c++ - How can I check if a type is an instantiation of a given class template? -

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• doing static_assert template type template 2 answers

is possible check type instantiation of particular template?

i have class template 1 of template parameter must either instantiation of particular template, or other type. instance, consider simple definition of typelist:

struct null_type;  template <typename head, typename tail> struct typelist {     // tail must typelist or null_type      typedef head head;     typedef tail tail; }; 

now, make sure type provided tail template parameter either instantiation of typelist or null_type. use partial specialization define template cases, this:

template <typename head, typename tail> struct typelist; // default, not defined  template <typename head, typename h, typename t> struct typelist< head, typelist<h,t> > // tail = typelist, ok {     typedef head head;     typedef typelist<h,t> tail; };  template <typename head> struct typelist< head, null_type > // tail = null_type, ok {     typedef head head;     typedef null_type tail; }; 

however, end duplicating code, avoid. ideally, i'd need trait test whether type instantiation of template, use enable_if or in static assertions:

#include <boost/mpl/or.hpp> #include <type_traits>  struct null_type;  template <typename head, typename tail> struct typelist {     static_assert(         boost::mpl::or_<             is_instantiation_of< typelist, tail >,             std::is_same< tail, null_type >         >::value,         "tail must typelist or null_type" );      typedef head head;     typedef tail tail; }; 

is such trait (is_instantiation_of) available in standard library or in boost? is possible write one?

i came following solution, using c++11 variadic templates , simple partial specialization:

#include <type_traits>  template < template <typename...> class template, typename t > struct is_instantiation_of : std::false_type {};  template < template <typename...> class template, typename... args > struct is_instantiation_of< template, template<args...> > : std::true_type {}; 

it adapted c++03 using preprocessor generate versions varying number of template parameters, there maybe simpler way.