Re: TR1 true_type and flase_type, how to combine?
On 3 Apr., 17:43, Michiel.Salt...@tomtom.com wrote:
For type_traits, classes true_type and false_type have been defined.
These types are the equivalent of true and false, but for template
meta-
programming. Hence, I'd also expect the equivanent of &&, || and !
Yet I can't find any mention of them?
The implementation seems trivial:
template <typename L, typename R>
struct and_type{ };
template <>
struct and_type<true_type, true_type> : public true_type { };
template <>
struct and_type<true_type, false_type> : public false_type { };
template <>
struct and_type<false_type, true_type> : public false_type { };
template <>
struct and_type<false_type, falsee_type> : public false_type { };
etcetera. This would allow us to assert properties like
and_type<is_base_of<T, U>, is_base_of<T, V> >
Have these been considered?
I cannot tell you, but can only guess here: Currently, the
C++ Standard Library has only little support for meta-programming
constructs, I think we are still at the beginning. Note also
that std::enable_if has just been introduced into the draft
(since ~three versions or so) and does this only via the very
minimalistic version using a bool non-type-template parameter
(which is named enable_if_c in boost), instead of the type-based
variant. For the current state this is ok, I think, there exists
the basic to implement the library *with* mp techniques, but
most of the building-blocks will be implementation-defined.
It probably needs some more time to introduce an MP sub-library
in the standard library than we have left for C++0x.
To conclude: I would say that for the *current* state of the
C++ Standard Library the above proposed MP operators would
take an outsider-position, because no further units, which
interact with them, exist.
Just my two Euro cent,
Daniel Kr?gler
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