Structs

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Goal

Define a custom type with named fields. After this page you will have the Item type the JSON endpoint returns.

Prerequisites

• Functions

What a struct is

A struct is a named bundle of fields. Each field has a name and a type. Once defined, you create values of the struct type and access fields with ..

Two rules worth knowing now:

• A field name that starts with an uppercase letter is exported — visible outside its package. Lowercase is package-private. The encoding/json package only sees exported fields, so JSON-bound structs use uppercase field names.
• Field tags in backticks (the `json:"id"` part below) tell other packages how to handle the field. The JSON package reads these to pick output key names.

Code

package main

import "fmt"

type Item struct {
	ID    int     `json:"id"`
	Name  string  `json:"name"`
	Price float64 `json:"price"`
}

func main() {
	pen := Item{
		ID:    1,
		Name:  "Pen",
		Price: 2.5,
	}

	fmt.Println(pen)
	fmt.Println("name:", pen.Name)

	// Mutate a field.
	pen.Price = 3.0
	fmt.Println("new price:", pen.Price)
}

Run it:

go run main.go

Output:

{1 Pen 2.5}
name: Pen
new price: 3

The json:"..." tags don't do anything in this file — they sit dormant until the JSON encoder reads them. We wire them up two pages from now.

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