fmt.Formatter is woefully under documented. The second argument (called c in
the interface definition) is the verb, i.e. 's', 'v', 'd' etc. The first
argument provides a state interface with functions to allow you to emit the
format of your type. The Flag function on this interface lets you test the type
of format requested. Despite this being an int (also confusingly called c) it's
actually a character, i.e. '#', '+', ' ' etc.
So if someone writes fmt.Sprintf("%+v", yourtype) you will be passed 'v' as the
verb and you must call Flag('+') to test if the caller wants the '+' form.
On Wed, 24 Jun 2020, at 10:32 PM, 'Ian Cottrell' via golang-nuts wrote:
> You want to implement fmt.Formatter
> <https://pkg.go.dev/fmt?tab=doc#Formatter>.
> In general I prefer implementing Formatter to either Stringer or GoStringer
> if I am only doing it for printing, even for the simple cases, as I feel it
> better reflects the intent.
> I reserve implementing the String method for when I really do want to be able
> to access the value programmatically as a string and it is possible to do so
> in a way that is more efficient than printing it.
>
> On Wed, Jun 24, 2020 at 4:33 PM yves baumes <ybau...@gmail.com> wrote:
>> When I want to override the '%v' format, I declare a String() method.
>> When I want to override the '%#v' format, I declare a GoString() method.
>>
>> But there is nothing about '%+v'? There is no PlusStringer() method ?
>>
>> Let's take the following exemple, the %v and %+v formatter prints the exact
>> same results. Which is annoying somehow..
>>
>> ```
>> package main
>>
>> import ("fmt")
>>
>> type page struct {
>> title string
>> body []byte
>> }
>>
>> func (p *page) String() string {
>> return fmt.Sprint("{ ", p.title, " ", string(p.body), " }")
>> }
>>
>> func main() {
>> const title = "helloworld"
>> const text = "Hello everyone"
>> p := &page{title: title, body: []byte(text)}
>> log.Printf("p = %v", p) // prints p = { helloworld Hello everyone }
>> log.Printf("p = %+v", p) // prints p = { helloworld Hello everyone }
>> log.Printf("p = %#v", p) // prints p = &main.page{title:"helloworld",
>> body:[]uint8{0x48, 0x65, 0x6c, [etc]
>> }
>>
>> ```
>>
>> Any clue on how to adapt the String() method to print the fields names?
>>
>>
>>
>>
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