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Interfaces

Learn about interfaces in Go.

We'll cover the following...

Checking if an interface variable is nil

This is certainly one of the most common traps in Go. An interface in Go is not simply a pointer to a memory location as in some other languages.

An interface has:

  • A static type (the type of the interface itself)
  • A dynamic type
  • A value

Note: A variable of an interface type is equal to nil when both its dynamic type and value are nil.”

Go (1.16.5)
package main
import (
    "fmt"
)
type ISayHi interface {
    Say()
}
type SayHi struct{}
func (s *SayHi) Say() {
    fmt.Println("Hi!")
}
func main() {
    // at this point variable “sayer” only has the static type of ISayHi
    // dynamic type and value are nil
    var sayer ISayHi
    // as expected sayer equals to nil
    fmt.Println(sayer == nil) // true
    // a nil variable of a concrete type
    var sayerImplementation *SayHi
    // dynamic type of the interface variable is now SayHi
    // the actual value interface points to is still nil
    sayer = sayerImplementation
    // sayer no longer equals to nil, because its dynamic type is set
    // even though the value it points to is nil
    // which isn't what most people would expect here
    fmt.Println(sayer == nil) // false
}

The interface value is set to a nil struct. The interface can’t be used for anything, so why isn’t it equal to nil? This is another ...