C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Union fun Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message John Max Skaller Guest Posted: Sun Oct 17, 2004 11:42 pm    Post subject: Union fun IMHO: union U1 { float a; int b; U1() : a(1.0), b(1); } x; x.a // undefined union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined g++ says his: initializations for multiple members of `U' I think g++ is wrong. Comments? --- [ 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 Ben Hutchings Guest Posted: Mon Oct 18, 2004 5:35 pm    Post subject: Re: Union fun "John Max Skaller" wrote: Quote: IMHO: union U1 { float a; int b; U1() : a(1.0), b(1); } x; x.a // undefined union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined g++ says his: initializations for multiple members of `U' I think g++ is wrong. Comments? It's right. The standard says in 12.6.2/3: "If a ctor-initializer specifies more than one mem-initializer for the same member, for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed." -- Ben Hutchings When in doubt, use brute force. - Ken Thompson --- [ 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 Jack Klein Guest Posted: Tue Oct 19, 2004 6:25 am    Post subject: Re: Union fun On Sun, 17 Oct 2004 23:42:21 GMT, [email]skaller (AT) nospam (DOT) com.au[/email] ("John Max Skaller") wrote in comp.std.c++: Quote: IMHO: union U1 { float a; int b; U1() : a(1.0), b(1); } x; x.a // undefined union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined g++ says his: initializations for multiple members of `U' I think g++ is wrong. Comments? You have neglected to provide your basis for concluding that g++ is wrong. There are two ways to look at this, first logically: What do you think it means to initialize multiple members of a union? All members of a union start at the same address and occupy the same space, or at least overlap the beginning of that space. When you assign a value to a member of a union, that member has a valid value, the values of the other members are indeterminate and accessing them is undefined. So what possible meaning could initializing more than one of them have? The second way to look at this is to see what the C++ standard says. 8.5.1 Aggregates paragraph 15: ======== 15 When a union is initialized with a brace-enclosed initializer, the braces shall only contain an initializer for the first member of the union. [Example: union u { int a; char* b; }; u a = { 1 }; u b = a; u c = 1; // error u d = { 0, "asdf" }; // error u e = { "asdf" }; // error —end example] [Note: as described above, the braces around the initializer for a union member can be omitted if the union is a member of another aggregate. ] ======== So I guess g++ isn't wrong. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html --- [ 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 Richard Smith Guest Posted: Tue Oct 19, 2004 6:26 am    Post subject: Re: Union fun John Max Skaller wrote: Quote: union U1 { float a; int b; U1() : a(1.0), b(1); } x; x.a // undefined union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined g++ says his: initializations for multiple members of `U' I think g++ is wrong. g++ is correct to issue a diagnostic: both code fragments are ill-formed. 12.6.2/2 says "if a ctor-initializer specifies more than one mem-initializer ... for multiple members of the same union ..., the ctor-initializer is ill-formed". -- Richard Smith --- [ 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 John Max Skaller Guest Posted: Tue Oct 19, 2004 6:28 am    Post subject: Re: Union fun On Mon, 18 Oct 2004 11:35:01 -0600, Ben Hutchings wrote: Quote: "John Max Skaller" wrote: union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined The standard says in 12.6.2/3: "If a ctor-initializer specifies more than one mem-initializer for the same member, for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed." Ah, thanks. Pity you can put non-POD types in unions then, I need to do that.. is there a proposal for a template like store_of<T> which has the alignment and size of T and which can be used in a union, for all value types T? If I had one of those, it might do as a poor man's replacement for unions of arbitrary types, the lack of which is a fairly serious defect. --- [ 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 Ben Hutchings Guest Posted: Tue Oct 19, 2004 2:47 pm    Post subject: Re: Union fun "John Max Skaller" wrote: Quote: On Mon, 18 Oct 2004 11:35:01 -0600, Ben Hutchings wrote: "John Max Skaller" wrote: union U2 { int b; float a; U1() : a(1.0), b(1); } y; y.a; // 1.0f, well defined The standard says in 12.6.2/3: "If a ctor-initializer specifies more than one mem-initializer for the same member, for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed." Ah, thanks. Pity you can put non-POD types in unions then, I need to do that.. is there a proposal for a template like store_of<T which has the alignment and size of T and which can be used in a union, for all value types T? No, but the Type Traits part of TR1 includes information about the required alignments (std::tr1::alignment_of Quote: If I had one of those, it might do as a poor man's replacement for unions of arbitrary types, the lack of which is a fairly serious defect. Boost.Variant <http://www.boost.org/libs/variant/> can do that for you. -- Ben Hutchings The world is coming to an end. Please log off. --- [ 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 Peter Dimov Guest Posted: Tue Oct 19, 2004 10:59 pm    Post subject: Re: Union fun [email]skaller (AT) nospam (DOT) com.au[/email] ("John Max Skaller") wrote in message news:<pan.2004.10.19.01.55.50.896756 (AT) nospam (DOT) com.au>... Quote: Ah, thanks. Pity you can put non-POD types in unions then, I need to do that.. is there a proposal for a template like store_of<T which has the alignment and size of T and which can be used in a union, for all value types T? TR1 includes aligned_storage aligned_storage<sizeof(T), alignment_of::type to obtain a storage type for T. It can be implemented using Boost's type traits as follows: template<size_t L, size_t A> struct aligned_storage { union type { unsigned char data_[ L ]; typename boost::type_with_alignment<A>::type align_; }; }; --- [ 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 Richard Smith Guest Posted: Wed Oct 20, 2004 12:46 am    Post subject: Re: Union fun John Max Skaller wrote: Quote: "If a ctor-initializer specifies more than one mem-initializer for the same