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From: James Kuyper <jameskuyper@alumni.caltech.edu>
Newsgroups: comp.lang.c
Subject: Re: Naming conventions (was Re: valgrind leak I can't find)
Date: Fri, 23 Aug 2024 11:57:20 -0400
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On 22.08.2024 16:01, Thiago Adams wrote:
> 
> C++ also made the use of "struct/union/enum" before tags optional.
> 
> For example:
> 
> struct X x;
> 
> In C++, we can write:
> 
> X x;
> 
> Consequence?

That's possible because C defines separate name spaces of identifiers
(not to be confused with C++ namespaces) for labels, tags, each struct
or union type, and attributes; all other identifiers are in the ordinary
name space. Because of the relevant grammar rules, names in the
different spaces can never occur in locations where it is ambiguous as
to which name space the identifier is from. Therefore, C allows you do
use the same identifier in different name spaces in the same scope with
different meanings. For example, identifiers in the tag name space are
always prefixed with struct, union or enum, so they can never be
confused with names from the ordinary name space.

In C++, there are separate name spaces only for macros and labels. All
other identifiers are in the same name space, including tags. Class,
struct, and union members are not controlled by name spaces, but by
scope rules that are different from those in C. That's what allows you
to use a tag without a preceding "class", "struct", or "union" keyword.
Note: The name space for macros isn't really comparable to the other
name spaces I've mentioned, which is why C doesn't have such a name
space. Macros are replaced with their expansions during translation
phase 3, whereas the other name spaces only become meaningful during
translation phase 8.

If C were changed to allow use of tags without "struct" or "union"
before them, the tag name space would have to be merged into the
ordinary name space, and that would break all kinds of legacy code that
uses the same identifier as a tag and as an identifier in the ordinary
name space. That would be a backwards incompatible change, and the
committee tends to avoid such changes.

I once spent some time creating an example program that could be
compiled in either C or C++, that displayed every construct that had
defined behavior in both C and C++, but had incompatibly different
behavior in the two languages. Most of the examples involved name
spaces. Most of the other differences have at least unspecified behavior
in one of the two languages; more often they are syntax errors or
constraint violations in the wrong language.