Why Can Value Receiver Methods Be Used on Pointers in Go, But Not the Other Way Around?

Methods on Value Receivers vs. Pointer Receivers in Go: Understanding the Design and Rationale

In Go, methods can be defined with either a value receiver (T) or a pointer receiver (T). While methods defined on T can affect copies of T, methods defined on T affect the actual data referenced by T.

Why Methods on Value Receivers Can Also Be Used By Pointer Receivers

Methods defined on T can be used on T because Go automatically dereferences pointers when passing arguments to functions. For example, if you have a method defined on int:

<code class="go">func (n *int) Add(x int) {
    *n += x
}</code>

You can call this method on both *int and int:

<code class="go">var n int
ptr := &n
ptr.Add(5) // Call Add on *int
n.Add(5) // Automatically dereferences ptr when calling Add on int</code>

Why Methods on Pointer Receivers Cannot Be Used By Value Receivers

Methods defined on *T cannot be used on T because Go does not automatically reference values when passing arguments to functions. To create a pointer to a value, you would need to explicitly take its address using the & operator:

<code class="go">var n int
ptr := &n</code>

However, this may not always be possible. For instance, if T is stored in a map or returned from a function, its address may not be static or accessible.

Pros and Cons of the Design

This design provides several benefits:

• It supports both shallow and deep copying: Methods on value receivers allow shallow copies that affect only the object in question, while methods on pointer receivers allow deep copies that also modify the referenced data.
• It ensures type safety: Go ensures that methods defined on *T cannot be called on T unless the value is explicitly dereferenced, preventing potential memory errors.
• It allows for optimizations: By limiting the addresses that can be taken, Go optimizers have more freedom to implement efficient data structures such as maps and interfaces.

However, it also has a drawback:

• It can be less intuitive: Beginners may be confused by the fact that methods on *T can be used on T, but the opposite is not true.

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