doucitao2944
2015-01-13 18:14
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由fmt.Sprint(e)在Error方法内部产生的无限循环

According to fortyforty's reply to this question:

fmt.Sprint(e) will call e.Error() to convert the value e to a string. If the Error() method calls fmt.Sprint(e), then the program recurses until out of memory.

You can break the recursion by converting the e to a value without a String or Error method.

This is still confusing to me. Why does fmt.Sprint(e) call e.Error() instead of String()? I tried using the Stringer interface, this is my code:

package main

import (
  "fmt"
  "math"
)

type NegativeSqrt float64

func (e NegativeSqrt) Error() string {
  fmt.Printf(".")
  return fmt.Sprint(e)
}

func (e NegativeSqrt) String() string {
  return fmt.Sprintf("%f", e)
}

func Sqrt(x float64) (float64, error) {
  if x < 0 {
    return 0, NegativeSqrt(x)
  }
  return math.Sqrt(x), nil
}

func main() {
  fmt.Println(Sqrt(2))
  fmt.Println(Sqrt(-2))
}

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