Alias Type Definitions do not Inherit Underlying Type Methods

Evan Hicks

The Problem

Type definitions do not inherit underlying type methods.
Compile Error: ./prog.go:17:3: b.Print undefined (type Bar has no field or method Print)

package main

import (
    "fmt"
)

type Foo struct{}

func (f Foo) Print() {
    fmt.Println("foo")
}

type Bar Foo

func main() {
    b := Bar{}
    b.Print()
}
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The Solution

Go does not have traditional inheritance like other languages, therefore the alias type declaration type Bar Foo works differently in Go. When you create a new type based on another type, your new type (Bar) doesn’t inherit the methods from Foo, which is why b.Print is undefined.

The fix:

package main

import (
    "fmt"
)

type Foo struct{}

func (f Foo) Print() {
    fmt.Println("foo")
}

type Bar struct {
    Foo
}

func main() {
    b := Bar{}
    b.Print()
}
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This creates a new type Bar, and embeds it with an implicit Foo object. In this case, when you call a method on a Bar object, it will check to see if Bar has that method, and if it doesn’t, it will check Foo for the method and use that.

Further Reading

• How slow is slow?
• Sentry’s Unified GO SDK
• The GO SDK (docs)