Functions as Struct Fields vs. Struct Methods: When to Use Which in Go?

Functions as Struct Fields vs. Struct Methods

In Go, functions can be embedded as fields within structs or defined as methods of those structs. Understanding the distinctions between these approaches can optimize your code design.

Fields of Function Type

Fields of function type are not true methods attached to the struct type. They store a reference to a function rather than being part of the struct's method set.

True Methods

True methods, declared with the struct type as the receiver, are integral to the struct's method set. They allow for implementing interfaces and operating on concrete types. Once defined, methods cannot be changed at runtime.

When to Use Fields of Function Type

• For callbacks, such as event handlers or network request handlers.
• To mimic virtual methods by assigning a function to a field and reassigning it at runtime.

When to Use True Methods

• When implementing interfaces
• When working with concrete types and requiring a consistent method set
• When security is a concern, as methods are immutable and field access can potentially be tampered with

Example

<code class="go">type Foo struct {
    Bar func()
}

func main() {
    f := Foo{
        Bar: func() { fmt.Println("initial") },
    }
    f.Bar()

    f.Bar = func() { fmt.Println("changed") }
    f.Bar()
}</code>

Output:

initial
changed

In this example, a function is embedded as a field in the Foo struct. By reassigning the field at runtime, we can change the behavior of the Bar method, demonstrating the flexibility of fields of function type.

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