Microsoft KB Archive/122675

= BUG: Wrong Operator Delete Called for Exported Class =

Article ID: 122675

Article Last Modified on 12/9/2003

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 1.51
 * Microsoft Visual C++ 1.52 Professional Edition
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 2.2
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 4.1 Subscription
 * Microsoft Visual C++ 4.2 Enterprise Edition
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 4.2 Professional Edition
 * Microsoft Visual C++ 5.0 Professional Edition

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This article was previously published under Q122675



SYMPTOMS
When allocating and deleting an object of a class that is exported from a DLL, you may find that the new operator in the EXE is called to allocate the memory but the delete operator in the DLL is called to delete the memory. Therefore, there is a new/delete mismatching problem, which may cause run-time errors.



CAUSE
This problem is a result of the way objects are allocated, constructed, destructed and deallocated. The following things occur when allocating and deallocating objects that have constructors and destructors:

Allocating:

A1. The memory that the object will reside in is allocated.

A2. The appropriate constructor for that object is called.

Deallocating:

D1. The destructor for the object is called.

D2. If the object is being deallocated via delete, call delete.

Step D2 is where the problem arises for objects created from classes exported from a DLL. Steps D1 and D2 are often carried out by a scalar or vector deleting destructor, which are a compiler generated helper functions. When this helper function is created in the DLL, any calls it makes to the delete operator will be in the context of the DLL. Therefore the delete operator overridden in the EXE will not be called as expected.

Note that the scalar deleting destructor is called when deallocating single objects, and the vector deleting destructor is called when deallocating arrays of one or more objects.



RESOLUTION
You can use one of the following workarounds:

 If your code does not use virtual destructors, make the constructor and destructor for the object inline, and put the actual work into your own helper functions. If the destructor is inline, the code and the compiler generated helper functions (if present) are in the EXE and so the proper operator delete is called.

If the destructor is virtual, this work-around won't work. This is because virtual functions can be called through base class pointers, usually known as run-time binding or dynamic binding. Which function is called is determined at runtime through the v-table. Since inlining is generated at compile time, runtime binding is not possible if the functions are actually inlined.

- or - Override the operators new and delete for the DLL classes so that the new/delete calls usage will be made within the DLL. A class with its own version of new and delete will override any global version of new and delete defined in either the EXE or the DLL. Therefore the proper new and delete will always be called. Please see the sample code below in the MORE INFORMATION section.

- or - If you are using Visual C++ 2.0 or later, try using a template wrapper class. By using this templated class, you are deriving a class "on the fly" from the imported class. The constructor and the destructor for the new class are in the context of the EXE and not in the context of the DLL and the problem will be avoided. Please see the sample code below in the MORE INFORMATION section.

If you are using Visual C++ 16-bit editions, you can derive a class from the DLL exported class in the EXE file, so the constructor and the destructor for the new class are in the context of the EXE and not in the context of the DLL and the problem also will be avoided.

- or - If you use Visual C++ 5.0 or later, a new implementation of new/delete is available which is virtually transparent. To invoke this new implementation you must:

 Specify _declspec(dllexport) in the class declaration when you implement the class. Specify _declspec(dllimport) in the class declaration when you use the class.</li> Use a virtual destructor.</li></ol>

If these requirements aren't met, the compiler generates code using the same new/delete implementation that was present in the previous version of the product.</li></ol>

<div class="status_section">

STATUS
Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

<div class="moreinformation_section">

MORE INFORMATION
The following sample code demonstrates the problem and provides the workarounds mentioned above in the RESOLUTION section.

1. Sample Code Illustrates the Problem and Workaround #2
Use the following files to build two projects, one for DLL and one for EXE. In the DLL project, include file testdll.cpp and define preprocessor identifier _DLL. In the EXE project, include file testapp.cpp. testdll.h is the shared header file for both DLL and EXE.

Make sure to define preprocessor identifier _WORKAROUND in both the EXE and the DLL projects if you want to work around the problem. For example, using the compiler switch /D_WORKAROUND.

The problem code produces the following output: <pre class="fixed_text">  In EXE global new In DLL class constructor In DLL class destructor In DLL global delete The workaround code produces the following output: <pre class="fixed_text">  In DLL class new In DLL class constructor In DLL class destructor In DLL class delete /* Compile options needed: *   testapp - /MD  and default settings for Console application. *   testdll - /MD, /D_DLL, and default setting for DLL project. *   Use /D_WORKAROUND in both the EXE and DLL projects to see the *   work-around. */

testdll.h
#include <iostream.h>  #include <stdlib.h>

#ifdef _DLL #define DLLEXP __declspec(dllexport) #else #define DLLEXP __declspec(dllimport) #endif

// For use with the 16-bit versions, you need to use the following code // to define DLLEXP. Also start by using a QuickWin application for // testapp and a DLL project for testdll

//#ifdef _DLL //#define DLLEXP __export //#else //#define DLLEXP //#endif

class DLLEXP testdll {    public:

testdll; virtual ~testdll;

#ifdef _WORKAROUND void* operator new( size_t tSize ); void operator delete( void* p ); #endif

};

testapp.cpp
#include <iostream.h>  #include "testdll.h"

void* operator new(size_t nSize) {     void* p=malloc(nSize); cout << "In EXE global new\n"; return p;  }

void operator delete( void *p ) {     free(p); cout << "In EXE global delete\n"; }

void main {

testdll *p = new testdll; delete p;  }

testdll.cpp
#include <iostream.h>  #include "testdll.h"

DLLEXP testdll::testdll {    cout << "In class DLL constructor\n"; }

DLLEXP testdll::~testdll {    cout << "In class DLL destructor\n"; }

void* operator new( size_t tSize ) {     void* p=malloc(tSize); cout << "In DLL global new\n"; return p;  }

void operator delete( void* p ) {     free(p); cout << "In DLL global delete\n"; }

#ifdef _WORKAROUND DLLEXP void* testdll :: operator new( size_t tSize ) {     void* p=malloc(tSize); cout << "In DLL class new\n"; return p;  }

DLLEXP void testdll :: operator delete( void* p ) {     free(p); cout << "In DLL class delete\n"; }  #endif

2. Sample Code for Workaround #3
The following code fragment demonstrates how you might implement a template wrapper class in the EXE to work around this problem: template <class T>  class DLLFix: public T   { public: DLLFix; virtual ~DLLFix; }; Assuming that ExportedClass is a class that is exported from a DLL, make the following change: //Old code ExportedClass * p = new ExportedClass; delete p; // wrong delete may be called here;

//new code DLLFix<ExportedClass> * p = new DLLFix<ExportedClass>; delete p;

Additional query words: _USRDLL _AFXDLL 8.00 9.00 GPF general protection fault GP

Keywords: kbbug kbcode kbcompiler KB122675

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