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Kobi Cohen-Arazi
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 Posted: Sun Aug 29, 2004 10:45 pm Post subject: Conditional expression between distinct pointer types error |
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Hi all,
Consider the following example:
------------- Start Code a.cpp -------------------
class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
return 0;
}
------------- End Code a.cpp -------------------
on my g++
Reading specs from /usr/lib/gcc-lib/i586-suse-linux/3.3.1/specs
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib
--enable-languages=c,c++,f77,objc,java,ada --disable-checking
--enable-libgcj --with-gxx-include-dir=/usr/include/g++
--with-slibdir=/lib --with-system-zlib --enable-shared
--enable-__cxa_atexit i586-suse-linux
Thread model: posix
gcc version 3.3.1 (SuSE Linux)
I get the following error:
| Quote: | g++ -c a.cpp
a.cpp: In function `int main()': |
a.cpp:9: error: conditional expression between distinct pointer types
`B*' and
`C*' lacks a cast
a.cpp:9: error: invalid conversion from `void*' to `A*'
Any idea why ?
A simple if will work, but why it doesn't work with the ?: operator
Thanks,
Kobi.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ivan Korotkov
Guest
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| Posted: Mon Aug 30, 2004 10:52 am Post subject: Re: Conditional expression between distinct pointer types er |
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| Quote: | ------------- Start Code a.cpp -------------------
class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
return 0;
}
------------- End Code a.cpp -------------------
I get the following error:
g++ -c a.cpp
a.cpp: In function `int main()':
a.cpp:9: error: conditional expression between distinct pointer types
`B*' and
`C*' lacks a cast
a.cpp:9: error: invalid conversion from `void*' to `A*'
Any idea why ?
A simple if will work, but why it doesn't work with the ?: operator
|
"If both expressions are of pointer types or if one is a pointer type and
the other is a constant expression that evaluates to 0, pointer
conversions are performed to convert them to a common type." (MSDN,
"Conditional operator ?:"
In this case, g++ thinks that the common type for C* and B* is void*
(though A* is a less general common type which would be more acceptable
here). Visual C++ 7.1 reports even a more dummy error:
error C2446: ':' : no conversion from 'C *' to 'B *'
so it tries to cast expr2 (&c) to the type of expr1 (B *).
If you want it to work, do this:
A *pa = static_cast<A *>((1) ? &b : &c));
--
Ivan
e-mail me at: korotkov2 at ztel dot ru
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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John Potter
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| Posted: Mon Aug 30, 2004 10:57 am Post subject: Re: Conditional expression between distinct pointer types er |
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On 29 Aug 2004 18:45:44 -0400, [email]kcarazi (AT) finjan (DOT) com[/email] (Kobi Cohen-Arazi)
wrote:
| Quote: | class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
|
A a;
A* pA2 = (1) ? &b : &a;
A* pA3 = (1) ? &a : &c;
| Quote: | }
Any idea why ?
A simple if will work, but why it doesn't work with the ?: operator
|
The requirements for ?: are that the two types are the same or that
one is convertable to the other. Both convertable to something else
is not supported.
John
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Sharad Kala
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| Posted: Mon Aug 30, 2004 11:49 pm Post subject: Re: Conditional expression between distinct pointer types er |
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"Kobi Cohen-Arazi" <kcarazi (AT) finjan (DOT) com> wrote in message
| Quote: | ------------- Start Code a.cpp -------------------
class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
return 0;
}
[snip]
g++ -c a.cpp
a.cpp: In function `int main()':
a.cpp:9: error: conditional expression between distinct pointer types
`B*' and
`C*' lacks a cast
a.cpp:9: error: invalid conversion from `void*' to `A*'
Any idea why ?
|
Because the second and third operands should be of the same type or a
suitable conversion should be present. A quote from 5.16/3 -
"if the second and third operand have different types, and either has
(possibly cv-qualified) class type, an attempt is made to convert each of
those operands to the type of the other". The section later defines the
conversions that are tried by the compiler.
-Sharad
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Motti Lanzkron
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| Posted: Tue Aug 31, 2004 12:03 am Post subject: Re: Conditional expression between distinct pointer types er |
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Kobi Cohen-Arazi wrote:
| Quote: | class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
return 0;
}
|
The second and third operands of the conditional expression must be
convertable to the same type. Since B* isn't convertable to C* and C*
isn't convertable to B* the compilation fails. If any one of the two
operands was an A* it would be OK.
A* pA = true? &b : static_cast<A*>(&c);
Ugly but OK.
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Michiel Salters
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| Posted: Tue Aug 31, 2004 12:12 am Post subject: Re: Conditional expression between distinct pointer types er |
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[email]kcarazi (AT) finjan (DOT) com[/email] (Kobi Cohen-Arazi) wrote in message news:<d23dc578.0408290553.70d542fa (AT) posting (DOT) google.com>...
| Quote: | Hi all,
Consider the following example:
------------- Start Code a.cpp -------------------
class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
return 0;
}
|
The problem is that &b and &c have different types. The compiler
will not try to find a common base type. This is not too hard for
single inheritance, but with MI it is really hard. The compiler
also will not check the left side of the operator=, because
that's how C++ works. The ?: operator is not dependent on a later
operation.
Just use the obvious cast.
Regards,
Michiel Salters
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Old Wolf
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| Posted: Tue Aug 31, 2004 10:19 am Post subject: Re: Conditional expression between distinct pointer types er |
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Ivan Korotkov <clcppm-poster (AT) this (DOT) is.invalid> wrote:
| Quote: |
class A {};
class B: public A {};
class C: public A {};
int main() {
B b;
C c;
A* pA = (1) ? &b : &c;
g++ -c a.cpp
a.cpp: In function `int main()':
a.cpp:9: error: conditional expression between distinct pointer types
`B*' and
`C*' lacks a cast
a.cpp:9: error: invalid conversion from `void*' to `A*'
In this case, g++ thinks that the common type for C* and B* is void*
(though A* is a less general common type which would be more acceptable
here).
|
I wonder why it doesn't find A*. In fact my version of G++ won't
even find void*, and bombs out. Anyway...
| Quote: | If you want it to work, do this:
A *pa = static_cast<A *>((1) ? &b : &c));
|
This is undefined behaviour (assuming that the compiler decided that
the conditional expression had type "void *"). Let's rewrite so that
it is clearer:
void *pv = (1) ? &b : &c;
A *pa = static_cast<A *>(pv);
You can only convert from (void *) to (A *) if the pointer was
originally an (A *) before it was converted to (void *), which is
not the case here.
For example, if B was actually multiply-derived from D and A,
then pv is just a pointer to the first byte of the 'b' object,
and the static cast to A has no idea that it is actually an MI
object so you will end up with a horrible mess.
I'd suggest instead:
A* pa = (1) ? static_cast<A *>(&b) : &c;
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ivan Korotkov
Guest
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| Posted: Tue Aug 31, 2004 7:57 pm Post subject: Re: Conditional expression between distinct pointer types er |
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| Quote: | I wonder why it doesn't find A*. In fact my version of G++ won't
even find void*, and bombs out. Anyway...
|
Visual C++ 7.1 won't either. It says that it cannot find any conversion
from C * to B *.
| Quote: | A* pa = (1) ? static_cast<A *>(&b) : &c;
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Yes, that's really better.
--
Ivan
e-mail me at: korotkov2 at ztel dot ru
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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