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Can Enum's be declared forward?

Posted on 1998-03-19
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Last Modified: 2011-10-03
I am trying to switch from Microsoft Vissual C++ to Borland C++ Builder 3.  In VC I have dozens of cases of foreward declared enum's (created by a utuility) that look like this

enum SomeEnum;

    *      *      *
enum SomeEnum
{
    EnumItem1,
   EnumItem2
};

This worked fine in VC, but is causing a problem for BC.  It complains that the

'SomeEnum' must be a previously defined enumeration tag.

So VC thinks it is legal and BC seems to think it is not.  Who is right?  Is there a way to get this past BC?  (Other than moving the enum declaration).
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Question by:nietod
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6 Comments
 
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Accepted Solution

by:
Nexial earned 150 total points
ID: 1183648
Per K&R (both 1 and 2) it is not legal.   See page 215 in K&R 2
-- "incomplete enumeration types do not exist"

However, if you encapsulate the enum in a typedef, then, since
incomplete typedefs do exist, you can get the same effect.
 
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Author Comment

by:nietod
ID: 1183649
Isn't K&R C, not C++?  
How do you encapsulate it in a typedef.?
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Expert Comment

by:yonat
ID: 1183650
I don't know what Nexial means by "incomplete typedef", but the new C++ standard uses the new keyword "typename" for that purpose:

    typename SomeEnum;

I don't know if BC supports this, though.
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LVL 22

Author Comment

by:nietod
ID: 1183651
Thanks.  That is vaguely familiar.  I'll look into that tomorrow.
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Expert Comment

by:Nexial
ID: 1183652
Encapsulate an enum in a typedef:

typedef enum enum_name {name1, name,...};

It has exactly the same syntax as a typedef on a struct or
union except for incomplete enumeration types.   The tag  (enum_name) without a following list must refer to an in-scope
specifier with a list.   So the enum list must be defined within the same scope, but may follow the typedef declaration.   I have used this in ANSI standard C, so I know it works (if the compiler didn't lie).

I think the same holds true for C++, but I am not absolutely sure.

Obviously, fail my answer if it doesn't work for you.


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LVL 22

Author Comment

by:nietod
ID: 1183653
Things have gotten weird.  Builder 3.0 definitily would not handled forward enum's like I showed above.  Both in my real code and in a small example.  I went to test both of your suggestions and now it works fine.  That is, without employing the suggestions.  i am confussed.  I'll fool with it some more.  I don't wish to accept an answer that I haven't tested.  But if I can't test it, I'll accept Nexial's answer assuming it is right.  
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