Structs, Receiver Methods & Composition

In Plain English

Go achieves object-oriented design without classes. Group fields into structs, attach value and pointer receiver methods, and leverage struct embedding for elegant composition.

The Mental Model

"A struct is like an architectural blueprint. Methods are buttons on the device. Embedding is fitting a standard motor inside a blender instead of rebuilding the motor from scratch."

Deep Dive: How It Works

Struct Definition: Custom compound type aggregating named fields.

Receiver Methods: func (r ReceiverType) Method() attaches behaviors directly to types.

Composition over Inheritance: Struct embedding promotes inner fields and methods to the outer struct.

Syntax Blueprint

type User struct { Name string }
func (u *User) Rename(n string) { u.Name = n }

Declare struct fields and bind methods using value or pointer (*T) receivers.

Core Rules to Remember

Pointer vs Value Receivers: Use pointer receiver (*T) when mutating struct state; use value receiver for read-only.

Struct Embedding: Anonymous embedded structs promote inner methods to the parent struct automatically.

Constructor Idiom: Constructors are regular factory functions named NewType(...) returning *Type.

Common Beginner Traps & How to Fix Them

Mistake:Using value receiver func (a Account) Deposit(amount) and wondering why balance never changes.

Why it happens: A value receiver operates on a local copy of the struct.

How to fix: Change receiver to pointer receiver: func (a *Account) Deposit(amount).

Pro Tip: Be consistent across your type: if any method on a struct requires a pointer receiver, use pointer receivers for all methods on that struct.

Live Interactive Example

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Bank Account with Pointer Receiver Methods

Go 1.22 / Runtime

Output Console

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How it works: Pointer receivers (*Account) allowed Deposit and Withdraw to update Balance directly.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Rectangle Area & Perimeter Methods

Define a Rectangle struct with Width and Height float64 fields.
Implement an Area() float64 value receiver method.
Implement a Perimeter() float64 value receiver method.
In main(), create a Rectangle with Width 5.0 and Height 3.0, and print "Area: %.1f, Perimeter: %.1f".

Sandbox Output

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Core Rules to Remember

Pointer vs Value Receivers: Use pointer receiver (*T) when mutating struct state; use value receiver for read-only.

Struct Embedding: Anonymous embedded structs promote inner methods to the parent struct automatically.

Constructor Idiom: Constructors are regular factory functions named NewType(...) returning *Type.