Implementing a simple shared_ptr class

my_shared_ptr.cpp

// In C++11 we got shared pointer. The standard implementation is much more complete and robust, 
// but here I show a barebones version of it to illustrate the main components.
// Behaviour: SharedPTR manages the lifetime of a pointer to a templated object
//            SharedPTR can be used to share the pointed object among other SharedPTRs.
//            When all of them have given up the pointed object, The SharedPTR deallocates the 
//            pointed object
//            For this we use reference counting. And the main point is that the counter should be a shared object itself
//            that would be shared among the SharedPTRs.

template<typename T>
struct SharedPTR{
  T* ptr = nullptr;

  // manages the life time count
  struct Counter{
    int counter = 0;
    int inc(){ return counter++;}
    int dec(){ return counter--;}
    bool isZero(){ return counter == 0;}  
  };

  Counter * C = new Counter();

  explicit SharedPTR(){} // explicit default constructor 
  explicit SharedPTR(T* t):ptr(t){ C->inc(); } // explicit parameterised default constructor

  // copy constructor
  SharedPTR(const SharedPTR<T>& other){
    if (this!=&other){ // avoid copying self
      C = other.C;
      ptr = other.ptr;
      C->inc();
    }
  }

  // copy assignment operator
  SharedPTR<T>& operator=(const SharedPTR<T>& other){
    if (this!=&other){ // avoid copying self
      C = other.C;
      ptr = other.ptr;
      C->inc();
    }
    return *this;
  }

  ~SharedPTR(){
    C->dec();
    if(C->isZero()){
      delete ptr;
      delete C;
    }
  }
};


struct dummy{
  dummy(){ std::cout << "constructed\n"; }
  ~dummy(){ std::cout << "destructed\n"; }
};

int main(){
  SharedPTR<dummy> p1;
  {
    SharedPTR<dummy> sp(new dummy());
    p1 = sp;
  }
  std::cout << "here\n";
}