trop tard, je sais, mais qu'en est-ce:
// MSVC++ 2010 SP1 x86
// boost 1.53
#include <tuple>
#include <memory>
// test
#include <iostream>
#include <boost/assert.hpp>
#include <boost/static_assert.hpp>
#include <boost/mpl/size.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/pair.hpp>
#include <boost/mpl/begin.hpp>
#include <boost/mpl/deref.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/placeholders.hpp>
#include <boost/mpl/unpack_args.hpp>
#include <boost/mpl/apply.hpp>
// test
#include <boost/range/algorithm/for_each.hpp>
/*! \internal
*/
namespace detail
{
/*! \internal
*/
namespace runtime_template
{
/*! \internal
fwd
*/
template <
typename Template
, typename Types
, typename Map // top level map iterator
, typename LastMap // top level map iterator
, int Index
, bool Done = std::is_same<Map, LastMap>::value
>
struct apply_recursive_t;
/*! \internal
fwd
*/
template <
typename Template
, typename Types
, typename Map // top level map iterator
, typename LastMap // top level map iterator
, typename First
, typename Last
, int Index
, bool Enable = !std::is_same<First, Last>::value
>
struct apply_mapping_recursive_t;
/*! \internal
run time compare key values + compile time push_back on \a Types
*/
template <
typename Template
, typename Types
, typename Map // top level map iterator
, typename LastMap // top level map iterator
, typename First
, typename Last
, int Index // current argument
, bool Enable /* = !std::is_same<First, Last>::value */
>
struct apply_mapping_recursive_t
{
typedef void result_type;
template <typename TypeIds, typename T>
inline static void apply(const TypeIds& typeIds, T&& t)
{ namespace mpl = boost::mpl;
typedef typename mpl::deref<First>::type key_value_pair;
typedef typename mpl::first<key_value_pair>::type typeId; // mpl::int
if (typeId::value == std::get<Index>(typeIds))
{
apply_recursive_t<
Template
, typename mpl::push_back<
Types
, typename mpl::second<key_value_pair>::type
>::type
, typename mpl::next<Map>::type
, LastMap
, Index + 1
>::apply(typeIds, std::forward<T>(t));
}
else
{
apply_mapping_recursive_t<
Template
, Types
, Map
, LastMap
, typename mpl::next<First>::type
, Last
, Index
>::apply(typeIds, std::forward<T>(t));
}
}
};
/*! \internal
mapping not found
\note should never be invoked, but must compile
*/
template <
typename Template
, typename Types
, typename Map // top level map iterator
, typename LastMap // top level map iterator
, typename First
, typename Last
, int Index
>
struct apply_mapping_recursive_t<
Template
, Types
, Map
, LastMap
, First
, Last
, Index
, false
>
{
typedef void result_type;
template <typename TypeIds, typename T>
inline static void apply(const TypeIds& /* typeIds */, T&& /* t */)
{
BOOST_ASSERT(false);
}
};
/*! \internal
push_back on \a Types template types recursively
*/
template <
typename Template
, typename Types
, typename Map // top level map iterator
, typename LastMap // top level map iterator
, int Index
, bool Done /* = std::is_same<Map, LastMap>::value */
>
struct apply_recursive_t
{
typedef void result_type;
template <typename TypeIds, typename T>
inline static void apply(const TypeIds& typeIds, T&& t)
{ namespace mpl = boost::mpl;
typedef typename mpl::deref<Map>::type Mapping; // [key;type] pair vector
apply_mapping_recursive_t<
Template
, Types
, Map
, LastMap
, typename mpl::begin<Mapping>::type
, typename mpl::end<Mapping>::type
, Index
>::apply(typeIds, std::forward<T>(t));
}
};
/*! \internal
done! replace mpl placeholders of \a Template with the now complete \a Types
and invoke result
*/
template <
typename Template
, typename Types
, typename Map
, typename LastMap
, int Index
>
struct apply_recursive_t<
Template
, Types
, Map
, LastMap
, Index
, true
>
{
typedef void result_type;
template <typename TypeIds, typename T>
inline static void apply(const TypeIds& /* typeIds */, T&& t)
{ namespace mpl = boost::mpl;
typename mpl::apply<
mpl::unpack_args<Template>
, Types
>::type()(std::forward<T>(t));
}
};
/*! \internal
helper functor to be used with invoke_runtime_template()
\note cool: mpl::apply works with nested placeholders types!
