Check for nil and nil interface in Go

Question

Currently I\'m using this helper function to check for nil and nil interfaces

func isNil(a interface{}) bool {
  defer func() { recover() }()
  return a == n         


        

Answer 1

Two solutions NOT using reflection:

Copy and paste code into editor at: https://play.golang.org/ to see in action.

1: Add an "IsInterfaceNil()" function to interface.

2: Use A "type switch"

CODE BELOW:

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EXAMPLE #1: IsInterfaceNil()

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//:Example #1:
//:I prefer this method because the 
//:TakesInterface function does NOT need to know
//:about all the different implementations of
//:the interface.
package main;
import "fmt";

func main()(){

    var OBJ_OK *MyStruct = &( MyStruct{} );
    var NOT_OK *MyStruct = nil;
    
    //:Will succeed:
    TakesInterface( OBJ_OK );
    
    //:Will fail:
    TakesInterface( NOT_OK );

}

func TakesInterface( input_arg MyInterface ){

    if( input_arg.IsInterfaceNil() ){
        panic("[InputtedInterfaceIsNil]");
    }
    
    input_arg.DoThing();
}

type MyInterface interface{
    DoThing()()
    IsInterfaceNil()(bool)
}
type MyStruct struct{}
func(f *MyStruct)DoThing()(){
    fmt.Println("[MyStruct.DoThing]");
}
func(f *MyStruct)IsInterfaceNil()(bool){
    if(nil==f){ return true; }
    return false;
}

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EXAMPLE #2: Type Switch

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//:Example #2:
//:This will also work, but the function taking
//:the interface needs to know about all 
//:implementations. This defeats a bit of the
//:decoupling from implementation that an
//:interface offers, but if you are just using
//:interfaces for polymorphism, it's probably
//:an okay way to go. (opinion)
package main;
import "fmt";

func main()(){

    //:Will succeed:
    var OBJ_OK *IMPLMENTS_INTERFACE_01 = 
             &( IMPLMENTS_INTERFACE_01{} );
    TakesInterface( OBJ_OK );
    
    //:Will fail:
    var NOT_OK *IMPLMENTS_INTERFACE_01 = nil;
    TakesInterface( NOT_OK );
}

func TakesInterface( hasDoThing MyInterface ){

    //:THIS WILL NOT WORK:
    if(nil==hasDoThing){
        panic("[This_Error_Message_Will_Never_Happen]");
    }
    
    //:TYPE SWITCH TO THE RESCUE:
    switch v := hasDoThing.(type){
    
        case (*IMPLMENTS_INTERFACE_01): 
        if(nil==v){ panic("[Nil_PTR_01]"); }
        
        case (*IMPLMENTS_INTERFACE_02): 
        if(nil==v){ panic("[Nil_PTR_02]"); }
        
        case (*IMPLMENTS_INTERFACE_03): 
        if(nil==v){ panic("[Nil_PTR_03]"); }
        
        default: 
            panic("[UnsupportedInterface]");
    }
    
    hasDoThing.DoThing();
    
}

type IMPLMENTS_INTERFACE_01 struct{};
type IMPLMENTS_INTERFACE_02 struct{};
type IMPLMENTS_INTERFACE_03 struct{};
func (f *IMPLMENTS_INTERFACE_01)DoThing()(){
    fmt.Println( "DoingTheThing_01" );
}
func (f *IMPLMENTS_INTERFACE_02)DoThing()(){
    fmt.Println( "DoingTheThing_02" );
}
func (f *IMPLMENTS_INTERFACE_03)DoThing()(){
    fmt.Println( "DoingTheThing_03" );
}

type MyInterface interface{
    DoThing()()
}

UPDATE: After implementing in my code base, I found #2 (type switch) to be best solution. Specifically because I DON'T want to EDIT the glfw.Window struct in the bindings library I am using. Here is a paste-bin of my use-case. Apologies for my non-standard coding style. https://pastebin.com/22SUDeGG

Answer 2

If neither of the earlier options works for you, the best I could came up so far is:

if c == nil || (reflect.ValueOf(c).Kind() == reflect.Ptr && reflect.ValueOf(c).IsNil())

At least it detects (*T)(nil) cases.

Answer 3

See for example Kyle's answer in this thread at the golang-nuts mailing list.

In short: If you never store (*T)(nil) in an interface, then you can reliably use comparison against nil, no need to use reflection. On the other hand, assigning untyped nil to an interface is always OK.

Answer 4

This is the interface definition for this exmaple solution:

package checker

import (
    "errors"

    "github.com/rs/zerolog"
)

var (
    // ErrNilChecker returned if Check invoked on a nil checker
    ErrNilChecker = errors.New("attempted Check with nil Checker")

    // ErrNilLogger returned if the Check function is provide a nil logger
    ErrNilLogger = errors.New("nil logger provided for Check")
)

// Checker defines the interface
type Checker interface {
    Check(logger *zerolog.Logger) error
}

One of our Checker implementations supports aggregation of Checkers. But testing uncovered the same issue as this thread. This solution uses the reflect package if the simple nil check fails, leveraging the reflect.Value type to resolve the question.

// AggregateChecker implements the Checker interface, and
//  supports reporting the results of applying each checker
type AggregateChecker struct {
    checkers []Checker
}

func (ac *AggregateChecker) Add(aChecker Checker) error {
    if aChecker == nil {
        return ErrNilChecker
    }

    // It is possible the interface is a typed nil value
    // E.g. checker := (&MyChecker)(nil)
    t := reflect.TypeOf(aChecker)
    if reflect.ValueOf(aChecker) == reflect.Zero(t) {
        return ErrNilChecker
    }

    ac.checkers = append(ac.checkers, aChecker)
    return nil
}