Simplest way of mapping an index to a string in C++

Question

Requirements:

Must be able to use C strings as well as C++ strings
Fast
No maps
No templates
No direct lookup, i.e. index might be out of bounds.
Index is not consecutive
Enums and strings contained in one header file
Only instantiate what you use.

This is what I have come up with so far:

- test.hh -

// Generic mapper
// 
// The idea here is to create a map between an integer and a string.
// By including it inside a class we prevent every module which
// includes this include file from creating their own instance.
//
struct Mapper_s
{
   int Idx;
   const char *pStr;
};

// Status
enum State_t
{
   Running = 1,
   Jumping = 6, 
   Singing = 12
};

struct State_s
{
   static const Mapper_s *GetpMap(void)
   {
       static Mapper_s Map[] = 
       {
            { Running,   "Running" },
            { Jumping,   "Jumping" },
            { Singing,   "Singing" },
            { 0,   0}
        };
        return Map;
    };
};

- test.cc -
// This is a generic function 
const char *MapEnum2Str(int Idx, const Mapper_s *pMap)
{
    int i;
    static const char UnknownStr[] = "Unknown";

    for (i = 0; pMap[i].pStr != 0; i++)
    {
        if (Idx == pMap[i].Idx)
        {
            return pMap[i].pStr;
        }
    }

    return UnknownStr;
}

int main()
{
   cout << "State: " << MapEnum2Str(State, State_s::GetpMap()) << endl;
   return 0;
}

Any suggestions on how to improve this ?

I feel that the header file looks slightly cluttered...

For loop termination should be pMap[i].IdX != INT_MAX; If array is stored sequencly, then you can directly reference state <= KNOWN_STATE ? pMap[state-1] : "Unknown" assuming KNOWN_STATE = 4; — lakshmanaraj, Dec 16 '08 at 09:19
Any more restrictions? Is there a reason for using State_s instead of just the Map on its own? Can we assume that Map is defined in such a way that the Idx is increasing? — mweerden, Dec 16 '08 at 09:58
Are you basically just trying to do string internment, only you want to be able to specify the values of the interned strings specifically? — singpolyma, Dec 16 '08 at 20:00

score 1 · Answer 1 · answered Jan 23 '09 at 10:25

Fastest execution: build a hash table. Since you know the indices ahead of time, you can even build a perfect hash table.

Easiest to program: use a massive switch statement and hope the compiler knows how to optimize for noncontiguous case integers. At least the strings will live in the .text segment of the executable itself so you don't need to instantiate anything:

// Status
enum State_t
{
   Running = 1,
   Jumping = 6, 
   Singing = 12
};

const char *StateToString(State_t state)
{
  switch(state)
  {
    case Running: return "Running";
    case Jumping: return "Jumping";
    case Singing: return "Singing";
    default: return "ERROR"; 
  }
}

You can hide all that inside macros (as Suma's link suggests) so it's not so much of a WTF.

score 1 · Accepted Answer · edited Dec 11 '11 at 17:47

Here is what I settled on. Using this technique all you need to do is to include a header file. You will only instantiate what you use. You could also store a perfect hash table instead of just Idx & pStr. This approach does not work in C.

file: e2str.hh

struct Mapper_s
{
    int Idx;
    const char *pStr;
};

#define ENUM2STR_BEGIN(x) struct x { static const Mapper_s *GetpMap(void) {  static const Enum2StrMap_s Map[] = 
#define ENUM2STR_END      return Map; }; }

const char *MapEnum2Str(int Idx, const Mapper_s *pMap);

file: mapper.hh

#include "e2str.hh"

ENUM2STR_BEGIN(State_s) 
{
        { Running,   "Running" },
        { Singing,   "Singing" },
        { Jumping,   "Jumping" },
        { 0, 0}
};
ENUM2STR_END;

file: test.cc

#include "mapper.hh"
int main()
{
   cout << "State: " << MapEnum2Str(State, State_s::GetpMap()) << endl;
   return 0;
}

score 0 · Answer 3 · edited Nov 14 '11 at 13:55

0

I can't do a direct mapping since one application might be feeding me values which are >out of range for my list of strings. A direct lookup into nothingness would be critical.

So add a count of objects to the array that your accessors check before returnng a value:

struct map {
const char *mapping[] = { "Running", "Jumping", "Singing" };
const int count = 3;
}

or if you want it automated

struct map {
map() { 
  for( count = 0; strlen( mapping[count] ); ++i )
}

const char *mapping[] = { "Running", "Jumping", "Singing", "" };
int count;
}

edited Nov 14 '11 at 13:55

Jason Plank

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answered Dec 16 '08 at 09:32

Patrick

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I'm sorry but I forgot to mention that the indexes might not be in consecutive order i.e. the enums might have holes in them. This is why I need to do the manual search. – tormod Dec 16 '08 at 09:46

score 0 · Answer 4 · edited May 23 '17 at 11:55

0

How about Easy way to use variables of enum types as string in C?

In this style enums and strings are not only contained in one file, they are also contained in one location. You can easily extend the factory to accept more "columns" in SOME_ENUM, in your case you might want the string not to be generated from name, but provided explicitely.

edited May 23 '17 at 11:55

Community

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answered Dec 16 '08 at 10:24

Suma

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Simplest way of mapping an index to a string in C++

4 Answers4