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"
);
