Re: How to do explicit specialization of non-type template function
Bill wrote:
On Feb 20, 1:01 am, Marcel M?ller <news.5.ma...@spamgourmet.com>
wrote:
Victor Bazarov schrieb:
Sorry, you can't do that. You're trying to define a partial
specialisation of a function template, which is not allowed.
Usually a work-around is to use function object and define a partial
specialization of this class.
Marcel
Thank you, could you send more details?
Example:
/*****************************************************************************
*
* cast traits class
*
*****************************************************************************/
template <typename D, typename S>
struct my_cast_traits
{ static D do_static_cast(S s)
{ return static_cast<D>(s);
}
static D do_dynamic_cast(S s)
{ return dynamic_cast<D>(s);
}
static D do_const_cast(S s)
{ return const_cast<D>(s);
}
// we will never overload the reinterpret_cast!
};
/*****************************************************************************
*
* template functions for convenience
*
*****************************************************************************/
// wee need wrappers here
template <typename D, typename S>
inline D my_static_cast(S s)
{ return my_cast_traits<D,S>::do_static_cast(s);
}
template <typename D, typename S>
inline D my_dynamic_cast(S s)
{ return my_cast_traits<D,S>::do_dynamic_cast(s);
}
template <typename D, typename S>
inline D my_const_cast(S s)
{ return my_cast_traits<D,S>::do_const_cast(s);
}
/*****************************************************************************
*
* Example for specialization for casting boost::shared_ptr
*
*****************************************************************************/
template <typename D, typename S>
struct my_cast_traits<boost::shared_ptr<D>, boost::shared_ptr<S> >
{ static boost::shared_ptr<D> do_static_cast(boost::shared_ptr<S> s)
{ return boost::static_pointer_cast<D>(s);
}
static boost::shared_ptr<D> do_dynamic_cast(boost::shared_ptr<S> s)
{ return boost::dynamic_pointer_cast<D>(s);
}
static boost::shared_ptr<D> do_const_cast(boost::shared_ptr<S> s)
{ return boost::const_pointer_cast<D>(s);
}
};
Now my_static_cast behaves exactly as static cast (see note below),
except that you can additionally write
boost::shared_ptr<X> x;
boost::shared_ptr<Y> y = my_static_cast<boost::shared_ptr<Y>>(x);
This static cast is normally not allowed. And this is an important
difference to C style pointers.
Practically I partially specialized the template function
my_static_cast. Syntactically only a helper class is partially specialized.
Note: since C++ knows nothing about side effect free functions they
cannot be evaluated at compile time even if their arguments are compile
time constants. This results in the difference that the above cast
replacement removes the compile time constant attribute from any
argument passed to it. The built-in static_cast function does not do this.
In my opinion this is a principal lack in the C++ language.
Marcel