static counter in c++

Question

I'm trying to create a Data class whose objects each hold a unique ID.

I want the 1st object's ID to be 1, the 2nd to be 2, etc. I must use a static int, but all the objects have the same ID, not 1, 2, 3...

This is the Data class:

class Data
{
private:
   static int ID;
public:
   Data(){
   ID++;
   }
};

How can I do it so the first one ID would be 1, the second would be 2, etc..?

Answer 1

This:

class Data
{
private:
   static int ID;
   const int currentID;
public:
   Data() : currentID(ID++){
   }
};

Besides a static counter, you also need an instance-bound member.

Answer 2

If the ID is static, then it will have the same value for all class instances.

If you want each instance to have sequential id values, then you could combine the static attribute with a class variable, like this:

class Data
{
private:
   static int ID;
   int thisId;
public:
   Data(){
   thisId = ++ID;
   }
};

int Data::ID = 0;

If the application will be multi threaded, then you'll have to synchronize it with something like a pthread mutex.