c++ - Pattern-match/SFINAE on parameterization of container for specific classes? -

i have base class , derived classes. bit patterns come in external code can make these, , there generic code builds them. generic code has use explicit template "factory":

auto d = factory<d>(data); // d derived class 

this factory sfinae-enabled classes derived base:

template<     class t,     typename = typename std::enable_if<         std::is_base_of<base, t>::value     >::type > t factory(externaldata &data) {     t result;     if (!result.initfromdata(&data))         throw std::runtime_error("data not set");     return result; } 

i'd add variant works std::experimental::optional<derived> cases when bit patterns indicate things unset. in case generic code passing in either type:

auto dod = factory<dod>(data); // dod can derived or std::optional<derived> 

using explicit parameterization this, how create sfinae variant "pattern match" existence for std::experimental::optional<any class base base>?

the desire if write:

template<     std::experimental::optional<class t>, // imaginary     typename = typename std::enable_if<         std::is_base_of<base, t>::value     >::type > std::optional<t> factory(externaldata &data) {     t result;     if (!result.initfromdata(&data))         return nullopt;     return result; } 

is there non-imaginary version of implementing desire? looked @ "using sfinae template class specialisation" (and several others) , seems maybe there ways cooperatively hack container if wrote (?), missing easy answer?

#include <type_traits>  template <typename t> struct extract_optional;  template <typename t> struct extract_optional<std::experimental::optional<t>> {     using type = t; };  template <typename t> using extract_optional_t = typename extract_optional<t>::type;  template <     typename t,     typename op = extract_optional_t<t>,     typename = typename std::enable_if<         std::is_base_of<base, op>::value     >::type > std::experimental::optional<op> factory(externaldata &data) {     op result;     if (!result.initfromdata(&data))         return nullopt;     return result; } 

demo