Type assertion and type switch

Sometimes, you may want to know the exact type of an interface variable. In this scenario, you can use type assertion:

x.(T)

x is the variable whose type must be interface, and T is the type which you want to check. For example:

package main

import "fmt"

func printValue(v interface{}) {
    fmt.Printf("The value of v is: %v", v.(int))
}

func main() {
    v := 10
    printValue(v)
}

The running result is:

The value of v is: 10

In the above example, using v.(int) to assert the v is int variable.

if the type assertion operation fails, a running panic will occur: change

fmt.Printf("The value of v is: %v", v.(int))  

into:

fmt.Printf("The value of v is: %v", v.(string))

Then executing the program will get following error:

panic: interface conversion: interface is int, not string

goroutine 1 [running]:
panic(0x4f0840, 0xc0820042c0)
......

To avoid this, type assertion actually returns an additional boolean variable to tell whether this operations holds or not. So modify the program as follows:

package main

import "fmt"

func printValue(v interface{}) {
    if v, ok := v.(string); ok {
        fmt.Printf("The value of v is: %v", v)
    } else {
        fmt.Println("Oops, it is not a string!")
    }

}

func main() {
    v := 10
    printValue(v)
}

This time, the output is:

Oops, it is not a string!

Furthermore, you can also use type switch which makes use of type assertion to determine the type of variable, and do the operations accordingly. Check the following example:

package main

import "fmt"

func printValue(v interface{}) {
    switch v := v.(type) {
    case string:
        fmt.Printf("%v is a string\n", v)
    case int:
        fmt.Printf("%v is an int\n", v)
    default:
        fmt.Printf("The type of v is unknown\n")
    }
}

func main() {
    v := 10
    printValue(v)
}

The running result is here:

10 is an int

Compared to type assertion, type switch uses keyword type instead of the specified variable type (such as int) in the parentheses.

References:
Effective Go;
Go – x.(T) Type Assertions;
How to find a type of a object in Golang?.