struct String() implemention causes stack overflow with Sprintf "+" flag

Question

Golang noob question: Why can I not use the "%+v" flag for a struct in the String() implementation method?

I have a struct where I want to implement a String() method for pretty print. I like the answer given here, but I don't like to type, so I'm trying to modify it to return a string using the "%+v" format flag for structs. from the fmt doc:

%v the value in a default format when printing structs, the plus flag (%+v) adds field names

This works fine if I simply call it with fmt.Printf("%+v", color), but if I try to put the + flag in the String() implementation, I get a stack overflow (my first chance to ask a "stack overflow" question on stackoverflow.com ;) )

I'm sure I'm not understanding a pointer reference here, or there is some recursion. I doubt this noob found my first Golang bug, so can someone please explain?

see go play demonstration here https://play.golang.org/p/13_qI8Iwwa

score 9 · Accepted Answer · 2016-09-27 18:47:47Z

9

See Package fmt Docs:

Except when printed using the verbs %T and %p, special formatting considerations apply for operands that implement certain interfaces. In order of application:

If an operand implements method String() string, that method will be invoked to convert the object to a string, which will then be formatted as required by the verb (if any).

To avoid recursion in cases such as
type X string 
func (x X) String() string { return Sprintf("<%s>", x) }
convert the value before recurring:
func (x X) String() string { return Sprintf("<%s>", string(x)) }
Infinite recursion can also be triggered by self-referential data structures, such as a slice that contains itself as an element, if that type has a String method. Such pathologies are rare, however, and the package does not protect against them.

Inside:

func (c Color) String() string {
    // THIS CAUSES STACK OVERFLOW
    return fmt.Sprint(c)
}

The call to

fmt.Sprint(c)

or fmt.Println(c) which calls func (c Color) String() string again recursively causes overflow: try it on The Go Playground

Also this works fine: https://play.golang.org/p/NYLtrxUeiA

edited Sep 27, 2016 at 18:47

answered Sep 27, 2016 at 18:26

user6169399

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1 Comment

Merl Over a year ago

Thanks @amd. Your initial answer pointed out the obvious recursion... But using the new string (string(x)) loses the struct reference, so there's still no way to use the "+" flag... however, your solution in your playground to use the "#" flag in the String() method uses Go's "syntax representation", gives me the labels, and avoids the recursion. Thanks!

Elmar Zander · Accepted Answer · 2022-01-04 16:05:06Z

2

You can get around the infinite recursion by redefining your type Color as a new local type (e.g. type _Color Color) and then applying Sprintf to the converted value _Color(color), since that cannot recursively call your String() function.

func (color Color) String() string {
    type _Color Color
    return fmt.Sprintf("%+v", _Color(color))
}

Note: don't use type aliases (e.g. type _Color = Color) because that doesn't introduce a new type, but just introduces a new name for the old type and you're back to that infinite recursion problem.

EDIT: I updated your example accordingly, so you can check that it works: https://go.dev/play/p/Cb1jQTcajc4

edited Jan 4, 2022 at 16:05

answered Jan 4, 2022 at 15:58

Elmar Zander

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