Answers by Sentry

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()
}

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()
}

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

Loved by over 4 million developers and more than 90,000 organizations worldwide, Sentry provides code-level observability to many of the world’s best-known companies like Disney, Peloton, Cloudflare, Eventbrite, Slack, Supercell, and Rockstar Games. Each month we process billions of exceptions from the most popular products on the internet.

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