Microsoft KB Archive/122217

= PRB: offsetof Macro Fails with Virtual Base Class =

Article ID: 122217

Article Last Modified on 7/5/2005

-

APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 4.1 Subscription
 * Microsoft Visual C++ 4.2 Professional Edition

-



This article was previously published under Q122217



SYMPTOMS
Use of the offsetof macro may cause the compiler to generate incorrect code when used with classes that have virtual base classes. This incorrect code results in a null pointer access, which causes an access violation when the application is executed.

This problem occurs only for members declared in the virtual base class.

Note that according to the ANSI C++ Draft Working Papers, clause 18.1.5, the offsetof macro is restricted to POD union and POD structure types. POD types may not have base classes. Using the macro contrary to this restriction results in undefined behavior.



RESOLUTION
The offsetof macro returns the offset in bytes of a member from the beginning of a class by casing NULL to a pointer of class/struct type. To work around this problem, cast the address of the object to a char *, cast the address of the member to a char *, and subtract the address of the object from the address of the member. Please see the Sample Code in the MORE INFORMATION section for an example.

While this workaround generates code that is as efficient as the offsetof macro, it does require that the class/struct be instanced, while offsetof does not.



STATUS
This is by design.



MORE INFORMATION
The following code demonstrates the problem and the workaround.

Sample Code
/* Compile options needed: none */

#include   #include    #include 

class A { public: int a; }; class B : virtual public A { public: int v_a; }; class C : virtual public A { public: int v_b; }; class D : virtual public B, virtual public C { public: int b; };

int main {

D xD;

xD.b  = 16; xD.v_a = 8; xD.v_b = 4; xD.a  = 2;

cout << "sizeof(D) == " << sizeof(D)  << endl; cout << "sizeof(C) == " << sizeof(C)  << endl; cout << "sizeof(B) == " << sizeof(B)  << endl; cout << "sizeof(A) == " << sizeof(A)  << endl << endl;

// The following code section works around the offsetof problem cout << "offsetof(D.b)  == " << (char *)&xD.b - (char *)&xD << endl; cout << "offsetof(D.v_a) == " << (char *)&xD.v_a - (char *)&xD << endl; cout << "offsetof(D.v_b) == " << (char *)&xD.v_b - (char *)&xD << endl; cout << "offsetof(D.a)  == " << (char *)&xD.a - (char *)&xD << endl; cout << endl;

// The following statement is generated correctly cout << "offsetof(D.b)  == " << offsetof(D, b)   << endl;

// The following three statements cause access violations when executed cout << "offsetof(D.v_a) == " << offsetof(D, v_a) << endl; cout << "offsetof(D.v_b) == " << offsetof(D, v_b) << endl; cout << "offsetof(D.a)  == " << offsetof(D, a)   << endl << endl;

cout << "Press Enter..."; cin.get;

return 0; }

Additional query words: kbVC400bug 8.00 9.00 buglist1.00 buglist2.00 10.00 10.10 10.20

Keywords: kbprb kbcodegen kbcpponly kbcompiler KB122217

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.