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go

Is an Interface a Pointer?

Suppose I have the following type definitions:

type ICat interface {
  Meow() string
} 

type Cat struct {   
  Name string
}

func (c Cat) Meow() string { 
  return "Meow" 
}

When I perform this operation:

var a Cat
a.Name = "Tom"

A struct of type Cat is allocated in memory and one of its fields gets assigned.

But, if perform the following operation:

var b ICat

What is exactly being allocated in memory? is a Golang Interface just an struct that holds a pointer to another struct? a "Boxed pointer"?.

Solution

  • An interface holds two things:

    • a pointer to the underlying data
    • the type of that data.

    So, when you declare

    var b ICat

    b contains those two elements.

    When you do:

    b := Cat{}

    b now contains a pointer to a copy of Cat{}, and the fact that the data is a struct Cat.

    When you do:

    b := &Cat{}

    b now contains a copy of the pointer to Cat{}, and the fact that it is a *Cat.