*/
template <typename Template>
struct make_runtime_template_t
{
typedef void result_type;
template <typename Base>
inline void operator()(std::unique_ptr<Base>* base) const
{
base->reset(new Template());
}
};
} // namespace runtime_template
} // namespace detail
/*! \brief runtime template parameter selection
\param Template functor<_, ...> placeholder expression
\param Maps mpl::vector<mpl::vector<mpl::pair<int, type>, ...>, ...>
\param Types std::tuple<int, ...> type ids
\param T functor argument type
\note all permutations must be compilable (they will be compiled of course)
\note compile time: O(n!) run time: O(n)
\sa invoke_runtime_template()
\author slow
*/
template <
typename Template
, typename Map
, typename Types
, typename T
>
inline void invoke_runtime_template(const Types& types, T&& t)
{ namespace mpl = boost::mpl;
BOOST_STATIC_ASSERT(mpl::size<Map>::value == std::tuple_size<Types>::value);
detail::runtime_template::apply_recursive_t<
Template
, mpl::vector<>
, typename mpl::begin<Map>::type
, typename mpl::end<Map>::type
, 0
>::apply(types, std::forward<T>(t));
}
/*! \sa invoke_runtime_template()
*/
template <
typename Template
, typename Map
, typename Base
, typename Types
>
inline void make_runtime_template(const Types& types, std::unique_ptr<Base>* base)
{
invoke_runtime_template<
detail::runtime_template::make_runtime_template_t<Template>
, Map
>(types, base);
}
/*! \overload
*/
template <
typename Base
, typename Template
, typename Map
, typename Types
>
inline std::unique_ptr<Base> make_runtime_template(const Types& types)
{
std::unique_ptr<Base> result;
make_runtime_template<Template, Map>(types, &result);
return result;
}
////////////////////////////////////////////////////////////////////////////////
namespace mpl = boost::mpl;
using mpl::_;
class MyClassInterface {
public:
virtual ~MyClassInterface() {}
virtual double foo(double) = 0;
};
template <int P1, int P2, int P3>
class MyClass
: public MyClassInterface {
public:
double foo(double /*a*/) {
// complex computation dependent on P1, P2, P3
std::wcout << typeid(MyClass<P1, P2, P3>).name() << std::endl;
return 42.0;
}
// more methods and fields (dependent on P1, P2, P3)
};
// wrapper for transforming types (mpl::int) to values
template <typename P1, typename P2, typename P3>
struct MyFactory
{
inline void operator()(std::unique_ptr<MyClassInterface>* result) const
{
result->reset(new MyClass<P1::value, P2::value, P3::value>());
}
};
template <int I>
struct MyConstant
: boost::mpl::pair<
boost::mpl::int_<I>
, boost::mpl::int_<I>
> {};
std::unique_ptr<MyClassInterface> Factor(const std::tuple<int, int, int>& constants) {
typedef mpl::vector<
MyConstant<0>
, MyConstant<1>
, MyConstant<2>
, MyConstant<3>
// ...
> MyRange;
std::unique_ptr<MyClassInterface> result;
invoke_runtime_template<
MyFactory<_, _, _>
, mpl::vector<MyRange, MyRange, MyRange>
>(constants, &result);
return result;
}
int main(int /*argc*/, char* /*argv*/[])
{
typedef std::tuple<int, int, int> Tuple;
const Tuple Permutations[] =
{
std::make_tuple(0, 0, 0)
, std::make_tuple(0, 0, 1)
, std::make_tuple(0, 1, 0)
, std::make_tuple(0, 1, 1)
, std::make_tuple(1, 0, 0)
, std::make_tuple(1, 2, 3)
, std::make_tuple(1, 1, 0)
, std::make_tuple(1, 1, 1)
// ...
};
boost::for_each(Permutations, [](const Tuple& constants) { Factor(constants)->foo(42.0); });
return 0;
}
Je voudrais vraiment savoir la raison au-delà de cette question. Pourriez-vous nous expliquer ce que vous essayez d'accomplir en utilisant cette construction étrange? –
Il existe un énorme algorithme qui peut être paramétré à l'aide de paramètres de type entier. En fonction des paramètres, la compilation génère du code hautement optimisé. Maintenant, je veux pouvoir utiliser ces différentes "versions" de l'extérieur sans se soucier de leur implémentation et en spécifiant les paramètres à l'exécution d'une manière supervisée par l'utilisateur. Malgré cette application, il s'agissait également d'une question théorique par pure curiosité. – Danvil
Notez qu'en raison de l'instanciation d'un nombre potentiellement important de spécialisations, l'énorme taille exécutable qui en résulte peut techniquement aller à l'encontre de vos optimisations en termes de performances. Un gros code signifie souvent un code lent, en particulier en présence de motifs de branchement irréguliers. (comme toujours, profil pour savoir ce qui se passe) –