Demonstrate a very simple safe pointer class. : template class « Class « C++






  1. C++
  2. Class
  3. template class

Demonstrate a very simple safe pointer class.

  
#include <iostream>
#include <string>

using namespace std;

// The exception type thrown by the safe pointer.
class bad_ptr {
public:
  string msg;

  bad_ptr(string str) { msg = str; }
};

// A class used to demonstrate the safe pointer.
class myclass {
public:
  int alpha;
  int beta;
  myclass(int p, int q) { alpha = p; beta = q; }
};

template <class T> class safe_ptr {
  T *ptr;
public:
  safe_ptr() { ptr = 0; }

  T *operator->() {
    if(!ptr != 0) throw bad_ptr("Attempt to use -> on null pointer.");
    else return ptr;
  }

  T &operator*() {
    if(!ptr) throw bad_ptr("Attempt to dereference null pointer.");
    else return *ptr;
  }

  operator T *() { return ptr; }

  T *operator=(T *val) { ptr = val; return ptr; }
};

int main()
{
  safe_ptr<int> intptr;

  try {
    *intptr = 23;
    cout << "The value pointed to by intptr is: " << *intptr << endl;
  } catch(bad_ptr bp) {
    cout << bp.msg << endl;
  }

  intptr = new int;

  try {
    *intptr = 23;
    cout << "The value pointed to by intpr is: " << *intptr << "\n\n";
  } catch(bad_ptr bp) {
    cout << bp.msg << endl;
  }

  safe_ptr<myclass> mcptr;

  try {
    mcptr = new myclass(100, 200);
    cout << "The values of alpha and beta for mcptr are: "
         << mcptr->alpha << " and " << mcptr->beta << endl;

    mcptr->alpha = 27;
    cout << "New value for mcptr->alpha: " << mcptr->alpha << endl;
    cout << "Same as (*mcptr).alpha: " << (*mcptr).alpha << endl;

    mcptr->beta = 99;
    cout << "New value for mcptr->beta: " << mcptr->beta << "\n\n";
  } catch(bad_ptr bp) {
    cout << bp.msg << endl;
  }

  safe_ptr<myclass> mcptr2;

  try {
    mcptr2->alpha = 88;
  } catch(bad_ptr bp) {
    cout << bp.msg << endl;
  }

  delete intptr;
  delete mcptr;

  return 0;
}








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