Dynamic memory allocation of objects in C++

Question

I'm trying to dynamically allocate (it's not so dynamic as it is right now, but eventually it will be) memory for objects in a very simple C++ program. I'm new to classes and have only recently started playing with C++, leaving C behind. Here's the code:

#include <iostream>
using namespace std;

class Test {
  private:
    int i;
  public:
    Test(int);
    ~Test();
    void print();
};

Test::Test(int ii) { i = ii; }
Test::~Test() { i=0; cout << "deconstructor called...value of i= " << i << endl; }
void Test::print() { cout << "value of i= " << i << endl; }

int main()
{
  Test a(10),*b,*c;
  //a.print(); // this works

  b = new Test(12);
  //b->print(); // this works as well

  for (int i=0; i<2; i++)
    c = new Test(i);

  c->print(); /* this shows that the value of i=1 .. should be 0? */
  c[0].print(); /* as expected (I guess), this prints i=1 as well... [expected because c->print() shows i=1 also */
  c[1].print(); /* shows value of i=0... */

  //delete []c; /* this fails miserably, but `delete c` works, why :( */

}

A lot of my confusion is actually included within comments in the code itself. I'm basically trying to have an array c where each element of the array is an object of itself.

The behavior of the code that I'm getting is described in the comments.

Answer 1

Perhaps we should look at the declarations, expanded you have:

Test a(10);
Test *b;
Test *c;

You have defined b and c as pointer-to-Test, but you seem to want c to be an array of pointer-to-test. The declaration for c you intended was likely:

Test **c;

which you would initialize:

c = new Test*[2];

for (int i=0; i<2; i++)
   c[i] = new Test(i);

and which you would access thus:

c[0]->print();
c[1]->print();

Answer 2

There are few serious problems with the given code.

1. Performing new on *b but missed to delete it.
2. You are overwriting *c few times in for loop, which will leak memory. Always deallocate resources before allocating a new one from a pointer.
3. If you are allocating with new/new[]/malloc then you must deallocate the pointer with delete/delete[]/free respectively. The same you are not maintaining with *c (that's why it fails).

Also, apart from learning dynamic allocation one should also be aware of STL containers, which provide a better way of handling dynamic resources. e.g. std::vector.

Answer 3

for (int i=0; i<2; i++)
    c = new Test(i);

The above code leaks the memory. c just point to the lastly constructed object in the loop iteration.

c->print(); /* this shows that the value of i=1 .. should be 0?

Here c points to location constructed on new Test(1);. So, the output.

Every new[] should be accompanied with delete[] and new with delete. You cannot intermix both.

Answer 4

That the delete[] doesn't work is perfectly normal: you never allocated c as an array, but as a pointer. You could store the address of an array in a pointer, but that's all. I'm actually wondering why exactly c[1] works, because your for loop just stores repeatedly pointers to newly allocated objects in the same pointer (you are not populating an array!).

Answer 5

delete c[]; Only deletes starting element. If you want to delete that array use dz delete c[] in for loop

You failed to allocate memory for c and keep on coding its wrong how can you get a output without allocating memory to a pointer variable?

Answer 6

See according to me , you have allocated memory for *c multiple times as

for (int i=0; i<2; i++)
c = new Test(i);

Have a look on this code, which will make everything clear

for (int i=0; i<2; i++)
{   c = new Test(i);    }       /*see , here the loop goes for i=0; then
                                for i=1; which basically overwrites what c will have
                                 i.e. finally       c = new test(1); */
c->print(); /* works fine , gives value of i=1 */
c[0].print(); /* as expected , this prints i=1 as well... */
c[1].print(); /*clearly, it will give a garbage value */
delete c;

But According to me , it would be more fine to replace

for (int i=0; i<2; i++)
{   c = new Test(i);    }

with

c = new Test(1);    //as the previous code is doing the same in for loop but that was consuming more resources

So if you want output as i=0 and then i=1 then do so-

c = new int(0);
c->print(); /* works fine , gives value of i=0 */
c[0].print(); /* as expected , this prints i=0 as well... */
delete c;

c = new int(1);
c->print(); /* works fine , gives value of i=1 */
c[0].print(); /* as expected , this prints i=1 as well... */
delete c;

The above code is what which will completely satisfy your need.