C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Problem with Visual C++ 7.1.3088 and bit-fields?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Dennis McCrohan Guest Posted: Wed Sep 29, 2004 8:32 pm    Post subject: Problem with Visual C++ 7.1.3088 and bit-fields? Hi- I'm wondering if anyone has any feedback or knowledge of the following problem. I have searched the MSDN Knowledge Base and not found anything relevant... I have an enum similar to: enum FruitType { apple, pear, peach, mango }; And a class FruitContainer that has a member variable of FruitType, declared with a bit-width of 2: FruitType m_myFruit : 2; and access functions: FruitType GetMyFruit () { return m_myFruit; } void SetMyFruit(FruitType myFruit) { m_myFruit = myFruit; } What we are seeing the Visual C++ 7.1 debugger is that if we call SetMyFruit() with a value of apple or pear, and then retrieve the value using GetMyFruit(), everything works as expected. But if we pass SetMyFruit() the value mango, it ends up getting interpreted as the integer value -1, and that is what is returned by GetMyFruit(). It appears that the compiler or run-time is confused and considering the enum value to be a signed quantity. The same code works correctly on Solaris (compiled using Workshop) or Linux (using gcc). If I remove the bit-width from the declaration, the code works correctly on Windows. Also works if I set the bit-width to 3. Thanks, -Dennis McCrohan Any biases, misguided opinions, or outright idiocy expressed in this message are absolutely my own, and do not represent those of my employer... Back to top Pete Becker Guest Posted: Wed Sep 29, 2004 8:56 pm    Post subject: Re: Problem with Visual C++ 7.1.3088 and bit-fields? Dennis McCrohan wrote: Quote: enum FruitType { apple, pear, peach, mango }; FruitType m_myFruit : 2; The underlying type for an enumeration has to be an integral type that's large enough to represent the values. There's no requirement that it be unsigned if there are no negative values. Looks like the compiler is using a signed type, which is allowed. So either change your bitfield width to 3, declare the bitfield type as unsigned (which will require some casts), or get rid of the bitfield entirely. That last one is probably best: bitfields are best used for things like hardware access, where bit positions matter. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Victor Bazarov Guest Posted: Wed Sep 29, 2004 9:04 pm    Post subject: Re: Problem with Visual C++ 7.1.3088 and bit-fields? Dennis McCrohan wrote: Quote: I'm wondering if anyone has any feedback or knowledge of the following problem. I have searched the MSDN Knowledge Base and not found anything relevant... I have an enum similar to: enum FruitType { apple, pear, peach, mango }; And a class FruitContainer that has a member variable of FruitType, declared with a bit-width of 2: FruitType m_myFruit : 2; and access functions: FruitType GetMyFruit () { return m_myFruit; } void SetMyFruit(FruitType myFruit) { m_myFruit = myFruit; } What we are seeing the Visual C++ 7.1 debugger is that if we call SetMyFruit() with a value of apple or pear, and then retrieve the value using GetMyFruit(), everything works as expected. But if we pass SetMyFruit() the value mango, it ends up getting interpreted as the integer value -1, and that is what is returned by GetMyFruit(). It appears that the compiler or run-time is confused and considering the enum value to be a signed quantity. The same code works correctly on Solaris (compiled using Workshop) or Linux (using gcc). If I remove the bit-width from the declaration, the code works correctly on Windows. Also works if I set the bit-width to 3. The underlying type for the 'enum FruitType' is apparently signed. You have no control over that, I believe. The compiler picks the underlying type for you (see 7.2/5). The compiler does not "get confused", it just picks whatever it thinks is right. If you want to make sure what type is used, do typedef unsigned char FruitType; FruitType const apple = 0, pear = 1, peach = 2, mango = 3; .... Of course it has a side effect: the type is not unique and does not differ from unsigned char, but at least you know it's unsigned... Victor Back to top Nils O. Selåsdal Guest Posted: Wed Sep 29, 2004 9:29 pm    Post subject: Re: Problem with Visual C++ 7.1.3088 and bit-fields? Dennis McCrohan wrote: Quote: Hi- I'm wondering if anyone has any feedback or knowledge of the following problem. I have searched the MSDN Knowledge Base and not found anything relevant... I have an enum similar to: enum FruitType { apple, pear, peach, mango }; And a class FruitContainer that has a member variable of FruitType, declared with a bit-width of 2: Then enums are probably integers (not unsigned integers). I'd guess if you make it of size 3, it would work. But why use a bitfield here ? Back to top Old Wolf Guest Posted: Thu Sep 30, 2004 9:32 pm    Post subject: Re: Problem with Visual C++ 7.1.3088 and bit-fields? Pete Becker <petebecker (AT) acm (DOT) org> wrote: Quote: bitfields are best used for things like hardware access, where bit positions matter. How can you say that, given that in C++ the alignment and allocation of bitfields is implementation-defined? In fact I think they don't even have to be in the order that they were declared. (C99 is a bit stricter: the bits have to be in the order declared and with no padding -- but it's still implementation- defined whether the bits number left-to-right or right-to-left, and what happens across unit boundaries) Back to top Pete Becker Guest Posted: Thu Sep 30, 2004 9:48 pm    Post subject: Re: Problem with Visual C++ 7.1.3088 and bit-fields? Old Wolf wrote: Quote: Pete Becker <petebecker (AT) acm (DOT) org> wrote: bitfields are best used for things like hardware access, where bit positions matter. How can you say that, given that in C++ the alignment and allocation of bitfields is implementation-defined? The compiler typically lines things up to match the hardware that it's targeting. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.