Extract template class default parameters - c++

Question

Is there a way to extract a template class' default parameters only knowing the unspecialized template class at compile time?
I know how to extract an instantiated template class' parameters, like this:
// Just an example class for the demonstration
template<class A, class B=void>
struct example {};
// Template parameters storage class
template<class...An>
struct args;
// MPL utility that extracts the template arguments from a template class
template<class C>
struct get_args
{
typedef args<> type;
};
template<template<class...> class C, class...An>
struct get_args< C<An...> >
{
typedef args<An...> type;
};
// And the assertion
static_assert(
std::is_same
< args<int,void>,
get_args< example<int> >::type
>::value,
"Check this out"
);
Now what I would like to know is if decltype or anything else could be used to retrieve the default template parameters only from the unspecialized template class:
// MPL utility that extract template default arguments from a template class
template<template<class...> class C>
struct get_default_args
{
typedef /* what goes here? */ type;
};
// And the assertion
static_assert(
std::is_same
< args<void>,
get_default_args< example >::type
>::value,
"Check this out"
);
For now, I only figured out how to extract the number of parameters of a template class, not their default value:
namespace detail {
template< template<class> class >
boost::mpl::size_t<1> deduce_nb_args();
template< template<class,class> class >
boost::mpl::size_t<2> deduce_nb_args();
template< template<class,class,class> class >
boost::mpl::size_t<3> deduce_nb_args();
/* ... and so on ... */
}
// MPL utility that extract the number of template arguments of a template class
template<template<class...> class C>
struct get_nb_args :
decltype(detail::deduce_nb_args<C>()) {};
// And the assertion
static_assert(
get_nb_args< example >::value == 2,
"Check this out"
);
Edit
It seems that at the end, and once again, MSVC prevents me to perform this operation.
Something like the following makes the compiler crash with a fatal error C1001: An internal error has occurred in the compiler.
template<template<class...> class D> static
boost::boost::mpl::int_<0> mandatory(D<>*)
{ return boost::boost::mpl::int_<0>(); }
template<template<class...> class D> static
boost::mpl::int_<1> mandatory(D<void>*)
{ return boost::mpl::int_<0>(); }
template<template<class...> class D> static
boost::mpl::int_<2> mandatory(D<void,void>*)
{ return boost::mpl::int_<0>(); }
template<template<typename...> class D> static
boost::mpl::int_<-1> mandatory(...)
{ return boost::mpl::int_<-1>(); }
int check()
{
return mandatory<example>(nullptr);
}
Trying next one results in error C2976: 'D' : too few template arguments
template<template<class,class> class D> static
boost::mpl::int_<0> mandatory2(D<>*)
{ return boost::mpl::int_<0>(); }
template<template<class,class> class D> static
boost::mpl::int_<1> mandatory2(D<void>*)
{ return boost::mpl::int_<0>(); }
template<template<class,class> class D> static
boost::mpl::int_<2> mandatory2(D<void,void>*)
{ return boost::mpl::int_<0>(); }
int check2()
{
return mandatory2<example>(nullptr);
}
So to me it seems that no matter the approach, MSVC forbids programmatic instantiation of a template class making use of default parameters.
In turn, it looks impossible to me to use a SFINAE technique to extract:
1. the mandatory number of parameters;
2. the types of default parameters.
Edit 2
Ok, after several tests it seems to be a bug with MSVC occurring when trying to programmatically instantiate a template class only using default arguments.
I filed a bug report here and another one here.
Here is a traits class allowing to check if a class is instantiable using given template parameters that does not make the compiler crash, but do not evaluates to true for fully default instantiable classes.
namespace detail {
typedef std::true_type true_;
typedef std::false_type false_;
template< template<class...> class D, class...An >
true_ instantiable_test(D<An...>*);
template< template<class...> class D, class...An >
false_ instantiable_test(...);
}
template< template<class...> class C, class...An >
struct is_instantiable : decltype(detail::instantiable_test<C,An...>(nullptr)) {};
That being said, it seems impossible with MSVC to retrieve the template type instantiated with default parameters. Typically the following does not compile:
template< template<class...> class T, class...An >
struct get_default_v0
{
typedef T<An...> type;
};
namespace detail {
template< template<class...> class T, class...An >
T<An...> try_instantiate();
} // namespace detail
template< template<class...> class T, class...An >
struct get_default_v1
{
typedef decltype(detail::try_instantiate<T,An...>()) type;
};
// C2976
static_assert(
std::is_same< get_default_v0<example,int> , example<int,void> >::value,
"v0"
);
// C2976
static_assert(
std::is_same< get_default_v1<example,int> , example<int,void> >::value,
"v1"
);

