Q132893: PRB: Exceptions Thrown During Construction Can Orphan Memory

Article: Q132893
Product(s): Microsoft C Compiler
Version(s): 2.0 2.1 4.0 4.2 5.0
Operating System(s): 
Keyword(s): kbcode kbCodeGen kbCompiler kbCPPonly kbVC
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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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[ASCII 146]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;
        }
     }

Additional query words: 9.0 9.1 9.00 9.10

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Keywords          : kbcode kbCodeGen kbCompiler kbCPPonly kbVC 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : 2.0 2.1 4.0 4.2 5.0
Issue type        : kbprb

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