Re: Accessing elements of static array by name

"BobR" <>
Mon, 27 Aug 2007 18:09:14 GMT
Paul Brettschneider <> wrote in message...

Thanks for your ideas. I know it's a common beginner mistake to care about
performance issues that can't even be measured, but looking up an
associative array at runtime, when the association can be resolved at
compile/linktime or at programm start "feels" wrong. In this case the
lookup will be orders of magnitude faster than the operation on the
element, so it wouldn't matter.

Anyway, I think I will use your idea, but instead of string-ids I will be
using an enum and be populating an id-to-object lookup-table at program
start. Similar to this:

#include <iostream>
#include <algorithm>
#include <iterator>

class A { public: int id; const char *s; };

class Array { public:
        enum Id { element_a = 0, element_b, element_c,
                element_last // Must be last
        static A items[];
        A *last;
        A *id2A[element_last];
        A &operator[](Id id) { return *id2A[id]; }
        A *begin() { return items; }
        A *end() { return last; }

A Array::items[] = {
        { element_a, "a" },
        { element_c, "c" }, // Order needn't be the same
        { element_b, "b" }, // as in enum
        { element_last, NULL } // Must be last

        A *it;
        for(it = items; it->id != element_last; ++it) id2A[it->id] = it;
        last = it;

std::ostream &operator<< (std::ostream &out, const A &a){
        return out << a.s;

static Array array;

int main(){
        // Access all elements
                  std::ostream_iterator<A>(std::cout, "\n"));
        // Access elements by id
        std::cout << array[Array::element_a] << '\n'
                  << array[Array::element_b] << '\n'
                  << array[Array::element_c] << std::endl;
        return 0;

Now it looks (to me) like you are re-inventing the wheel.
The 'wheel' in this case is std::vector.

// includes <iostream>, <vector>, <algorithm>
int main(){
   enum Id{ element_a = 0, element_b, element_c,
         element_last // Must be last
         }; // Id

   std::vector<std::string> Array( element_last ); element_a ) = "a"; element_b ) = "b"; element_c ) = "c";
   // element_last ) = ""; // error. not needed, it's default.

   std::copy( Array.begin(), Array.end(),
         std::ostream_iterator<std::string>(std::cout, "\n") );
   // - Access elements by id -
   std::cout << element_a ) << '\n'
         << element_b ) << '\n'
          << element_c ) << std::endl;
   if ( element_b ) != element_c ) ){
          std::swap( element_b ), element_c ) );
         } // if(b!=c)
   std::copy( Array.begin(), Array.end(),
         std::ostream_iterator<std::string>(std::cout, "\n") );
   return 0;
   } // main()

/* - output: Paul vectors -
a // - Access elements by id -

Bob R

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