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Empty Interface

Empty Interface

Sometimes we have no idea about the types that can be used by the code. Therefore, Go provides us with ‘empty interfaces’ that let us deal with these values. This lesson shows us how to work with them.

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Introduction

The empty or minimal interface has no methods, and so it doesn’t make any demands at all.

type Any interface{}

Any variable, any type implements it (not only reference types that inherit from Object in Java/C#), and any or Any is a good name as an alias and abbreviation! (It is analogous to the class Object in Java and C#, the base class of all classes. So, Obj would also fit.) A variable of that interface type var val interface{} can through assignment receive a variable of any type.

This is illustrated in the program:

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package main
import "fmt"
var i = 5
var str = "ABC"
type Person struct {
	name string
	age  int
}
type Any interface{}	// empty interface
func main() {
	var val Any
	val = 5	// assigning integer to empty interface
	fmt.Printf("val has the value: %v\n", val)
	val = str // assigning string to empty interface
	fmt.Printf("val has the value: %v\n", val)
	pers1 := new(Person)
	pers1.name = "Rob Pike"
	pers1.age = 55
	val = pers1 // assigning *Person type variable to empty interface
	fmt.Printf("val has the value: %v\n", val)
	switch t := val.(type) {	// cases defined on type of val
	case int:	// if val is int
		fmt.Printf("Type int %T\n", t)
	case string: // if val is string
		fmt.Printf("Type string %T\n", t)
	case bool: // if val is bool
		fmt.Printf("Type boolean %T\n", t)
	case *Person:	// if val is *Person
		fmt.Printf("Type pointer to Person %T\n", *t)
	default:	// None of the above types
		fmt.Printf("Unexpected type %T", t)
	}
}

In the above code, at line 4 and line 5, we make two global variables i and str, respectively, and set their values. Then, at line 7, we define a Person type struct with two fields name (a string) and age (an integer). Next, at line 12, we make an empty interface Any.

Now, look at main. We make Any type variable val at line 15. In the next line, we set val to 5. At line 18, val gets the value of str, so in the next line, ABC (value of str) will be printed.

At line 20, we make pers1 a Person pointer-type variable. In the next two lines, we give a value to the internal fields ( Rob Pike as name and 55 as age) of pers1. At line 23, val1 is given the value of pers1. In the next line, val is printed, which prints the pers1 struct as &{Rob Pike 55}.

Now, we have switch cases ...