为什么以下Go程序需要函数的名称为String（）而不是其他名称？

Question

I'm following a book which shows the following example.

package main

import (
    "fmt"
)

const (
    KB = 1024
    MB = 1048576          //KB * 1024
    GB = 1073741824       //MB * 1024
    TB = 1099511627776    //GB * 1024
    PB = 1125899906842624 //TB * 1024
)

type ByteSize float64

func (b ByteSize) String() string {
    switch {
    case b >= PB:
        return "Very Big"
    case b >= TB:
        return fmt.Sprintf("%.2fTB", b/TB)
    case b >= GB:
        return fmt.Sprintf("%.2fGB", b/GB)
    case b >= MB:
        return fmt.Sprintf("%.2fMB", b/MB)
    case b >= KB:
        return fmt.Sprintf("%.2fKB", b/KB)
    }
    return fmt.Sprintf("%dB", b)
}

func main() {
    fmt.Println(ByteSize(2048))
    fmt.Println(ByteSize(3292528.64))
}

When I run this program it gives me the following output (in human readable data size units).

2.00KB
3.14MB

But when I change the name of the function called String() to anything else, or if I lower-case the S in String, it gives me the following output.

2048
3.29252864e+06

What is the reason behind that? Is there some String() function attached to some interface and our ByteSize type satisfies that interface? I mean what the hell?

Answer 1

When you define a method named String with no parameters returning a string, you implement the Stringer interface, which is documented here: https://golang.org/pkg/fmt/#Stringer.