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Matthias Hofmann
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 Posted: Sat Aug 16, 2003 1:43 pm Post subject: Overloading template parameters |
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Hello!
I am trying to write a template function that performs a dynamic_cast and
performs an error check. As the error checking is different for pointers and
references, I am trying to overload the template functions. Here's the code,
which does not work (at least on VC++ 6.0) but should clarify what I am
trying to achieve:
// Template function for pointers.
template <class D, class S> D* AssertCast( S* ps )
{
D* pd = dynamic_cast<D*>( ps );
if ( pd != NULL ) { /* Handle error. */}
return pd;
}
// Template function for references.
template <class D, class S> D& AssertCast( S& rs )
{
try { return dynamic_cast<D&>( rs ); }
catch ( bad_cast& ) { /* Handle error. */ }
}
class A {};
class B : public A {};
int main()
{
B* pb = new B;
A* pa = AssertCast<A*>( pb ); // Should call pointer version.
A& ra = AssertCast<A&>( *pb ); // Should call reference version.
delete pb;
return 0;
}
I would like the compiler to invoke the template function for pointers in
the first case and the template function for references in the second case.
However, VC++6.0 rejects the code. I don't know if this is one of the usuall
template problems on this compiler or wether I oversaw anything.
Please help.
Best regards,
Matthias
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Matt Messina
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| Posted: Wed Sep 17, 2003 10:16 am Post subject: Re: Overloading template parameters |
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On 16 Aug 2003, Matthias Hofmann <hofmann (AT) anvil-soft (DOT) com> wrote:
| Quote: | I am trying to write a template function that performs a dynamic_cast and
performs an error check. As the error checking is different for pointers and
references, I am trying to overload the template functions. Here's the code,
which does not work (at least on VC++ 6.0) but should clarify what I am
trying to achieve:
|
| Quote: | // Template function for pointers.
template <class D, class S> D* AssertCast( S* ps )
|
If D is explictly specified as A*, then the return type would be A**.
That's not what you want.
| Quote: | class A {};
class B : public A {};
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Note that dynamic_cast requires a polymorphic type.
| Quote: | A* pa = AssertCast<A*>( pb ); // Should call pointer version.
A& ra = AssertCast<A&>( *pb ); // Should call reference version.
I would like the compiler to invoke the template function for pointers in
the first case and the template function for references in the second case.
However, VC++6.0 rejects the code. I don't know if this is one of the usuall
template problems on this compiler or wether I oversaw anything.
|
Here is a solution using Loki to combine overloading and explicit
function template arguments.
#include <cstddef>
#include <typeinfo>
#include "Loki/TypeManip.h"
#include "Loki/TypeTraits.h"
using std::bad_cast;
using Loki::Int2Type;
using Loki::TypeTraits;
template<class D, class S> D AssertCastAux(S* ps, Int2Type<false>)
{
D pd = dynamic_cast<D>(ps);
if (pd == NULL) { /* Handle error. */ }
return pd;
}
template<class D, class S> D AssertCastAux(S& rs, Int2Type<true>)
{
try { return dynamic_cast<D>(rs); }
catch (bad_cast&) { /* Handle error. */ }
}
template<class D, class S> D AssertCast(S& s)
{
return AssertCastAux<D>(s, Int2Type<TypeTraits());
}
struct A { virtual ~A() { } };
struct B : public A { };
struct C : public A { };
int main()
{
A* pa = new B;
B* pb = AssertCast<B*>(pa);
B& rb = AssertCast<B&>(*pa);
/*
C* pc = AssertCast<C*>(pa);
C& rc = AssertCast<C&>(*pa);
*/
delete pa;
}
I don't have access to MSVC, but Andrei's web site
<http://www.moderncppdesign.com/> references "the most complete port of
Loki to Microsoft Visual C++ 6".
You can use Boost rather than Loki if you prefer:
"Loki/TypeManip.h" => "boost/mpl/bool.hpp"
"Loki/TypeTraits.h" => "boost/type_traits.hpp"
Loki::Int2Type => boost::mpl::bool_
Loki::TypeTraits<>::isReference => boost::is_reference<>::value
Or you could roll your own Int2Type and isReference.
--
Matt Messina
[email]messina (AT) yahoo (DOT) com[/email]
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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White Wolf
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| Posted: Wed Sep 17, 2003 8:33 pm Post subject: Re: Overloading template parameters |
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Matt Messina wrote:
| Quote: | class A {};
class B : public A {};
Note that dynamic_cast requires a polymorphic type.
|
Not always. See 5.2.7 in the standard.
--
WW aka Attila
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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