http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33101
Keith Thompson <Keith.S.Thompson at gmail dot com> changed:
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--- Comment #10 from Keith Thompson <Keith.S.Thompson at gmail dot com>
2012-03-16 19:26:45 UTC ---
This came up in a question on stackoverflow.com:
http://stackoverflow.com/q/9742135/827263
I believe the code is valid. Here's a simpler test case:
typedef void VOID;
int main(VOID) { }
which g++ 4.5.2 rejects with the following:
void_test.cpp:2:10: error: ‘<anonymous>’ has incomplete type
void_test.cpp:2:14: error: invalid use of ‘VOID’
Quoting N3290, a draft of the 2011 ISO C++ standard, section 8.3.5 [dcl.fct]:
The parameter-declaration-clause determines the arguments that can
be specified, and their processing, when the function is called.
[ ... ] If the parameter-declaration-clause is empty, the function
takes no arguments. The parameter list (void) is equivalent to
the empty parameter list. Except for this special case, void
shall not be a parameter type (though types derived from void,
such as void*, can).
Earlier versions of the standard have similar or identical wording.
The last sentence implies that "void" here is being used as a type
name, not as some special-case syntactic use of the keyword. I believe
the phrase "special case" is meant to exclude declarations like
void foo(void, int);
or
void bar(void param);
not to prohibit the use of a typedef. Since a typedef creates an
*alias* for an existing type, the name "VOID" should be usable as a
replacement for "void" whenever it's used as a type name.
Note that the syntax for a parameter-declaration does not refer
specifically to the "void" keyword; rather, "void" is valid in that
context because it's a type name.
Practically speaking, the purpose of allowing "void" as a parameter
declaration is for compatibility with C. Quoting the latest draft
of the C standard, N1570 6.7.6.3p10:
The special case of an unnamed parameter of type void as the only
item in the list specifies that the function has no parameters.
It refers to the *type* void, not the keyword, implying that a
typedef is acceptable in this context -- and gcc 4.5.2 accepts the
above program without complaint when compiling it as C.
I agree that the C++ standard's wording is ambiguous, and *could*
be read as requiring the keyword "void", but I believe the intent
is to permit a typedef. (And if you want to argue that the use of
a typedef is silly, I agree completely.)
I believe that the resolution of DR 577:
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#577
confirms this. The resolution, as of August, 2011, suggests changing
the wording to refer to the *type* void rather than the keyword.
(That change doesn't appear in the N3270 draft; I don't know whether
it appears in the final ISO C++ 2011 standard.)
At the very least, since there is (unfortunately) real-world code that
depends on this, I suggest changing it from a fatal error to a warning,
though I think removing the diagnostic altogether would be better.
