C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  empty POD struct?          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Bobo Guest Posted: Mon Jul 14, 2003 9:57 pm    Post subject: empty POD struct? Hello all. I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Now let's write the following: struct derStruct : maybePOD { int foo; }; By means of the empty base optimization the maybePOD subobject in derStruct may have no memory assigned to it. But so, the following code won't work. template <typename T> void copyPOD(T &a, const T&b) { memcpy(&a, &b, sizeof(T)); } int main() { maybePOD noData, *p; derStruct obj1; copyPOD(obj1, non); } The first byte of obj1.foo will be overwritten by the unused byte from noData!!! What's wrong with this code? O am I missing something? TIA Bobo. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Tue Jul 15, 2003 3:03 am    Post subject: Re: empty POD struct? "Bobo" <pinzo (AT) correo (DOT) nu> wrote... Quote: I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Now let's write the following: struct derStruct : maybePOD { int foo; }; By means of the empty base optimization the maybePOD subobject in derStruct may have no memory assigned to it. But so, the following code won't work. template <typename T> void copyPOD(T &a, const T&b) { memcpy(&a, &b, sizeof(T)); } int main() { maybePOD noData, *p; derStruct obj1; copyPOD(obj1, non); } The first byte of obj1.foo will be overwritten by the unused byte from noData!!! What's wrong with this code? O am I missing something? The Standard does not allow using memcpy to copy objects into subobjects. What you're allowed to do is to copy a _complete_ object into another _complete_ object (see 3.9/2 and 1.8/2). Victor --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Bobo Guest Posted: Tue Jul 15, 2003 11:11 pm    Post subject: Re: empty POD struct? [email]v.Abazarov (AT) attAbi (DOT) com[/email] ("Victor Bazarov") wrote in message news:<vh6benh93jp6cc (AT) corp (DOT) supernews.com>... [snip] Quote: The Standard does not allow using memcpy to copy objects into subobjects. What you're allowed to do is to copy a _complete_ object into another _complete_ object (see 3.9/2 and 1.8/2). THX for the answer. I'm sorry but don't have a copy of the standard, but following the draft, the 3.9/2 is about copying the object into an array of chars. 3.9/3 is about copying from a T* into another T*, and in this paragraph there is no reference to _complete_ object. And what has 1.8/2 to do with this? It's about _implementation_defined_. Best regards. Bobo. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Wed Jul 16, 2003 1:25 am    Post subject: Re: empty POD struct? "Bobo" <pinzo (AT) correo (DOT) nu> wrote... Quote: [...] THX for the answer. I'm sorry but don't have a copy of the standard, but following the draft, [...] Get a copy of the Standard. Otherwise you have no idea what people here are talking about. Victor --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Ron Natalie Guest Posted: Wed Jul 16, 2003 2:04 am    Post subject: Re: empty POD struct? "Bobo" <pinzo (AT) correo (DOT) nu> wrote Quote: Hello all. I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. It does. The only odd thing is that it is NOT a legal C struct (but that has no bearing on C++). Quote: I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Perhaps, might be 4 or 8 for that matter. Quote: The first byte of obj1.foo will be overwritten by the unused byte from noData!!! What's wrong with this code? O am I missing something? You shouldn't use memcpy. The assignment operator would have done the right thing. