• Victor Porton (8/8) Aug 19 2020 This declaration does compile:
• FeepingCreature (4/12) Aug 19 2020 It's an enum type whose members we don't know.
• Steven Schveighoffer (12/27) Aug 19 2020 I use it as a symbol for UDAs.
• Victor Porton (7/15) Aug 19 2020 Oh, found: "An empty enum body (For example enum E;) signifies an
• FeepingCreature (5/28) Aug 19 2020 It means exactly what it says. The compiler doesn't know what

Victor Porton <porton narod.ru> writes:

This declaration does compile:

enum x;

But what is it? Is it an equivalent of

enum x { }

?

What in the specification allows this looking a nonsense

enum x;

?

FeepingCreature <feepingcreature gmail.com> writes:

On Wednesday, 19 August 2020 at 14:06:16 UTC, Victor Porton wrote:
 This declaration does compile:

 enum x;

 But what is it? Is it an equivalent of

 enum x { }

 ?

 What in the specification allows this looking a nonsense

 enum x;

 ?

It's an enum type whose members we don't know.

So we can't declare "x var;" but we can declare "x* var;".

It's the enum version of "struct SomeExternCStruct;".

Steven Schveighoffer <schveiguy gmail.com> writes:

On 8/19/20 10:06 AM, Victor Porton wrote:
 This declaration does compile:
 
 enum x;
 
 But what is it? Is it an equivalent of
 
 enum x { }
 
 ?
 
 What in the specification allows this looking a nonsense
 
 enum x;
 
 ?

I use it as a symbol for UDAs.

enum required;

struct S
{
     required int x;
}

which can then easily be found.

What is it? I have no idea. I'm just using the name.

I think it's treated as a forward declaration. Kind of like

void foo();

-Steve

Victor Porton <porton narod.ru> writes:

On Wednesday, 19 August 2020 at 14:06:16 UTC, Victor Porton wrote:
 This declaration does compile:

 enum x;

 But what is it? Is it an equivalent of

 enum x { }

 ?

 What in the specification allows this looking a nonsense

 enum x;

 ?

Oh, found: "An empty enum body (For example enum E;) signifies an 
opaque enum - the enum members are unknown."

But what this "unknown" does mean? How "unknown" differs from 
"none" in this context?

The specification is unclear. It does not define the meaning of 
unknown. I will submit a bug report.

FeepingCreature <feepingcreature gmail.com> writes:

On Wednesday, 19 August 2020 at 14:43:22 UTC, Victor Porton wrote:
 On Wednesday, 19 August 2020 at 14:06:16 UTC, Victor Porton 
 wrote:
 This declaration does compile:

 enum x;

 But what is it? Is it an equivalent of

 enum x { }

 ?

 What in the specification allows this looking a nonsense

 enum x;

 ?

 Oh, found: "An empty enum body (For example enum E;) signifies 
 an opaque enum - the enum members are unknown."

 But what this "unknown" does mean? How "unknown" differs from 
 "none" in this context?

 The specification is unclear. It does not define the meaning of 
 unknown. I will submit a bug report.

It means exactly what it says. The compiler doesn't know what 
members are in the enum. So you can't declare a variable of it, 
you can't use it directly.. you can p much only use it in 
pointers.