fmt: stack overflow

package main

import (
"fmt"
)

type Str string

func (s Str) String() string {
return fmt.Sprintf("Str: %s", s)
}

func main() {
var s Str = "hi"
fmt.Println(s)
}

RunCode:

runtime: goroutine stack exceeds 250000000-byte limit

fatal error: stack overflow

runtime stack:

runtime.throw(0x4e33b0, 0xe)

D:/go/src/runtime/panic.go:530 +0x7f

runtime.newstack()

D:/go/src/runtime/stack.go:940 +0x9a7

runtime.morestack()

D:/go/src/runtime/asm_386.s:382 +0x6f

goroutine 1 [stack growth]:

runtime.heapBitsSetType(0x1270c200, 0x8, 0x8, 0x4c1860)

D:/go/src/runtime/mbitmap.go:679 fp=0x18b10a7c sp=0x18b10a78

.....

原因：

You are implementing

Str.String

in
terms of itself.

return fmt.Sprintf("Str: %s", s)

will
call s.String(), resulting in infinite recursion. Convert s to string first.

This
is working as intended, you are using the %s verb to call Str's String method, which uses fmt.Sprint to call Str's String method, and so on.

修改后代码：

package main

import (
"fmt"
)

type Str string

func (s Str) String() string {
return fmt.Sprintf("Str: %s", string(s))
}

func main() {
var s Str = "hi"
fmt.Println(s)
}