Type Assertion in Go: Why 'Cannot Type Switch on Non-Interface Value' and How to Fix?

Type Assertion in Go: Resolving the "Cannot Type Switch on Non-Interface Value" Error

When working with type assertions in Go, it's possible to encounter the error "cannot type switch on non-interface value." Let's delve into what this means and how to resolve it, with an example using a custom type.

Consider the following code snippet:

package main

import "fmt"
import "strconv"

type Stringer interface {
    String() string
}

type Number struct {
    v int
}

func (number *Number) String() string {
    return strconv.Itoa(number.v)
}

func main() {
    n := &Number{1}
    switch v := n.(type) {
    case Stringer:
        fmt.Println("Stringer:", v)
    default:
        fmt.Println("Unknown")
    }
}

When you run this code, you'll encounter the error "cannot type switch on non-interface value." This indicates that the type assertion is being performed on a non-interface value. In this case, n is a pointer to a Number struct, which is not an interface.

The solution is to cast the value to an interface{} before attempting the type assertion. This is because type assertions can only be performed on interface values. Here's the corrected code:

func main() {
    n := &Number{1}
    switch v := interface{}(n).(type) {
    case Stringer:
        fmt.Println("Stringer:", v)
    default:
        fmt.Println("Unknown")
    }
}

Now, when you run the code, it will print "Stringer: 1." This is because casting n to interface{} allows it to be treated as an interface value for type assertion.

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