Question

What’s a good policy for when to use “new” to make an instance of a class? I’ve been hobby programming C++ for a while but I’m still not for sure when is the best time to do this:

MyClass thing(param1, param2);

over this:

MyClass* thing;
thing = new MyClass(param1, param2);

Any advice?

Answer 1

 MyClass thing(param1, param2); //memory for thing is allocated on the process stack(static allocation)

 MyClass* thing;
 thing = new MyClass(param1, param2); //memory is allocated dynamically on the heap(free store) for thing

The difference lies here:

 int main()
 { 
   {
    MyClass thing(param1, param2); //thing is local to the scope
   } //destructor called for thing
   //cannot access thing (thing doesn't exist)
 } 

 int main()
 {
   {
     MyClass* thing;
     thing = new MyClass(param1, param2);
   }
  //the object pointed to by thing still exists
   //Memory leak
 }  

For large objects you must allocate memory dynamically(use new) because the process stack has a limited size.