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RE: two 'const' questions
- From: "Ciaran O'Riordan" <ciaran_o_riordan at hotmail dot com>
- To: me at rupey dot net
- Cc: gcc-help at gcc dot gnu dot org
- Date: Mon, 09 Sep 2002 17:18:34 +0000
- Subject: RE: two 'const' questions
- Bcc:
On <earlier> "Rupert Wood" <me@rupey.net> wrote:
(I don't have all the answers, but I feel compelled to reply because
you wrote "moo cow" :-) )
my favourite meaningless variable names.
> #1 Why does GCC allow writing to 'const' globals and statics?
Could you give us an example of this?
const int cow = 10; | int main() | int main()
| { | {
int main() | const static cow = 6; | const cow = 6;
{ | cow = 7; | cow = 7;
cow = 11; | return 0; | return 0;
return 0; | } | }
} | |
When compiled and run (with gcc 2.95.3 or 3.1.1), the first and second
column will segfault. The third column runs without crashing (and a
larger example shows that 'cow' does received the value 7). All
produce warnings.
Now I don't have time to check the standards right now, but I recall
that C's definition of 'const' is less rigarous than C++'s;
From the standard (section 6.7.3 Type Qualifiers):
@quotation
[...] they specifiy the assumtions a compile can and must make when
accessing an object through an lvalue.
[...]
The syntax and semantics of 'const' were adapted from C++
[...]
const No writes through this lvalue. In the absense of this
qualifier, writes may occur through this lvalue.
@end quotation
pTo me, that says you cannot write to a const variable and by "cannot"
I mean "it is an error to".
> #2 Why doesn't GCC allow the use of 'const' variables as
> initialisation values?
> I get an error message saying "initialiser element is not
> constant".
g++ does allow this but GCC's C does not. Again, I expect this is
related to the C/C++ definitions of const, e.g. vs their definitions
of literal.
hmm, I think I'll send a mail to one of the lists the developers use
and file a bug report. If nothing else, the error message is
confusing.
For comparison, Sun's Forte C compiler rejects it too (but does give
an error for your first point):
Interesting.
I haven't found an answer to #2 in the standard yet...
thanks
Ciaran O'Riordan
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