MyPoint can be an io.Reader at compile time. It works.

var _ io.Reader = (*MyPoint)(nil)

But that isn't the solution I wanted. Tries like below fails, too. I think it's because of the reason above, isn't it?

i := interface{}(new(MyPoint))
    if _, ok := i.(io.Reader); ok {
        fmt.Println("i is an io.Reader")
}
pType := reflect.TypeOf(p)
if _, ok := pType.(io.Reader); ok {
    fmt.Println("The type of p is compatible to io.Reader")
}

readerType := reflect.TypeOf((*io.Reader)(nil)).Elem()
fmt.Printf("p impl. Reader %t \n", pType.Implements(readerType))

Exists one solution to check whether p implements the interface without compiling? I hope that someone can help me.

[0] http://play.golang.org/p/JCsFf7y74C (fixed) http://play.golang.org/p/cIStOOI84Y (old)

[1] Explanation of checking if value implements interface. Golang

editing your "fixed" [0] link to make it work play.golang.org/p/zjPUT_POF- — Dave C
– Dave C, Commented Aug 20, 2015 at 15:27

Didier Spezia · Accepted Answer · 2015-08-20 09:59:18Z

8

It is perfectly possible to do what you want with the reflect package. Here is an example:

package main

import (
    "fmt"
    "io"
    "reflect"
)

type Other int

type MyPoint struct {
    X, Y int
}

func (pnt *MyPoint) Read(p []byte) (n int, err error) {
    return 42, nil
}

func check(x interface{}) bool {
    // Declare a type object representing io.Reader
    reader := reflect.TypeOf((*io.Reader)(nil)).Elem()
    // Get a type object of the pointer on the object represented by the parameter
    // and see if it implements io.Reader
    return reflect.PtrTo(reflect.TypeOf(x)).Implements(reader)
}

func main() {

    x := MyPoint{0, 0}
    y := Other(1)

    fmt.Println(check(x)) // true
    fmt.Println(check(y)) // false
}

The tricky point is to pay attention to how pointers are handled.

edited Aug 20, 2015 at 9:59

answered Aug 20, 2015 at 9:43

Didier Spezia

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1 Comment

Dave C Over a year ago

What's wrong with just doing _, ok := x.(io.Reader)? Then ok is true if x implements io.Reader and false otherwise.

Volker · Accepted Answer · 2015-08-20 10:12:30Z

5

Exists one solution to check whether p implements the interface without compiling?

Yes: Careful reading of the code :-)

How could that be possible? p implements some interface i if the method set of p covers i. You will always have to compile the code.

I assume you want to fail not during compilation but just print during runtime. The trick is getting a non-nil interface type which can be tested with the Implements method of reflect.Type:

pt := reflect.TypeOf(&MyPoint{})
ioReaderType := reflect.TypeOf((*io.Reader)(nil)).Elem()
fmt.Println(pt.Implements(ioReaderType))  //  ==> true

http://play.golang.org/p/2Qcpfjm4ft

answered Aug 20, 2015 at 10:12

Volker

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