Why Can\'t I Take the Address of a Value Embedded Within a Go Interface?

Taking the Address of Values Within Interfaces

In Go, an interface represents a set of methods that a type must implement. When a type implements the methods declared by an interface, it signifies that it can participate in that interface.

Storing a value of a specific type in an interface instance involves the concept of type embedding. The interface value consists of two components: the value of the embedded type and a pointer to the type definition.

Consider the following example:

<code class="go">import "container/list"

type Retry struct{}

func main() {
    el := list.New().Front()
    el.Value = Retry{}
    p := &el.Value.(Retry) // Attempt to take the address of the embedded value
}</code>

However, you may encounter an error:

cannot take the address of el.Value.(Retry)

Understanding the Error

This error occurs because Go prohibits taking the address of values embedded within interfaces. This restriction stems from the way Go manages interface values.

When an interface variable is created, it allocates two words of memory. The first word stores the type information of the embedded value, and the second word contains the value itself or a pointer to the value, depending on its size.

Assigning a new value to an interface variable may reuse the storage allocated for the previous value. This mechanism ensures efficient memory management but poses a challenge when attempting to take the address of an embedded value. If the storage is reused for a new value, the pointer to the previous value becomes invalid.

Consequences and Alternatives

This restriction has significant implications:

• It prevents direct access to the address of values embedded within interfaces.
• It ensures type safety and prevents corruption of data structures.

If you require access to the address of the embedded value, consider the following alternatives:

• Store pointers in the interface: Instead of storing values directly, store pointers to the values within the interface.
• Pass list.Element values: Pass references to the embedded values using list.Element values.

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