c++ - C++11: Implementing a Selector between std::array and std::unordered_map according to item count -


i'm trying create mapping class automatically select proper implementation according possible value range. if value range 256, use std::array, otherwise, use std::unordered_map.

template<typename keytype, typename valtype> class mapimpl : public std::unordered_map<keytype,valtype> {   void foo() { std::cout << "map foo" << std::endl; } };  template<typename valtype, std::size_t size> class arrimpl : public std::array<valtype,size> { public:   void foo() { std::cout << "arr foo" << std::endl; } };  template<typename keytype, typename valtype,          keytype min_value = boost::integer_traits<keytype>::const_min,          keytype max_value = boost::integer_traits<keytype>::const_max> class mappingselector { public:   void bar() { map.foo(); } private:   typedef std::conditional<(max_value-min_value < 256),                            arrimpl<valtype,max_value-min_value+1>,                            mapimpl<keytype,valtype>                           > mappingtype;   mappingtype map; };

so far good(?) instantiate with:

mappingselector<unsigned char, double> ms; ms.bar();

however, compiler doesn't seem , tells me mappingtype has no member named foo.

std::conditional doesn't define directly resulting type, defines struct has typedef type corresponding type.

this means must qualify specific type, typedef should 

typedef std::conditional<(max_value-min_value < 256), arrimpl<valtype,max_value-min_value+1>, mapimpl<keytype,valtype> > mappingtype; typename mappingtype::type map;     ^ requires typename unambiguation                        ^ ::type real typedef

or can typedef mappingtype directly qualifying ::type, same result. actually, since working c++11 can use using, eg:

using mappingtype = typename std::conditional<(max_value-min_value < 256), arrimpl<valtype,max_value-min_value+1>, mapimpl<keytype,valtype> >::type;  mappingtype map;

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