PRB: Exceptions Thrown During Construction Can Orphan Memory

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 4.0, 4.2, 5.0

SYMPTOMS

Memory that is dynamically allocated in a constructor can be orphaned when an exception is thrown in the constructor.

CAUSE

When inside a constructor, the object is partially constructed, so the destructor is not called. While automatic data is freed during stack unwinding, memory that is dynamically allocated is not properly cleaned up.

RESOLUTION

Two-phased construction of objects resolves this problem (see the following code sample). This way, the object is fully constructed, so when it is deleted, cleanup is handled correctly.

Sample Code

The following sample code shows both single- and two-phased construction for a class.

   /* Compile options needed: /MT /GX
   */

   // Change the following line into a comment to show the problem:
   #define TWO_PHASED_CONSTRUCTION

   // Include the MFC debug memory allocation functions. You will see the
   // memory leaks reported by MFC when the application terminates when you
   // run the program in the Visual C++ debugger.
   #include "afx.h"
   #define new DEBUG_NEW

   class A
   {
      char *x;

   public:

   #ifndef TWO_PHASED_CONSTRUCTION
      A() // This code doesn’t clean up.
      {
         x = new char[10]; // x will be orphaned.
         throw int(1);
      }
   #else
      A() // This code cleans up fine.
      {
         // Initialize automatic variables here.
      }
      void Create()
      {
         // Initialize dynamic members here.
         x = new char[10];
         throw int(1);
      }
   #endif

      ~A()
      {
         delete [] x;
      }
   };

   void main()
   {
      A *a;
      try
      {
         a=0;
         a=new A;
   // Do memory allocation in Create() when doing two-phased construction.
   #ifdef TWO_PHASED_CONSTRUCTION
         a->Create();
   #endif
      }
      catch(int)
      {
         delete a;
      }
   }