[go-nuts] "_ struct{}" ? What does it mean?

On Wed, Sep 2, 2015 at 2:11 PM wrote:

At
https://go.googlesource.com/go/+/master/src/cmd/internal/obj/link.go#254

we have:

type ProgInfo struct {
Flags uint32 // flag bits
Reguse uint64 // registers implicitly used by this instruction
Regset uint64 // registers implicitly set by this instruction
Regindex uint64 // registers used by addressing mode
_ struct{} // to prevent unkeyed literals
}

Could someone explain to me what that "_ struct{}" does? I don't know that
idiom. What is it for?

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On Wed, Sep 2, 2015 at 5:34 PM, 'Chris Manghane' via golang-nuts wrote:

In this struct, it is a field identified by the blank identifier with a
type size of 0 (the size of an empty struct type). Because the field is the
blank identifier, it cannot be accessed directly; it only serves as a
placeholder. A consequence of this is, as the comment states, unkeyed
literals of ProgInfo cannot be constructued e.g. ProgInfo{1, 2, 3, 4} must
be ProgInfo{Flags: 1, Reguse: 2, Regset: 3, Regindex: 4} or the compiler
will complain that there is a missing field in the struct literal. I'd
imagine this is done to avoid bugs that might arise from specifying the
arguments in an incorrect order.
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On Wed, Sep 2, 2015 at 5:33 PM, 'Chris Manghane' via golang-nuts wrote:

In this struct, it is a field identified by the blank identifier with a
type size of 0 (the size of an empty struct type). Because the field is the
blank identifier, it cannot be accessed directly; it only serves as a
placeholder. A consequence of this is, as the comment states, unkeyed
literals of ProgInfo cannot be constructued e.g. ProgInfo{1, 2, 3, 4} must
be ProgInfo{Flags: 1, Reguse: 2, Regset: 3, Regindex: 4} or the compiler
will complain that there is a missing field in the struct literal. I'd
imagine this is done to avoid bugs that might arise from specifying the
arguments in an incorrect order.

On Wednesday, September 2, 2015 at 4:11:49 PM UTC-5, oju...@gmail.com wrote:

At
https://go.googlesource.com/go/+/master/src/cmd/internal/obj/link.go#254

we have:

type ProgInfo struct {
Flags uint32 // flag bits
Reguse uint64 // registers implicitly used by this instruction
Regset uint64 // registers implicitly set by this instruction
Regindex uint64 // registers used by addressing mode
_ struct{} // to prevent unkeyed literals
}

Could someone explain to me what that "_ struct{}" does? I don't know that
idiom. What is it for?

On Wed, Sep 2, 2015 at 7:08 PM Jacob Hands wrote:

Is this a good/common practice?


On Wednesday, September 2, 2015 at 4:11:49 PM UTC-5, oju...@gmail.com
wrote:
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On Thursday, September 3, 2015 at 1:08:20 AM UTC+2, Jacob Hands wrote:

Is this a good/common practice?

On Wednesday, September 2, 2015 at 4:11:49 PM UTC-5, oju...@gmail.com
wrote: