Template type member variables in c++ -

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i'm trying change type of membervariable, based on whats written within template.

for example a<64, 64> should make member int_128 sadly, have no idea how work template types, , every tutorial find helps template functions.

my class looks like

template<int x, int y> class a{      private:     typetobegeneric m_variable }

is there way in constructor like

if( x+y <= 64){ typetobegeneric = int_64 }     else{typetobegeneric = int_128}

i don't want add specific type within template<>. structure a<64, 64> should untouched.

constexpr bool lessthan64(int a,int b) {     return (a+b) < 64; }  template<int x, int y> class a{     using type = typename std::conditional<lessthan64(x,y),int_64,int_128>::type;     private:     type m_variable; }

using constexpr functions compile time metaprogramming can evaluate values @ compile time , use std::conditional chose between 2 types.

edit: more 2 types can use variadic templates or explicit specialization.

template<unsigned int i,typename... sizes> struct select;  template<unsigned int i,typename t,typename... sizes> struct select<i,t,sizes...>:select<n-1,cases...> { }      template<unsigned int i,typename t,typename... sizes> struct select<o,t,sizes...> {     using type =t }  need constexpr function differentiate between types.

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