* Address of a cast expression in C++
@ 2002-07-25 13:47 Matt Austern
2002-07-25 14:30 ` Gabriel Dos Reis
0 siblings, 1 reply; 4+ messages in thread
From: Matt Austern @ 2002-07-25 13:47 UTC (permalink / raw)
To: gcc
With both 3.1 and TOT, the following program is rejected by
the C front end but accepted by the C++ front end. g++
compiles it without error or warning.
#include <stdio.h>
int main() {
int n = 0;
char* p = &(char)n;
*p = 0x7f;
printf("%x\n", n);
}
As I read the C++ Standard, this is incorrect; a diagnostic is
required. 5.4/1 says that the result of (char)n is an rvalue,
and 5.3.1/2 says that you can't take the address of an rvalue.
Question: is this violation of the C++ Standard deliberate,
or is it a bug?
The reason I ask, of course, is that gcc has a generalized
lvalue extension. The manual says that even with this
extension it's illegal to write &(int)f. Who's wrong: the manual,
or the compiler?
--Matt
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Address of a cast expression in C++
2002-07-25 13:47 Address of a cast expression in C++ Matt Austern
@ 2002-07-25 14:30 ` Gabriel Dos Reis
2002-08-06 16:27 ` Matt Austern
0 siblings, 1 reply; 4+ messages in thread
From: Gabriel Dos Reis @ 2002-07-25 14:30 UTC (permalink / raw)
To: Matt Austern; +Cc: gcc
Matt Austern <austern@apple.com> writes:
| With both 3.1 and TOT, the following program is rejected by
| the C front end but accepted by the C++ front end. g++
| compiles it without error or warning.
| #include <stdio.h>
|
| int main() {
| int n = 0;
| char* p = &(char)n;
| *p = 0x7f;
| printf("%x\n", n);
| }
|
| As I read the C++ Standard, this is incorrect; a diagnostic is
| required. 5.4/1 says that the result of (char)n is an rvalue,
| and 5.3.1/2 says that you can't take the address of an rvalue.
|
| Question: is this violation of the C++ Standard deliberate,
| or is it a bug?
I would say it is a result of a bug -- I suspect another instance of
code duplication where the problem is fixed in C but not in C++ :-(
Indeed, the C++ front-end's copy (in typeck.c) of the C front-end's
build_unary_op() (in c-typeck.c) doesn't not handle the case you
spotted. Yet another instance of "code duplication is evil".
| The reason I ask, of course, is that gcc has a generalized
| lvalue extension. The manual says that even with this
| extension it's illegal to write &(int)f. Who's wrong: the manual,
| or the compiler?
The compiler is buggy.
-- Gaby
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Address of a cast expression in C++
2002-07-25 14:30 ` Gabriel Dos Reis
@ 2002-08-06 16:27 ` Matt Austern
2002-08-06 16:37 ` Gabriel Dos Reis
0 siblings, 1 reply; 4+ messages in thread
From: Matt Austern @ 2002-08-06 16:27 UTC (permalink / raw)
To: Gabriel Dos Reis; +Cc: gcc
On Thursday, July 25, 2002, at 11:38 AM, Gabriel Dos Reis wrote:
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Address of a cast expression in C++
2002-08-06 16:27 ` Matt Austern
@ 2002-08-06 16:37 ` Gabriel Dos Reis
0 siblings, 0 replies; 4+ messages in thread
From: Gabriel Dos Reis @ 2002-08-06 16:37 UTC (permalink / raw)
To: Matt Austern; +Cc: gcc
Matt Austern <austern@apple.com> writes:
| On Thursday, July 25, 2002, at 11:38 AM, Gabriel Dos Reis wrote:
|
| > | As I read the C++ Standard, this is incorrect; a diagnostic is
| > | required. 5.4/1 says that the result of (char)n is an rvalue,
| > | and 5.3.1/2 says that you can't take the address of an rvalue.
| > |
| > | Question: is this violation of the C++ Standard deliberate,
| > | or is it a bug?
| >
| > I would say it is a result of a bug -- I suspect another instance of
| > code duplication where the problem is fixed in C but not in C++ :-(
|
| You're right that code duplication is evil, but the problem here
| turns out to be different from what we thought.
Thanks for the update. Next time, I'll look much harder :-) :-)
-- Gaby
^ permalink raw reply [flat|nested] 4+ messages in thread
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2002-07-25 13:47 Address of a cast expression in C++ Matt Austern
2002-07-25 14:30 ` Gabriel Dos Reis
2002-08-06 16:27 ` Matt Austern
2002-08-06 16:37 ` Gabriel Dos Reis
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