Overview

Golang provides format identifiers to print the underlying type and underlying value represented by the interface value.

• %T can be used to print the concrete type of the interface value
• %v can be used to print the concrete value of the interface value.

Assume we have an interface animal as below

type animal interface {
    breathe()
    walk()
}

We also have a lion struct implementing this animal interface

type lion struct {
    age int
}

Code

package main

import "fmt"

type animal interface {
    breathe()
    walk()
}

type lion struct {
    age int
}

func (l lion) breathe() {
    fmt.Println("Lion breathes")
}

func (l lion) walk() {
    fmt.Println("Lion walk")
}

func main() {
    var a animal
    a = lion{age: 10}
    fmt.Printf("Underlying Type: %T\n", a)
    fmt.Printf("Underlying Value: %v\n", a)
}

Output

Concrete Type: main.lion
Concrete Value: {10}