go 35 lines · 5 steps

Implementing fmt.Stringer in Go

How custom String methods let your types control their own formatted output and compose cleanly.

Explained by highlit
Beginner interfaces stringer composition formatting structs
1package geometry
2 
3import "fmt"
4 
5// Point represents a coordinate in 2D space.
6type Point struct {
7 X, Y int
8}
9 
10// String satisfies the fmt.Stringer interface, controlling how a Point
11// is rendered by fmt verbs like %v, %s, and by Println.
12func (p Point) String() string {
13 return fmt.Sprintf("(%d, %d)", p.X, p.Y)
14}
15 
16// Color models an RGB color.
17type Color struct {
18 R, G, B uint8
19}
20 
21func (c Color) String() string {
22 return fmt.Sprintf("#%02X%02X%02X", c.R, c.G, c.B)
23}
24 
25// Shape composes other Stringer values; its own String method delegates
26// to them, demonstrating how Stringer composes naturally.
27type Shape struct {
28 Name string
29 Origin Point
30 Fill Color
31}
32 
33func (s Shape) String() string {
34 return fmt.Sprintf("%s at %v filled %v", s.Name, s.Origin, s.Fill)
35}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Any type with a String() string method automatically satisfies fmt.Stringer and controls its own rendering under %v, %s, and Println.
  2. 2Implementing Stringer once means every fmt verb and logging call formats your type consistently, without callers writing format strings.
  3. 3Stringer composes: an outer type's String method can use %v on its Stringer fields, so formatting nests automatically.

Related explainers

go 
package main
 
import (
	"errors"

Parsing and validating CLI flags in Go

cli-parsing validation error-handling
Intermediate 8 steps
ruby 
module DurationFormatter
  UNITS = [
    ['week', 604_800],
    ['day', 86_400],

Turning seconds into human-readable durations in Ruby

greedy-decomposition modular-arithmetic formatting
Intermediate 7 steps
go 
package cache
 
import (
	"container/list"

Building a generic LRU cache in Go

lru-cache generics linked-list
Intermediate 8 steps
go 
package model
 
import (
	"encoding/json"

Custom JSON marshaling in Go

json serialization interfaces
Intermediate 5 steps
go 
func (h *TransactionHandler) ExportCSV(c *gin.Context) {
	ctx := c.Request.Context()
	filters := parseTransactionFilters(c)
 

Streaming a CSV export in Gin

streaming csv-export database-cursor
Intermediate 8 steps
go 
package store
 
import (
	"database/sql"

Wrapping and inspecting errors in Go

error-handling error-wrapping sentinel-errors
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

Implementing fmt.Stringer in Go — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code