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#include <string>
#include <functional>
#include <unordered_set>
 
template<typename T> class BaseB {
 
    protected:
        std::string ms;
 
    private:
        struct hashstruct {
            std::size_t operator()(const T & b) const {
                return b.hash();
            }
        };
 
    public:
        bool operator==(const T & b) const {
            return ms == b.ms;
        }
 
        virtual std::size_t hash() const = 0;
 
        typedef std::unordered_set<T, hashstruct> Set;
};
 
class B01 : public BaseB<B01> {
 
    public:
        std::size_t hash() const {
            return std::hash<std::string>{}(ms) ^ 1;
        }
};
 
class B02 : public BaseB<B02> {
 
    public:
        std::size_t hash() const {
            return std::hash<std::string>{}(ms) ^ 2;
        }
};
class B03 : public B01 , public B02, public BaseB<B03>{
 
	    public:
        std::size_t hash() const {
            return 0;
        }
};
 
int main(int iArgs, char * args[]) {
 
    B01::Set setOfB01;
    setOfB01.insert(B01());
 
    B02::Set setOfB02;
    setOfB02.insert(B02());
 
    B03::Set setOfB03;
    setOfB03.insert(B03());
 
    return 0;
};

prog.cpp: In function 'int main(int, char**)':
prog.cpp:58:5: error: reference to 'Set' is ambiguous
     B03::Set setOfB03;
     ^
prog.cpp:24:51: note: candidates are: typedef class std::unordered_set<B03, BaseB<B03>::hashstruct, std::equal_to<B03>, std::allocator<B03> > BaseB<B03>::Set
         typedef std::unordered_set<T, hashstruct> Set;
                                                   ^
prog.cpp:24:51: note:                 typedef class std::unordered_set<B02, BaseB<B02>::hashstruct, std::equal_to<B02>, std::allocator<B02> > BaseB<B02>::Set
prog.cpp:24:51: note:                 typedef class std::unordered_set<B01, BaseB<B01>::hashstruct, std::equal_to<B01>, std::allocator<B01> > BaseB<B01>::Set
prog.cpp:59:5: error: 'setOfB03' was not declared in this scope
     setOfB03.insert(B03());
     ^

Standard output is empty