Restrict the number of object instantiations from a class to a given number

Question

Given a class, I would like to limit the number of objects created from this class to a given number, say 4.

Is there a method to achieve this?

Answer 1

You're looking for the instance manager pattern. Basically what you do is restrict instantiations of that class to a manager class.

class A
{
private: //redundant
   friend class AManager;
   A();
};

class AManager
{
   static int noInstances; //initialize to 0
public:
   A* createA()
   {
      if ( noInstances < 4 )
      {
         ++noInstances;
         return new A;
      }
      return NULL; //or throw exception
   }
};

A shorter way is throwing an exception from the constructor, but that can be hard to get right:

class A
{
public:
   A()
   {
       static int count = 0;
       ++count;
       if ( count >= 4 )
       {
           throw TooManyInstances();
       }
   }
};