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fmt: an unknown verb on a struct should just print each field with that verb, same as for arrays #2889

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Sajmani opened this issue Feb 6, 2012 · 1 comment
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@Sajmani
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@Sajmani Sajmani commented Feb 6, 2012

What steps will reproduce the problem?
1. go run fmt.go (attached)

What is the expected output? What do you see instead?

We have:
type foo struct {
    n int
    t time.Time
    p *foo
}

We have this time:

--- t = time.Now() ---
%#v == time.Time{sec:63464136979, nsec:565853000, loc:(*time.Location)(0x4e8620)}
%s == Mon Feb  6 09:56:19 -0500 EST 2012
%v == Mon Feb  6 09:56:19 -0500 EST 2012
%+v == Mon Feb  6 09:56:19 -0500 EST 2012

And this instance of foo containing that time:

--- y = &foo{t: t} ---
%#v == &main.foo{n:0, t:time.Time{sec:63464136979, nsec:565853000,
loc:(*time.Location)(0x4e8620)}, p:(*main.foo)(nil)}
%s == &{%!s(int=0) {%!s(int64=63464136979) %!s(int32=565853000) 0x4e8620}
<nil>}
%v == &{0 {63464136979 565853000 0x4e8620} <nil>}
%+v == &{n:0 t:{sec:63464136979 nsec:565853000 loc:0x4e8620} p:<nil>}

None of these verbs call time's String method, whereas %s, %v, and %+v do when t is
passed directly.

This all works fine if the struct type defines its own String method that prints the
time member with %s; see type bar in the attached program.

Attachments:

  1. fmt.go (946 bytes)
@rsc
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@rsc rsc commented Jun 26, 2012

Comment 1:

This happens because the fields are unexported. Reflect won't let you call methods on a
value obtained by use of an unexported field, so fmt cannot call these String methods.

Status changed to Unfortunate.

@golang golang locked and limited conversation to collaborators Jun 24, 2016
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