Go - Interfaces

Implicit Implementation

type Stringer interface{ String() string }

type User struct{ Name string }
func (u User) String() string { return u.Name }

func Print(s fmt.Stringer){ fmt.Println(s.String()) }

Interface Composition

type ReadWriteCloser interface{
  io.Reader
  io.Writer
  io.Closer
}

Tips

• Depend on small interfaces; accept interfaces, return concrete types.
• Use interface values sparingly in hot paths.

Common errors

• Returning interfaces from constructors—prefer returning concrete types and accept interfaces.
• Nil interfaces vs typed nil: a nil interface value has no dynamic type; beware comparisons.

Practice

• Define a small interface (e.g., Sayer) and implement it for two distinct concrete types.
• Write a function that accepts your interface and works with both types.

Quick quiz

1. How does a type implement an interface in Go?