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fmt: support printing reflect.Value directly #8965

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rsc opened this issue Oct 20, 2014 · 4 comments
Closed

fmt: support printing reflect.Value directly #8965

rsc opened this issue Oct 20, 2014 · 4 comments
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@rsc
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@rsc rsc commented Oct 20, 2014

A reflect.Value obtained from an unexported field does not support .Interface().
If you hold such a value there is no way to pass it to fmt.Print to get the underlying
value printed.
(The usual way is fmt.Print(v.Interface()).)

Rog asks for a way to print such a reflect.Value. He has cited two unsafe packages that
do this by modifying the Value to remove the 'obtained from unexported field' bit.

Two possibilities.

1) Add a new type, in either reflect or fmt, that fmt recognizes and prints. 
For example, type RawValue struct { V reflect.Value }.

2) Make fmt recognize reflect.Value and print the underlying value instead
of printing the reflect.Value struct as it does today.
@robpike
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@robpike robpike commented Oct 20, 2014

Comment 1:

Ugh.
@rogpeppe
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@rogpeppe rogpeppe commented Oct 20, 2014

Comment 2:

I considered option 2, but I'm not sure it's a viable solution,
as %v will currently produce moderately useful output
when presented with a reflect.Value, because reflect.Value
implements String. People are probably relying on that.
Ugh, agreed. FWIW I would not have agreed to include this hack
in production code, but it's very useful in tests. On reflection,
I should probably clone/reimplement that part of fmt, but the above
RawValue fix would be preferable.
@rsc rsc added accepted labels Oct 20, 2014
@bradfitz bradfitz modified the milestone: Go1.5 Dec 16, 2014
@bradfitz bradfitz removed the release-go1.5 label Dec 16, 2014
@robpike
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@robpike robpike commented Apr 9, 2015

I don't understand the issue. This program prints 3.

package main

import (
"fmt"
"reflect"
)

type t struct { a int }

func main() {
var x = t{3}
fmt.Println(reflect.ValueOf(x.a).Interface())
}

@robpike
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@robpike robpike commented Apr 9, 2015

This shows the problem:

package main

import (
"fmt"
"reflect"
)

type t struct{ a int }

func main() {
var x = t{3}
fmt.Println(reflect.ValueOf(x).Field(0).Interface())
}

@rsc rsc removed accepted labels Apr 14, 2015
@robpike robpike closed this in 049b89d Apr 15, 2015
@golang golang locked and limited conversation to collaborators Jun 25, 2016
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