What Does the Receiver in Go Function Declarations Do?

What's Inside the Parentheses in Go Function Declarations?

In Go, you might encounter function declarations with syntax like this:

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    ...
}

func (s *GracefulServer) BlockingClose() bool {
    ...
}

What's inside the parentheses, such as (h handler) and (s *GracefulServer), is known as the "receiver". The way it works in Go differs from other languages.

The Receiver as a Class

The receiver type, like a class in object-oriented programming, represents the entity on which methods are invoked. For instance, if A is a method within class Person, an instance of Person is required to call A.

Receiver as Value or Pointer

In the examples above, h is a value type while s is a pointer. This distinction affects how the method operates:

• Value Type (e.g., handler): Modifications to the receiver within the method do not affect the original instance.
• Pointer Type (e.g., *GracefulServer): Changes made to the receiver persist in the original instance.

When to Use Pointers

Use pointers as receivers when you want to modify the state of the receiver. For example, if you need BlockingClose to alter the state of GracefulServer, s *GracefulServer is necessary.

Spec Reference:

For more details, refer to the official Go specification: https://golang.org/ref/spec#Method_sets

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