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To: gcc@gcc.gnu.org
From: Thomas Immich <immich@ergosign.de>
Subject: Getting the byte offset of a member variable pointer type
Date: Wed, 22 Jun 2005 13:48:00 -0000
X-SW-Source: 2005-06/txt/msg00968.txt.bz2

Hi all,

I need to get the exact byte offset of a member variable inside a  
class in order to map a member variable pointer type to a concrete  
member variable instance. This worked fine until I ported my code to  
GCC 4.0, which is much stricter regarding type casts.

Unfortunately I cannot use the offsetof() macro, since I do not have  
enough information to do so.
The only information I have is:
     - The member variable type (in template parameter T, for example  
int)
     - A member variable pointer (m_memberVariablePtr, for example  
int MyClass::*)
     - The class type (in template parameter C, for example MyClass)
     - A concrete instance of type C (obj)

I want to get
     - The address of obj's concrete member variable (represented by  
m_memberVariablePtr)

Here is how I calculated this address in GCC 3.0:

const C* addressOfObject = static_cast<const C*>(&obj);
T* result = (T*)((std::size_t)addressOfObject + (size_t) 
m_memberVariablePtr);

However, this snippet does not compile in GCC 4.0, since I am not  
allowed to cast a member variable pointer to a size_t type. Is there  
any elegant possibility to get the address correctly?

Thanks in advance,

Thomas