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Adrian Sandor
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 Posted: Wed Sep 15, 2004 12:43 am Post subject: problems with templates and function pointers |
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Hello,
Here is a simplified example that shows my problem:
//------8<--------
template
{
template<void(*f)(g&)>void e()
{
f(*this);
}
};
template<class T>void f1(g<T>&x)
{
x.e<f1>(); /* here gcc 3.3.1 says "syntax error before `)' token" */
}
template void f1<bool>(g<bool>&x); /* without this, VC++ at Dinkumware
has an unresolved symbol error; with this line added, it compiles and
links */
int main()
{
g<bool> x;
x.e<f1>(); /* here my VC++ says "cannot convert from 'void (__cdecl
*)(g<T> &)' to 'void (__cdecl *const )(g<T> &)' with [T=bool]" */
}
//------>8----------
Please ignore the infinite recursion and the uselessness of the
program. I'm just talking about compiling/linking errors here. I don't
know if the program can be simplified more while keeping the same
behavior; if you can then please do :)
I tested this program with gcc 3.3.1 (mingw) on my computer, VC++ at
http://www.dinkumware.com/exam/, and VC++.NET 2003 Standard on my
computer.
I included the errors in comments.
Here's what I'd like to know:
- is the program valid C++?
- is it supposed to compile and link correctly without the explicit
f1<bool> instantiation?
- what could be the reason that my VC++ behaves differently from the
one at Dinkumware? (AFAIK it's the same version: .NET 2003)
- are those errors caused by compiler bugs?
- what happens if you try this code on other compilers (e.g. gcc 3.4)?
Thanks
Adrian
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Ben Hutchings
Guest
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| Posted: Wed Sep 15, 2004 7:34 pm Post subject: Re: problems with templates and function pointers |
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Adrian Sandor wrote:
| Quote: | Hello,
Here is a simplified example that shows my problem:
//------8<--------
template
{
template<void(*f)(g&)>void e()
{
f(*this);
}
};
template<class T>void f1(g<T>&x)
{
x.e<f1>(); /* here gcc 3.3.1 says "syntax error before `)' token" */
|
You need to add the keyword "template" before "e". See
<http://womble.decadentplace.org.uk/c++/template-faq.html#disambiguation>
for an explanation.
| Quote: | }
template void f1<bool>(g<bool>&x); /* without this, VC++ at Dinkumware
has an unresolved symbol error; with this line added, it compiles and
links */
int main()
{
g<bool> x;
x.e<f1>(); /* here my VC++ says "cannot convert from 'void (__cdecl
*)(g<T> &)' to 'void (__cdecl *const )(g<T> &)' with [T=bool]" */
}
|
That's bizarre. I think the recursive references between the
templates may be confusing it.
| Quote: | //------>8----------
Please ignore the infinite recursion and the uselessness of the
program. I'm just talking about compiling/linking errors here. I don't
know if the program can be simplified more while keeping the same
behavior; if you can then please do :)
I tested this program with gcc 3.3.1 (mingw) on my computer, VC++ at
http://www.dinkumware.com/exam/, and VC++.NET 2003 Standard on my
computer.
I included the errors in comments.
Here's what I'd like to know:
- is the program valid C++?
|
No, but I believe it would be with the addition of "template" as I
noted above.
| Quote: | - is it supposed to compile and link correctly without the explicit
f1<bool> instantiation?
|
I believe so.
| Quote: | - what could be the reason that my VC++ behaves differently from the
one at Dinkumware? (AFAIK it's the same version: .NET 2003)
|
Perhaps they are using different compiler options, though I can't
think of any that would affect template instantiation.
| Quote: | - are those errors caused by compiler bugs?
|
I believe VC++'s error message indicates a bug.
| Quote: | - what happens if you try this code on other compilers (e.g. gcc 3.4)?
|
Comeau 4.3.3 beta accepts the code with the addition of "template".
--
Ben Hutchings
Quantity is no substitute for quality, but it's the only one we've got.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Adrian Sandor
Guest
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| Posted: Thu Sep 16, 2004 11:08 am Post subject: Re: problems with templates and function pointers |
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So it is "x.template e<f1>();"
Thank you very much, that solved it in gcc!
I didn't know about this syntax, always learning 
And by the way, good FAQ, it's bookmarked now :)
| Quote: | - is it supposed to compile and link correctly without the explicit
f1<bool> instantiation?
I believe so.
|
gcc agrees with that, VC++ at Dinkumware doesn't
Thanks
Adrian
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jon hanson
Guest
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| Posted: Fri Sep 17, 2004 2:13 am Post subject: Re: problems with templates and function pointers |
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[email]aditsu (AT) gmail (DOT) com[/email] (Adrian Sandor) wrote in message news:<83dae161.0409141137.79437303 (AT) posting (DOT) google.com>...
| Quote: | Hello,
Here is a simplified example that shows my problem:
//------8<--------
template
{
template<void(*f)(g&)>void e()
{
f(*this);
}
};
template<class T>void f1(g<T>&x)
{
x.e<f1>(); /* here gcc 3.3.1 says "syntax error before `)' token" */
}
|
i've encountered this before and believe it to be a bug in gcc. I
couln't find a workaround either.
<...>
jon
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