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From: blodgett@apollo.HP.COM (Bruce Blodgett)
Newsgroups: comp.std.c
Subject: Re: How soon can `sizeof (tagged_type)' be used?
Message-ID: <519a28cf.20b6d@apollo.HP.COM>
Date: 16 May 91 19:44 GMT
References: <5506@lupine.NCD.COM> <1991May13.175830.7143@zoo.toronto.edu>
Sender: root@apollo.HP.COM
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In-reply-to: henry@zoo.toronto.edu's message of 13 May 91 17:58 GMT

In article <1991May13.175830.7143@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>In article <5506@lupine.NCD.COM> rfg@lupine.ncd.com (Ron Guilmette) writes:
>>Are either or both of these examples legal?
>>
>>enum E { red, green = sizeof (enum E), blue };
>>struct S { int i; int j : sizeof (struct S); int k; };
>
>I think neither.
>
>In the first one, the argument of sizeof is meaningless, because E is not
>declared until the end of the brace-enclosed list.  Enums, unlike structs
>and unions, cannot be incomplete types.

Section 3.1.2.1 states:
"enumeration tags have scope that begins just aftert the appearance of
the tag in a type specifier that declares the tag."

So enum tag E is in scope at the sizeof(enum E).

Section 3.3.3.4 lists the following Constraint:
"The sizeof operator shall not be applied to an expression that has ...
an incomplete type"

The question becomes, "does enum E have incomplete type inside its
enumerator-list?"

Section 3.1.2.5 defines incomplete types as:
"types that describe objects but lack information needed to determine
their sizes".

The question becomes, "does enum E lack information needed to
determine its size inside its enumerator-list?"

Section 3.5.2.2 states:
"Each enumerated type shall be compatible with an integer type; the
choice of type is implementation-defined."

The implementation could choose exactly one integral type with which
to make all enumerated types compatible.  In this case, enum E has
enough information to determine its size inside its enumerator-list.

Alternately, the implementation could tailor each enumerated type to
be the smallest integral type sufficient to store any of its member's
values (to save on disk space).  In this case, enum E does NOT have
enough information to determine its size inside its enumerator-list.

Therefore the definition
   enum E { red, green = sizeof (enum E), blue };

relies upon implementation-defined behavior.
Bruce Blodgett
blodgett@apollo.hp.com
(508) 256-0176 x4037