c++ - Overload copy assignment operator for a member struct of a non-type template struct -

i have following non-type template:

template<size_t max_size> struct path{     struct point{         float x;         float y;        }     };     point segment[max_size]; }; 

if declare 2 different paths, cannot assign elements of different segments each other, structs may have same structure, of different type :

path<10> path_a ; path<30> path_b ; path_a.segment[0].x = 1; path_a.segment[0].y = 2; path_b.segment[0] = path_a.segment[0]; // <- error c2679 in visual studio) 

of course, if separate definition of point , path, assignment work:

struct point{         float x;         float y;        };  template<size_t max_size> struct path{     point segment[max_size]; }; 

but that's not want (this mwe), wondering how can overload copy assignment operator make work. i've tried numerous variants, example:

template<size_t max_size> struct path{     struct point{         float x;         float y;         template<size_t other_size>         point & operator = (const typename path<other_size>::point & that)         {             x = that.x;             y = that.y;             return *this;         }     };     point segment[max_size]; }; 

but same error. question is: possible overload = in way allows assignment of following form without changing layout of structs?

path_b.segment[0] = path_a.segment[0]; 

yes, such setup possible. @ core, need assignment operator template accept types:

template<class t> point & operator = (const t & that) 

as basic solution, enough. work types have members x , y of compatible types, , produce (usually) ugly error message types don't.

if that's enough you, we're done.

if have other overloads of assignment operator, want selectively disable template one. this, need instrument point classes , use sfinae:

template<size_t max_size> struct path{     struct point{         float x;         float y;         struct enableassignment {};     };     point segment[max_size]; }; 

the instrumentation used this:

template<class t, class u = typename t::enableassignment> point & operator = (const t & that) 

[simplified live example]

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the code above uses default template argument in function template, introduced in c++11. before that, have invoke sfinae in other way:

template <class l, class r> struct sfinaethenright {   typedef r type; };  template <class t> typename sfinaethenright<typename t::enableassignment, point&>::type operator = (const t & that) 

[simplified c++98 live example]