Mapping enum to enum

Question

I need to map from one enum type to another. Here is the enum I am given:

enum CfgFruitType { CFGNoFruit, CFGApple, CFGApricot, CFGBanana,
   CFGMango, CFGKiwi, CFGFig, CFGGrape, CFGMelon, CFGLemon,
   CFGNectarine, CFGPineapple, CFGWatermelon, CFGCherry, CFGPear,
   CFGStrawberry };

and I need to map this to a second enum:

enum FAddressType { AddressTypeUnknown, AddressTypeCherry,
  AddressTypeApple, AddressTypePear, AddressTypePlum,
  AddressTypeStrawberry };

Here is my implementation:

FAddressType maptype(CfgFruitType cfgtype) {
   int mapper[16][2] =
   {
      { CFGNoFruit,          AddressTypeUnknown },
      { CFGApple,            AddressTypeApple },
      { CFGApricot,          AddressTypeUnknown },
      { CFGBanana,           AddressTypeUnknown },
      { CFGMango,            AddressTypeUnknown },
      { CFGKiwi,             AddressTypeUnknown },
      { CFGFig,              AddressTypeUnknown },
      { CFGGrape,            AddressTypeUnknown },
      { CFGMelon,            AddressTypeUnknown },
      { CFGLemon,            AddressTypeUnknown },
      { CFGNectarine,        AddressTypeUnknown },
      { CFGPineapple,        AddressTypeUnknown },
      { CFGWatermelon,       AddressTypeUnknown },
      { CFGCherry,           AddressTypeCherry },
      { CFGPear,             AddressTypePear },
      { CFGStrawberry,       AddressTypeStrawberry }
     };

   return cfgtype < 16 ? (FAddressType)mapper[cfgtype][1] : AddressTypeUnknown;
}

Is this the best approach? How can it be improved?

David · Accepted Answer · 2013-03-21 12:05:28Z

You made a look-up table. Here's 2 things to note:

Your look up table is between 2 types, but your table is only 1 type. You break type safety. They are enums and it's 'safe' if the enums have the same underlying type, but dirty either way.
A switch frequently generates a look up table for you, and in this case definitely will on any decent compiler. It will not generate an if-if else-else chain. See the following:

:

FAddressType maptype(CfgFruitType cfgtype)
{
    switch(cfgtype)
    {
    case CFGNoFruit:    return AddressTypeUnknown;
    case CFGApple:      return AddressTypeApple;
    case CFGApricot:    return AddressTypeUnknown;
    case CFGBanana:     return AddressTypeUnknown;
    case CFGMango:      return AddressTypeUnknown;
    case CFGKiwi:       return AddressTypeUnknown;
    case CFGFig:        return AddressTypeUnknown;
    case CFGGrape:      return AddressTypeUnknown;
    case CFGMelon:      return AddressTypeUnknown;
    case CFGLemon:      return AddressTypeUnknown;
    case CFGNectarine:  return AddressTypeUnknown;
    case CFGPineapple:  return AddressTypeUnknown;
    case CFGWatermelon: return AddressTypeUnknown;
    case CFGCherry:     return AddressTypeCherry;
    case CFGPear:       return AddressTypePear;
    case CFGStrawberry: return AddressTypeStrawberry;
    default:            assert(!"Not a valid CfgFruitType!"); return AddressTypeUnknown;
    }
}

Shorter code, very clear, easy to add new cases, very likely to outperform (by a negligible margin) your code.

Or, you could be less explicit and make it really short:

switch(cfgtype)
{
case CFGApple:      return AddressTypeApple;
case CFGCherry:     return AddressTypeCherry;
case CFGPear:       return AddressTypePear;
case CFGStrawberry: return AddressTypeStrawberry;
default:            return AddressTypeUnknown;
}

SylvainD · Accepted Answer · 2013-03-20 14:24:37Z

If I were you I'd write a function to convert from one to the other which would perform a switch on your input.

My motivation to do so would be to :

have warnings/errors whenever a case is not handled. If you do so and you update the enum, you can't forget to update your code performing handling the conversion (-Wswitch : Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration)
you can easily update your code if you need to.

And the overhead in terms of code and

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