Answer 1

I'd try something like this:
template <typename ...> struct Get2;
template <template <typename...> class Tmpl,
typename A, typename B, typename ...Rest>
struct Get2<Tmpl<A, B, Rest...>>
{
using type = B;
};
template <template <typename...> class Tmpl> struct GetDefault2
{
using type = typename Get2<Tmpl<void>>::type;
};

Answer 2

I realize this is a long answer, but here is a possible approach:
#include <type_traits>
namespace tmpl
{
namespace detail
{
template<template<typename...> class C, typename... T>
struct is_valid_specialization_impl
{
template<template<typename...> class D>
static std::true_type test(D<T...>*);
template<template<typename...> class D>
static std::false_type test(...);
using type = decltype(test<C>(0));
};
} // namespace detail
template<template<typename...> class C, typename... T>
using is_valid_specialization = typename detail::is_valid_specialization_impl<C, T...>::type;
} // namespace tmpl
The following is a partial copy/paste from my github repository, dont worry too much about it, most of the code is to find the minimum/maximum number of template arguments required (in this case we only care about the minimum number):
#if !defined(TEMPLATE_ARGS_MAX_RECURSION)
#define TEMPLATE_ARGS_MAX_RECURSION 30
#endif
namespace tmpl
{
namespace detail
{
enum class specialization_state {
invalid,
valid,
invalid_again
};
template<bool, template<typename...> class C, typename... T>
struct num_arguments_min
: std::integral_constant<int, sizeof...(T)>
{ };
template<template<typename...> class C, typename... T>
struct num_arguments_min<false, C, T...>
: num_arguments_min<is_valid_specialization<C, T..., char>::value, C, T..., char>
{ };
template<specialization_state, template<typename...> class C, typename... T>
struct num_arguments_max;
template<template<typename...> class C, typename... T>
struct num_arguments_max<specialization_state::invalid, C, T...>
: num_arguments_max<
is_valid_specialization<C, T..., char>::value
? specialization_state::valid
: specialization_state::invalid,
C,
T..., char
>
{ };
template<template<typename...> class C, typename... T>
struct num_arguments_max<specialization_state::valid, C, T...>
: std::conditional<
((sizeof...(T) == 0) || (sizeof...(T) == TEMPLATE_ARGS_MAX_RECURSION)),
std::integral_constant<int, -1>,
num_arguments_max<
is_valid_specialization<C, T..., char>::value
? specialization_state::valid
: specialization_state::invalid_again,
C,
T..., char
>
>::type
{ };
template<template<typename...> class C, typename... T>
struct num_arguments_max<specialization_state::invalid_again, C, T...>
: std::integral_constant<int, (sizeof...(T) - 1)>
{ };
} // namespace detail
template<template<typename...> class C>
struct template_traits
{
constexpr static int args_min = detail::num_arguments_min<is_valid_specialization<C>::value, C>::value;
constexpr static int args_max = detail::num_arguments_max<is_valid_specialization<C>::value
? detail::specialization_state::valid
: detail::specialization_state::invalid,
C>::value;
constexpr static bool is_variadic = (args_max < args_min);
template<typename... T>
using specializable_with = is_valid_specialization<C, T...>;
};
} // namespace tmpl
Some helper types specifically for your question:
template<typename... Ts>
struct type_sequence { };
namespace detail
{
template<int N, typename...>
struct skip_n_types;
template<int N, typename H, typename... Tail>
struct skip_n_types<N, H, Tail...>
: skip_n_types<(N - 1), Tail...> { };
template<typename H, typename... Tail>
struct skip_n_types<0, H, Tail...>
{
using type = type_sequence<H, Tail...>;
};
} // namespace detail
template<int N, typename... T>
using skip_n_types = typename detail::skip_n_types<N, T...>::type;
namespace detail
{
template<typename T>
struct get_default_args;
template<template<typename...> class T, typename... A>
struct get_default_args<T<A...> >
{
using type = typename skip_n_types<
tmpl::template_traits<T>::args_min,
A...>::type;
};
} // namespace detail
template<typename T>
using get_default_args = typename detail::get_default_args<T>::type;
To put it all together:
template<typename>
struct dependant { };
template<typename T, typename U = void>
struct example { };
template<typename T, typename U = dependant<T> >
struct dependant_example { };
template<typename T>
void print_type(T)
{
std::cout << __PRETTY_FUNCTION__ << std::endl;
}
int main(int argc, char** argv)
{
{
using example_type = example<int>;
using default_args = get_default_args<example_type>;
print_type(example_type{});
print_type(default_args{});
}
{
using example_type = dependant_example<int>;
using default_args = get_default_args<example_type>;
print_type(example_type{});
print_type(default_args{});
}
}
Output:
void print_type(T) [T = example<int, void>]
void print_type(T) [T = type_sequence<void>]
void print_type(T) [T = dependant_example<int, dependant<int> >]
void print_type(T) [T = type_sequence<dependant<int> >]