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From: roger@valid.UUCP
Newsgroups: comp.lang.c++
Subject: Re: Structures want anonymity, too.
Message-ID: <1062@valid.UUCP>
Date: Wed, 11-Mar-87 18:16:54 EST
Article-I.D.: valid.1062
Posted: Wed Mar 11 18:16:54 1987
Date-Received: Fri, 13-Mar-87 03:43:49 EST
References: <165700008@uiucdcsb>
Organization: Valid Logic, San Jose, CA
Lines: 84

We (Valid Logic Systems, Inc.) have modified our 1.1 C++ translator to
allow anonymous structures as well as anonymous unions.  Our primary motivation
for this enhancement was for use with our Pascal-to-C++ translator that we
are developing.  In Pascal there is a construct known as a "variant record"
that takes the following form (from Jensen & Wirth, 2nd Edition, pp. 42 & 45):

<record type> ::= record <field list> end
<field list> ::= <fixed part> | <fixed part> ; <variant part> | <variant part>
<fixed part> ::= <record section> { ; <record section>}
<record section> ::= <field identifier> {, <field identifier>} : <type>|<empty>
<variant part> ::= case <tag field> <type identifier> of <variant> {; <variant>}
<variant> ::= <case label list> : ( <field list> ) | <empty>
<case label list> ::= <case label> {, <case label>}
<case label> ::= <constant>
<tag field> ::= <identifier> : | <empty>

Thus, one can have types such as:

variant_record_example = record
	a: integer;
	case boolean of
		FALSE: (b, c, d: integer; e: real);
		TRUE: (f: array [0 .. 10] of char; g: random_type_name)
	end;

var q: variant_record_example;
begin
q.a := 1;
q.b := 2;
q.e := 3.0;
<etc.>

where all of the names of the various variant and non-variant fields are in
the same scope.  The "equivalent" type in C would look like:

typedef struct {
	int a;
	union {
		struct {
			int b, c, d; float e;
		} _pointless_name_1;
		struct {
			char f[11]; random_type_name g;
		} _pointless_name_2;
	} _pointless_name_3;
} variant_record_example;

variant_record_example q;
q.a = 1;
q._pointless_name_3._pointless_name_1.b = 2;
q._pointless_name_3._pointless_name_1.e = 3;
<etc.>

Standard C++ allows you to eliminate _pointless_name_3, but still requires
_pointless_name_1 and _pointless_name_2.  Our extension allows you to declare
this type as:

typedef struct {
	int a;
	union {
		struct {
			int b, c, d; float e;
		};
		struct {
			char f[11]; random_type_name g;
		};
	};
} variant_record_example;

Once you have anonymous structures AND unions you begin to see their more
abstract function, which is to group a collection of variables for storage
allocation purposes; a struct allocates them in series and a union allocates
them in parallel.  Naming a struct or union then becomes merely a scope issue,
allowing you to create many simultaneous, independent name spaces for
variables.
I have discussed this issue with Bjarne and he said that he is considering
adopting this change in the standard.  Others who believe it is a useful
generalization might want to drop Bjarne a line to that effect.

Roger H. Scott
Valid Logic Systems, Inc.
2820 Orchard Parkway
San Jose, CA 95134
(408) 432-9400 x2484