Re: memory leak in the code?

On Jan 5, 10:04 am, George2 <george4acade...@yahoo.com> wrote:


I have doubts, so lets find out, Mr Holmes:

#include <iostream>
#include <stdexcept>

class A
{
  static int count;
public:
  A()
  {
    std::cout << "A()\n";
    if(2 == count)
      throw std::bad_alloc();
    count++;
  }
  ~A() { std::cout << "~A()\n"; }
};

int A::count;

class B
{
public:
  B() { std::cout << "B()\n"; }
  ~B() { std::cout << "~B()\n"; }
};

int main()
{
  A* p_a = 0;
  B* p_b = 0;
  try {
    p_b = new B[5];
    p_a = new A[5];
  } catch (const std::bad_alloc& e)
  {
    std::cout << "error: ";
    std::cout << e.what() << std::endl;
  }
}

/*
B()
B()
B()
B()
B() // obviously, something is wrong
A()
A()
A()
~A()
~A()
error: St9bad_alloc
*/

So we definitely have a problem.
Is the solution to delete [] p_a?
no. thats the wrong solution, there is a simpler, better way.
A std::vector own its elements unless these are pointers.
also, a std::vector invokes copy construction to populate elements at
creation time.
So to track what is happening, we need to modify A's copy ctor to
increment a static counter.

....
#include <vector>

class A
{
  static int copy_count;
  static int count;
public:
  ...
  // A's copy ctor
  A::A( const A& copy)
  {
    std::cout << "copy A\n";
    if(2 == copy_count)
      throw std::bad_alloc();
    copy_count++;
  }
  ...
};

int A::count;
int A::copy_count;

int main()
{
  try
  {
    std::vector< B > vb(5);
    std::vector< A > va(5);
  }
  catch (const std::bad_alloc& e)
  {
    std::cout << "error: ";
    std::cout << e.what() << std::endl;
  }
}

/*
B()
~B() // the entire vector vb is initialized with a copy
A() // default ctor
copy A // copy_count = 0
copy A
copy A // copy_count = 2, kaboom!
~A()
~A()
~A()
~B() // automatic
~B()
~B()
~B()
~B()
error: St9bad_alloc
*/

moral: When you aren't sure, test it.
And prove the solution.
Don't use pointers, you could use a boost::scoped_array.