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Jay
Guest
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 Posted: Tue Mar 22, 2005 2:01 am Post subject: Can template specialization be based on a base class? |
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I'd like to change the behaviour of a template class if the template
parameters are derived from a certain class e.g.
class Base
{
....
};
class Derived1 : public Base
{
....
};
class Derived2 : public Base
{
....
};
template <class T> class MyTemplateClass
{
public:
void f () {...} ;
};
so what I'd like to do is something like:
template <> class MyTemplateClass <derived from Base>
{
public:
void f () {/* do something different */ };
}
So far the nearest I've got is to do run-time checking i.e.
template <class T> void MyTemplateClass<T>::f ()
{
Base* b = dynamic_cast <Base*> (this);
if (b)
{
...
}
}
I'm not sure how much a compiler would optimize this and I'd rather not
modify the original template class if possible.
Any better ways of doing this?
Thanks,
Jay.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Jonathan Turkanis
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Carl Barron
Guest
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| Posted: Tue Mar 22, 2005 9:17 am Post subject: Re: Can template specialization be based on a base class? |
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In article <d1neb6$bk$1 (AT) nyytiset (DOT) pp.htv.fi>, Jay
<jay (AT) aleka (DOT) freeserve.co.uk> wrote:
| Quote: | I'd like to change the behaviour of a template class if the template
parameters are derived from a certain class e.g.
class Base
{
...
};
class Derived1 : public Base
{
...
};
class Derived2 : public Base
{
...
};
template <class T> class MyTemplateClass
{
public:
void f () {...} ;
};
so what I'd like to do is something like:
template <> class MyTemplateClass <derived from Base
{
public:
void f () {/* do something different */ };
}
So far the nearest I've got is to do run-time checking i.e.
template
{
Base* b = dynamic_cast <Base*> (this);
if (b)
{
...
}
}
I'm not sure how much a compiler would optimize this and I'd rather not
modify the original template class if possible.
Any better ways of doing this?
Thanks,
Non abiguous interitence boost::is_complatable<T *,Base *> or |
is_base_and_derived<T,Base> should provide a compile time type and
value
that is different if T is derived from Base and when it is not. So a
simple solution is tag dispatching via a bool template parameter of a
base class and using a struct to inherit the proper base class, the
struct only has forwarding constructors that are needed,
// the general class
template <class T,bool B> class MyTemplateBase
{
public:
static const bool general = true;
void f(){};
};
// this is the specialize class.
template <class T> class MyTemplateBase<T,true>
{
public:
static const bool general = false;
void f();
};
template <class T>
struct MyTemplate: MyTemplateBase
<
T,
boost::is_compatible
<
T *,
Base *
// forwarding constructors here
};
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ryan Gallagher
Guest
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| Posted: Tue Mar 22, 2005 9:24 am Post subject: Re: Can template specialization be based on a base class? |
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Jay wrote:
| Quote: | I'd like to change the behaviour of a template class if the template
parameters are derived from a certain class
|
Check out boosts type_traits library: www.boost.org. Sorry, but you're
going to have to do a bit of metaprogramming for this (just a bit  .
boost::enable_if can be used to select which member functions make it
into your function template's instance. Something like (not compiled):
#include <boost/type_traits/is_base_and_derived.hpp>
#include <boost/enable_if.hpp>
template<class T>
boost::enable_if<boost::is_base_and_derived
void>::type
MyTemplateClass<T>::f()
{
// do something special for types derived from Base
}
HTH,
-Ryan
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Maxim Yegorushkin
Guest
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| Posted: Tue Mar 22, 2005 10:47 pm Post subject: Re: Can template specialization be based on a base class? |
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Jay <jay (AT) aleka (DOT) freeserve.co.uk> wrote:
| Quote: | I'm not sure how much a compiler would optimize this and I'd rather not
modify the original template class if possible.
Any better ways of doing this?
|
There was recently a similar (I believe) question and a solution regarding
a template specialization deriving a primary template:
http://groups-beta.google.com/group/comp.lang.c++.moderated/msg/eb7a5e89f9bcf7b4
--
Maxim Yegorushkin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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