static const map

[elhoushi]

Dear all,

I have found a turnaround solution to the problem of creating a static const map for those who want to make large lookup tables.

Form a class and make the map an instance member of that class. Fill the map in the class' constructor.

After that, make an static instance of that class.

In this way the data is filled only once. And you may define a method to return a value from the map.

[weaknessforcats]

Actually, you do not wnat a static instance of the map<>.

What you need is a Singleton class that manages the map<>.

Read this: http://bytes.com/topic/c/insights/65...erns-singleton.

[elhoushi]

Yes. I have used a singleton.
Here is the implementation:

ObjectMap.h file:

#include <string>
#include <map>
using namespace std;

class ObjectMap
{
public:
static ObjectMap* GetInstance();
string GetName(unsigned int number) {return mapInstance->m_map[number];}

protected:
ObjectMap();
map<unsigned int, string> m_map;

private:
static ObjectMap* mapInstance;
};

ObjectMap.cpp file:

#include "ObjectMap.h"

ObjectMap* ObjectMap::mapInstance = NULL;

//_________________________________________________
ObjectMap* ObjectMap::GetInstance()
{
if (mapInstance == NULL)
mapInstance = new ObjectMap;
return mapInstance;
}

//_________________________________________________
ObjectMap::ObjectMap()
{
m_map[0] = "Hello World0";
m_map[1] = "Hello World1";
m_map[2] = "Hello World2";
}

You may then create an instance of the ObjectMap class using the new statement and don't delete the pointer so that it will be available all the time and won't need to re-create it.

#include ObjectMap.h

void main()
{
ObjectMap* c = new ObjectMap;
c->GetName(2);
}

[weaknessforcats]

ObjectMap is not a singleton because you can create many instances of ObjectMap. Each of these instances will compete for your static map.

A Singleton is a class that can have only one instance.

Again, I say, the map should not be static. It should be just a regular member variable. ObjectMap needs to be set up so that only one instance can be created and that way you have one map.

You have no Instance() method to get a pointer to the Singleton.

Read this and pay special attention to a Singleton registry: http://bytes.com/topic/c/insights/65...erns-singleton.

[elhoushi]

Yes, you are right considering the point that there is no need to form a static instance of the class.

You have no Instance() method to get a pointer to the Singleton.

However, the class I have formed is a singleton. I have a GetInstance() method which is the Instance() method you are looking for.

[elhoushi]

I am sorry, this part is wrong:

#include ObjectMap.h

void main()
{
ObjectMap* c = new ObjectMap;
c->GetName(2);
}

It should be:

#include ObjectMap.h

void main()
{
ObjectMap* c = ObjectMap::GetInstance();
c->GetName(2);
}

I wanted to edit the original post but I couldn't.

[weaknessforcats]

That being the case, the map need not be static since there is only one ObjectMap object.