Creating Structs

Go provides several ways to create struct values: keyed literals, positional literals, zero values and pointers to struct literals.

Prefer keyed literals such as Point{X: 3, Y: 5} because the field names explain each value. A zero-valued struct contains the zero value of each field; its fields can be set later.

A pointer literal, &Point{X: 3}, creates a pointer to a struct value. Using new(Point) instead produces a pointer to a zero-valued Point. Neither option requires a constructor function.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

type Point struct{ X, Y int }

func main() {
	a := Point{X: 3, Y: 5}
	var b Point
	b.X = 9
	c := &Point{X: 2, Y: 4}
	fmt.Println("A:", a)
	fmt.Println("B:", b)
	fmt.Println("C:", *c)
}

Output

A: {3 5}
B: {9 0}
C: {2 4}

The keyed literal creates (3,5), the zero struct b starts at (0,0) then becomes (9,0), and c is a pointer to a value (2,4).

• Prefer keyed literals when a struct has multiple fields or may evolve.
• The zero value of a struct is a valid Go value even if a particular application expects extra initialization.

Create a Rectangle struct with Width and Height using both a keyed literal and a zero value. Print both.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Creating Structs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.