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top MJ Guest Posted: Fri Jul 18, 2003 8:29 pm    Post subject: Re: empty POD struct? [email]ron (AT) sensor (DOT) com[/email] ("Ron Natalie") wrote in message news:<3f1403c2$0$24573$9a6e19ea (AT) news (DOT) newshosting.com>... Quote: "Bobo" <pinzo (AT) correo (DOT) nu> wrote Hello all. I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. It does. The only odd thing is that it is NOT a legal C struct (but that has no bearing on C++). I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Perhaps, might be 4 or 8 for that matter. In fact my compiler produces a size of 8 for that case. For a class with a char only, the size is 1: class a { }; // size = 8 class b { char c }; // size = 1 What may be the reason for the compiler to produce a size of 8 for an empty class? Michael --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Andy Sawyer Guest Posted: Sat Jul 19, 2003 9:51 pm    Post subject: Re: empty POD struct? In article <423c153c.0307180005.1304d0a3 (AT) posting (DOT) google.com>, on Fri, 18 Jul 2003 20:29:47 +0000 (UTC), [email]c141592653589 (AT) hotmail (DOT) com[/email] (MJ) wrote: Quote: In fact my compiler produces a size of 8 for that case. For a class with a char only, the size is 1: class a { }; // size = 8 class b { char c }; // size = 1 What may be the reason for the compiler to produce a size of 8 for an empty class? Because it wants to? The standard certainly doesn't preclude doing so, although if it bothered me, I'd probably complain to my compiler vendor. Regards, Andy S. -- "Light thinks it travels faster than anything but it is wrong. No matter how fast light travels it finds the darkness has always got there first, and is waiting for it." -- Terry Pratchett, Reaper Man --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top llewelly Guest Posted: Mon Jul 21, 2003 6:31 pm    Post subject: Re: empty POD struct? [email]v.Abazarov (AT) attAbi (DOT) com[/email] ("Victor Bazarov") writes: Quote: "Bobo" <pinzo (AT) correo (DOT) nu> wrote... I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Now let's write the following: struct derStruct : maybePOD { int foo; }; By means of the empty base optimization the maybePOD subobject in derStruct may have no memory assigned to it. But so, the following code won't work. template <typename T> void copyPOD(T &a, const T&b) { memcpy(&a, &b, sizeof(T)); } int main() { maybePOD noData, *p; derStruct obj1; copyPOD(obj1, non); } The first byte of obj1.foo will be overwritten by the unused byte from noData!!! What's wrong with this code? O am I missing something? The Standard does not allow using memcpy to copy objects into subobjects. What you're allowed to do is to copy a _complete_ object into another _complete_ object (see 3.9/2 and 1.8/2). The Standard also says memcpy has undefined behavior for non-POD - such as derStruct. (9/4 says POD are aggregates, 8.5.1/1 says aggregates have no base classes.) --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.jamesd.demon.co.uk/csc/faq.html ] Back to top Victor Bazarov Guest Posted: Tue Jul 22, 2003 10:56 pm    Post subject: Re: empty POD struct? "llewelly" <llewelly.at (AT) xmission (DOT) dot.com> wrote... Quote: v.Abazarov (AT) attAbi (DOT) com ("Victor Bazarov") writes: "Bobo" <pinzo (AT) correo (DOT) nu> wrote... I am a bit confused about if this is a POD or not: struct maybePOD { }; I think it has all the requirements for POD structs. I know C++ doesn't allow zero sized objects, so sizeof(maybePOD) shall be 1. Now let's write the following: struct derStruct : maybePOD { int foo; }; By means of the empty base optimization the maybePOD subobject in derStruct may have no memory assigned to it. But so, the following code won't work. template <typename T> void copyPOD(T &a, const T&b) { memcpy(&a, &b, sizeof(T)); } int main() { maybePOD noData, *p; derStruct obj1; copyPOD(obj1, non); } The first byte of obj1.foo will be overwritten by the unused byte from noData!!! What's wrong with this code? O am I missing something? The Standard does not allow using memcpy to copy objects into subobjects. What you're allowed to do is to copy a _complete_ object into another _complete_ object (see 3.9/2 and 1.8/2). The Standard also says memcpy has undefined behavior for non-POD - such as derStruct. (9/4 says POD are aggregates, 8.5.1/1 says aggregates have no base classes.) I don't think this is relevant. The code attempts to copy 'maybePOD' part of 'derStruct' (put aside the fact that the code is not working). 'maybePOD' _is_ a POD. Victor --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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