Embed a Struct or a Pointer to a Struct in Go: When Should You Choose What?

Embedding Struct vs Pointer to Struct in Struct Used as Pointer

In Go, it's often necessary to reference a struct as a pointer, particularly when working with object-oriented patterns. However, when embedding a struct within another struct that is used as a pointer, there's a choice to be made: embed the struct itself or a pointer to it. This decision has subtle implications that can affect code behavior.

Embedding the Struct

Consider the following code:

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

type A struct {
    B
}</code>

Here, the B struct is embedded directly within the A struct. The zero value of A includes an embedded object of type B, which also has its zero value. As a result, we can safely call the Print method of the embedded B object:

<code class="go">var a A
a.Print() // prints 0</code>

Embedding the Pointer to the Struct

In contrast, consider the following code:

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

type A struct {
    *B
}</code>

Here, the B struct is embedded as a pointer within the A struct. The zero value of A has a nil pointer value for the embedded *B. Attempting to call the Print method on the embedded *B object will result in a panic:

<code class="go">var a A
a.Print() // panics</code>

Copying Differences

When creating new objects of these types, the behavior differs:

• For A objects, a copy of the embedded B object is created, meaning any modifications made to the embedded field will not affect the original object.
• For A objects with embedded pointers, a copy of the *B pointer is created, meaning any modifications made to the underlying B object will affect both the original object and any copies that share the same pointer.

Conclusion

The choice between embedding the struct directly or using a pointer depends on the intended use case. Embedding the struct provides the convenience of accessing the embedded fields directly, while embedding a pointer allows for modifications to the embedded struct to affect multiple copies. Understanding these implications ensures proper usage and avoids potential pitfalls in your code.

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