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how to make this work in gcc-3.4.5

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This is a well known trick of finding out some structure's header size, specially when the content of the structure might be changed by several programmers.

#define Offset(M,S)   ((&(((S*)1)->M)) - 1)

struct PacketData {
   PacketHeader Header;
   char*        pData;
};


enum HeaderSizes {
    PacketDataHeaderSize = Offset( pData, PacketData ),   
};


But apparently this trick doesn't work in >=GCC-3.4.5 anymore, cuz the compiler doesn't allow setting anything other than "constant literals" for enumerations. Hence, last night i had to do the nasty quick fix by making PacketDataHeaderSize a "const int" global variable. But is there any elegant way to do this in >=GCC-3.4.5 ?

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Have you tried searching the headers for the "offsetof" macro? It should be located in "stddef.h".

The C version is slightly different than yours:


#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

// and the C++ version more different still:

#define offsetof(TYPE, MEMBER)
(__offsetof__ (reinterpret_cast <size_t>
(&reinterpret_cast <const volatile char &>
(static_cast<TYPE *> (0)->MEMBER))))





That macro formatting might not be preserved.

At any rate, the problem isn't likely to be the macro but how you're using it with the enum.

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goodness me !! that CPP version is kinda complicated !! hehehe... well i was compiling
Torque Engine code in Gentoo (gcc-3.4.5).. and it wouldn't compile the following



#ifndef Offset
#if defined(TORQUE_COMPILER_GCC) && (__GNUC__ > 3) || ((__GNUC__ == 3) && (__GNUC_MINOR__ >= 1))
#define Offset(m,T) ((int)(&((T *)1)->m) - 1)
#else
#define Offset(x, cls) ((dsize_t)((const char *)&(((cls *)0)->x)-(const char *)0))
#endif
#endif

enum HeaderSizes
{
/// Byte offset to payload of a PacketReceiveEvent
PacketReceiveEventHeaderSize = Offset(data,PacketReceiveEvent),

/// Byte offset to payload of a ConnectedReceiveEvent
ConnectedReceiveEventHeaderSize = Offset(data,ConnectedReceiveEvent),

/// Byte offset to payload of a ConsoleEvent
ConsoleEventHeaderSize = Offset(data,ConsoleEvent)
};




As you can see the Torque guys wrote their own Offset macro. Later on i had to declare those
enums as Global constant integer variable. Anyway, the engine gives seg fault in gentoo.. i have no idea why... i remember it ran well in my old mandrake-10 box.

But i really dint know that offsetof macros was already defined in stddef.h !! just wondering why did the Torque guys re-wrote it then !!??

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