On Saturday, January 11, 2003, at 09:42 AM, Beman Dawes wrote:
>Or maybe even just:Despite the fact that I suggested it, I don't find it a killer example. ;-)
>
> template <class T>
> struct my_container
> : if_<is_POD<T>::value, impl1, impl2>::type
> {
> ...
> };
These are the examples that resonate with me, particularly Howard's version. It looks so easy, and has obvious practical uses. It would motivate me.
But it needs to be a complete, compilable, runable, program so I can try it, modify it, etc.
I already code stuff like this, with this other pattern:
template <class T, bool> struct impl;
template<class T> struct impl<T, true> { }; // impl1, optimized
template<class T> struct impl<T, false> { }; // impl2, not optimized
template <class T>
struct my_container
: private impl<T, is_POD<T>::value>
{
...
};
I'm not convinced that the version using if_ is significantly simpler. :-\
template<class T> struct impl1 { }; // optimized
template<class T> struct impl2 { }; // not optimized
template <class T>
struct my_container
: private if_<is_POD<T>, impl1<T>, impl2<T> >::type
{
...
};
if_ really shines when it is inconvenient to partially specialize an entire class just to get one little different type:
template <class T>
struct some_example
{
typedef typename if_<some_condition<T>::value, type1, type2>::type the_type;
...
};
Good example:
http://www.cuj.com/experts/1901/austern.htm?topic=experts
-Howard
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