c++ - Template assignment operator in template class copying inner class issue -

i have template class containing inner class. on operator=() overload want copy instance of inner class error:

no known conversion 'const foo<double>::bar' 'const foo<int>::bar' 

the code looks (you can find example here):

template<typename t> class foo { public:     template<typename>     friend class foo;      class bar {         std::string inner_str;     public:         bar const& operator=(bar const& r) {             inner_str = r.inner_str;             return *this;         }     };      template<typename f>     foo const& operator=(foo<f> const& r) {         value = static_cast<t>(r.value);         str_container = r.str_container;         return *this;     }  private:     bar str_container;     t value; }; 

moving bar outside foo works fine, inner class should part of foo.

how solve issue? thought making bar friend foo friend specializations of foo. don't know how introduce them.

_{sidenote: please feel free change title since don't know how title specific problem.}

foo<double>::bar unrelated type const foo<int>::bar, cannot convert between them implicitly.

since bar appears not depend on template argument, seem sensible solution:

moving bar outside foo works fine

problem solved :)

but inner class should part of foo.

considering how use it, argue no, should not part of foo.

alternatively, make assignment operator of bar template did assignment operator of foo:

template<typename b> bar const& operator=(const b& r) {     inner_str = r.inner_str;     return *this; } template<typename u> friend class foo<u>::bar; // bar must friends other bars 

if definition of bar depend on template argument of foo, use approach. otherwise, wouldn't make sense define identical, separate inner class inside every foo.