Re: Initialising a map

On Jun 27, 7:06 am, Angus <anguscom...@gmail.com> wrote:


   To be honest, I don't know of a simple way to do what you want
using both "const" and a non-pointer. But if you don't mind using a
const pointer, you can do it this way:

#include <iostream>
#include <map>
#include <string>

enum RequestType
{
   AOpenEx,
   AClose,
   ARegister,
   AUnregister
};

int main(int argc, char ** argv)
{
   static const std::map<RequestType,std::string> * RequestMap = NULL;
   if (!RequestMap)
   {
      static std::map<RequestType,std::string> tmpMap;
      tmpMap[AOpenEx] = "AOpenEx";
      tmpMap[AClose] = "AClose";
      tmpMap[ARegister] = "ARegister";
      tmpMap[AUnregister] = "AUnregister";

      RequestMap = &tmpMap;
   }

   std::map<RequestType,std::string>::const_iterator it
      = RequestMap->find(AOpenEx);
   std::cout << "RequestMap[AOpenEx] = \""
             << it->second << '"'
             << std::endl;

   // This next line shouldn't compile (due to "const"):
   // (*RequestMap)[AOpenEx] = "blah";

   return 0;
}

   This approach requires you to use RequestMap as a pointer, which
might make your syntax a little messier.

   But if you'd rather not use a pointer (and are fine with not using
"const"), you might prefer this next approach:

#include <iostream>
#include <map>
#include <string>

enum RequestType
{
   AOpenEx,
   AClose,
   ARegister,
   AUnregister
};

int main(int argc, char ** argv)
{
   static std::map<RequestType,std::string> RequestMap;
   if (RequestMap.empty())
   {
      RequestMap[AOpenEx] = "AOpenEx";
      RequestMap[AClose] = "AClose";
      RequestMap[ARegister] = "ARegister";
      RequestMap[AUnregister] = "AUnregister";
   }

   std::cout << "RequestMap[AOpenEx] = \""
             << RequestMap[AOpenEx] << '"'
             << std::endl;

   // Unfortunately, this next line DOES compile:
   RequestMap[AOpenEx] = "blah";

   return 0;
}

   The syntax in the second approach is simpler, I think. However,
this approach suffers from the fact that RequestMap is not declared
"const", so it can be freely populated/modified, which is probably not
what you want.

   However, both approaches benefit from the fact that RequestMap is
only populated once no matter how many times the function it is in is
called.

   I hope this helps, Angus.

   -- Jean-Luc