cf/main.go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/l...

// See page 43.
//!+

// Cf converts its numeric argument to Celsius and Fahrenheit.
package main

import (

"fmt"
"os"
"strconv"

"gopl.io/ch2/tempconv"

)

func main() {

for _, arg := range os.Args[1:] {
    t, err := strconv.ParseFloat(arg, 64)
    if err != nil {
        fmt.Fprintf(os.Stderr, "cf: %v\n", err)
        os.Exit(1)
    }
    f := tempconv.Fahrenheit(t)
    c := tempconv.Celsius(t)
    fmt.Printf("%s = %s, %s = %s\n",
        f, tempconv.FToC(f), c, tempconv.CToF(c))
}

}

//!-

tempconv/tempconv.go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/l...

//!+

// Package tempconv performs Celsius and Fahrenheit conversions.
package tempconv

import "fmt"

type Celsius float64
type Fahrenheit float64

const (

AbsoluteZeroC Celsius = -273.15
FreezingC     Celsius = 0
BoilingC      Celsius = 100

)

func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
func (f Fahrenheit) String() string { return fmt.Sprintf("%g°F", f) }

//!-

tempconv/conv.go
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/l...

// See page 41.

//!+

package tempconv

// CToF converts a Celsius temperature to Fahrenheit.
func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }

// FToC converts a Fahrenheit temperature to Celsius.
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }

//!-

上面程序中的
f := tempconv.Fahrenheit(t)
c := tempconv.Celsius(t)
是怎么调用的以下方法呢：
func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
func (f Fahrenheit) String() string { return fmt.Sprintf("%g°F", f) }
这两个方法都是有函数名String的。不太明白这个调用过程。