feadd3c8   
   From: marc.glisse@gmail.com   
      
   Daryle Walker  wrote:   
      
   > template < typename T, unsigned N >   
   > class my_math_vector   
   > {   
   > public:   
   >     constexpr my_math_vector() : c{} {}   
   >     constexpr my_math_vector( T const &first ) : c{first} {}   
   >     constexpr my_math_vector( T const &val, unsigned index );   
   >       // make this->c[index] == val, all other this->c[x] == T()   
   >     //...   
   > private:   
   >     T  c[ N ];   
   > };   
   >   
   > It's easy to fill in an array from the front during a constexpr constructor.   
   > Is it possible to initialize a single array element that is NOT the first   
   > one?  You can't know statically how many zero (or "T()") to put before your   
   > desired value, which prevents a direct solution.  But maybe there's some   
   > sort of recursive template metaprogramming that can do it?...   
      
   (completely untested)   
   You can delegate to:   
   template constexpr   
   my_math_vector(T const&val,unsigned index,index_type)   
   : c{((I==index)?val:T())...} {}   
      
   by passing an index_type<0,1,...,N-1>.   
      
   I am afraid the copies/moves will not be elided though. A factory with   
   index passed as a template argument might work better.   
      
      
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