Embed a Struct or a Pointer to a Struct: When and Why?

Embedding Struct or Pointer to Struct as Pointer: Key Differences

When a struct type A acts as a pointer with only pointer receivers and constructors returning A,embedding another struct type B presents two options: embedding B directly or embedding B.

Zero Value Differences:

The zero values of A with embedded B versus embedded *B differ. When B is embedded directly, the zero value of A contains an embedded zero value of B, allowing safe usage without initialization:

<code class="go">type B struct {
    X int
}

func (b *B) Print() { fmt.Printf("%d\n", b.X) }

type AObj struct {
    B
}

var aObj AObj
aObj.Print() // prints 0</code>

However, embedding a nil pointer value in APtr's zero value makes direct usage impossible:

<code class="go">type APtr struct {
    *B
}

var aPtr APtr
aPtr.Print() // panics</code>

Object Copying:

Objects are copied as expected. Creating a new AObj copies the embedded B:

<code class="go">aObj2 := aObj
aObj.X = 1
aObj2.Print() // prints 0, due to the copy</code>

Conversely, creating a new APtr copies the *B, preserving the shared concrete object:

<code class="go">aPtr.B = &B{}
aPtr2 := aPtr
aPtr.X = 1
aPtr2.Print() // prints 1, due to shared reference</code>

Example:

https://play.golang.org/p/XmOgegwVFeE provides a runnable example demonstrating these differences.

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