Re: why std::vector<T>&& is not a universal reference?
On 2012-10-10 10:48, rogeeva@googlemail.com wrote:
On Tuesday, October 9, 2012 2:19:29 PM UTC-4, Daniel Kr?gler wrote:
struct is_vector<std::vector<T>> : std::true_type {};
While we on the subject, why doesn't standard type traits provide
this?
I don't think that this trait should be standardized, because it is
neither very complicated to write, nor do I see any general pattern in
Ok. That's fair. Though I'd say since there is some container concept
defined in standard, there should be is_container trait defined to match
it, no?
Maybe, but I think we should first look for language concepts that allow
you this kind of concept checking in a more natural and (hopefully)
simpler way. Language concepts didn't get it into C++11, but there was
already a lot of work involved for them.
Generally useful traits provide answer to questions that are related to
type families that satisfy concepts.
Ok. How would I go about writing a trait for types which match a concept
of a container which I can forward iterate over? I'll need .begin(), .size(),
and ::const_iterator.
#include <type_traits>
#include <utility>
struct do_is_my_container
{
template<class T,
class = decltype(std::declval<const T&>().size()),
class = decltype(std::declval<T>().begin()),
class = typename T::const_iterator
>
static std::true_type test(int);
template<class>
static std::false_type test(...);
};
template<class T>
struct is_my_container : decltype(do_is_my_container::test<
typename std::remove_reference<T>::type>(0))
{
};
If you want a bit more advanced tests, you can impose more constraints,
e.g. you might want to verify that the result of the begin() member
function is also convertible to the member type const_iterator. Here a
slightly revised form of do_is_my_container that realizes this and gives
you an idea of the powerful capabilities of expression-based tests:
struct do_is_my_container
{
template<class T,
class = decltype(std::declval<const T&>().size()),
class = typename std::enable_if<
std::is_convertible<decltype(std::declval<T>().begin()),
typename T::const_iterator>::value
>::type
>
static std::true_type test(int);
template<class>
static std::false_type test(...);
};
HTH & Greetings from Bremen,
Daniel Kr?gler
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