struct-as-type-of-map-keys.go

package main

import (
	"encoding/json"
	"fmt"
	"strings"
)

const DASH = "-"

type Pair struct {
	Base, Quote string
}

func (p *Pair) String() string {
	return p.Base + DASH + p.Quote
}

func (p Pair) MarshalText() ([]byte, error) {
	return []byte(p.String()), nil
}

func (p *Pair) UnmarshalText(bytes []byte) error {
	xs := strings.SplitN(string(bytes), DASH, 2)
	if len(xs) != 2 {
		return fmt.Errorf("invalid pair: %s", string(bytes))
	}
	p.Base, p.Quote = xs[0], xs[1]
	return nil
}

func main() {
	// do NOT use pointer as map key
	m1 := map[Pair]float64{}
	m1[Pair{Base: "BTC", Quote: "USDT"}] = 1
	m1[Pair{Base: "ETH", Quote: "USDT"}] = 2
	fmt.Printf("%+v\n", m1)
	bs, e := json.Marshal(m1)
	fmt.Println(string(bs), e)

	// CAN NOT use pointer as map key
	m2 := map[Pair]float64{}
	e = json.Unmarshal(bs, &m2)
	fmt.Println(m2, e)
}

/*
OUTPUT:
map[{Base:BTC Quote:USDT}:1 {Base:ETH Quote:USDT}:2]
{"BTC-USDT":1,"ETH-USDT":2} <nil>
map[{BTC USDT}:1 {ETH USDT}:2] <nil>
*/

1. https://stackoverflow.com/questions/21064244/structs-as-keys-in-go-maps

2. Slice, map, and function types are not comparable.

3. https://stackoverflow.com/questions/55335296/problem-with-marshal-unmarshal-when-key-of-map-is-a-struct