member, for the same base class or for multiple members of the same union (including members of anonymous unions), the ctor-initializer is ill-formed." Ah, thanks. Pity you can put non-POD types in unions then, I need to do that. Why don't you use boost::variant, <http://www.boost.org/doc/html/variant.html>? This allows you to safely use non-POD types in what is effectively a union. -- Richard Smith --- [ 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 John Max Skaller Guest Posted: Wed Oct 20, 2004 2:39 pm    Post subject: Re: Union fun On Wed, 20 Oct 2004 00:46:51 +0000, Richard Smith wrote: Quote: Why don't you use boost::variant, http://www.boost.org/doc/html/variant.html>? This allows you to safely use non-POD types in what is effectively a union. Three possible reasons: correct me if I'm wrong. (1) It undoubtedly pulls in a heap of boost header files. This isn't acceptable. .. WOW .. I just looked, and the raw includes go off my screen. Its huge! (2) The number of variants is limited. (3) It's a template, which means type recursion doesn't work. One of the main uses of variants is to implement recursively defined inductive types. (Yeah .. I can see 'recursive_wrapper' there ..) Basically, there's a heap of machinery there of no use to me. I'm not writing C++ .. I'm generating it. Here's my current variant type: //VARIANTS struct _uctor_ { int variant; void *data; _uctor_() : variant(-1), data(0) {} _uctor_(int i, void *d) : variant(i), data(d) {} }; I'm forced to use the tagged pointer implementation contrary to the design requirements. I'd love to replace that void* with an actual union. In some other places, I need just an actual union without a variant tag (the program counter is enough). In both cases, it would work if I just stuck to supporting C. --- [ 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 John Max Skaller Guest Posted: Thu Oct 21, 2004 1:56 am    Post subject: Re: Union fun On Tue, 19 Oct 2004 22:59:53 +0000, Peter Dimov wrote: Quote: TR1 includes aligned_storage<Len, Align>. OK, thanks. You can use Quote: aligned_storage<sizeof(T), alignment_of::type to obtain a storage type for T. It can be implemented using Boost's type traits as follows: template<size_t L, size_t A> struct aligned_storage { union type { unsigned char data_[ L ]; typename boost::type_with_alignment<A>::type align_; }; }; Gak! That boost stuff is pretty tricky :) OK, I think that might work. Thanks. --- [ 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 John Max Skaller Guest Posted: Thu Oct 21, 2004 7:08 am    Post subject: Re: Union fun On Tue, 19 Oct 2004 06:25:32 +0000, Jack Klein wrote: Quote: On Sun, 17 Oct 2004 23:42:21 GMT, [email]skaller (AT) nospam (DOT) com.au[/email] ("John Max Skaller") wrote in comp.std.c++: You have neglected to provide your basis for concluding that g++ is wrong. It is sometimes the case the Standard contains contradictory statements, and you have to actually know that one is intended as a special case .. and you have to actually know it exists as well, otherwise you can draw the wrong conclusion. At least part of the problem is the really bad English word 'is' which sometimes means 'has the property' and sometimes means 'exactly is'. Here, I wasn't able to find any appropriate wording, even though I was looking at a nearby paragraph at the time :) FYI my example was based on the belief that all class members must be initialised in order of writing, perhaps trivially... but that belief is wrong, it isn't what the Standard says at all. --- [ 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 Helium Guest Posted: Mon Oct 25, 2004 6:22 pm    Post subject: Re: Union fun You come from a functional language that supports discriminated unions, right? Quote: Basically, there's a heap of machinery there of no use to me. I'm not writing C++ .. I'm generating it. Here's my current variant type: //VARIANTS struct _uctor_ { int variant; void *data; _uctor_() : variant(-1), data(0) {} _uctor_(int i, void *d) : variant(i), data(d) {} }; I'm forced to use the tagged pointer implementation contrary to the design requirements. I'd love to replace that void* with an actual union. In some other places, I need just an actual union without a variant tag (the program counter is enough). In both cases, it would work if I just stuck to supporting C. What about something like: You use a strcut containing a char-array. It's size is that of the largest object you want to store (-> sizeof()). Than you create objects using placement new in this array (and you destroy them manually calleng thir destructor). --- [ 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 Frank Birbacher Guest Posted: Tue Oct 26, 2004 10:00 pm    Post subject: Re: Union fun Hi! Helium wrote: Quote: What about something like: You use a strcut containing a char-array. It's size is that of the largest object you want to store (-> sizeof()). Than you create objects using placement new in this array (and you destroy them manually calleng thir destructor). After some days of debugging you resort to boost::any, which already does that. Frank --- [ 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 John Max Skaller Guest Posted: Wed Oct 27, 2004 4:33 pm    Post subject: Re: Union fun On Mon, 25 Oct 2004 18:22:01 +0000, Helium wrote: Quote: You come from a functional language that supports discriminated unions, right? Well, I come from a background like FORTRAN -> Pascal -> C++ -> C -> Tcl -> Python -> Ocaml (with various assmeblers thrown in) where my involvement with C++ was fairly intense (WG21 NB member for many years), and my training is in pure maths. My intent now is to provide C++ programmers with a new platform that is both source and binary compatible with C and C++, but which fixes many of the problems and extends capability considerably, particularly with functional programming and microthreading. The compatibility model is to abandon C syntax, whilst retaining enough of the flavour of C/C++ that a competent C++ programmer can easily pick it up. Source compatibility is then achieved with various source code embedding techniques. So the intent is an upgrade path with a number of migration options -- and in particular ensuring that there is no need to abandon existing code bases. In fact at present, to use Felix effectively, you're encouraged to mix-and-match it with C++ and C. Quote: In both cases, it would work if I just stuck to supporting C. What about something like: You use a strcut containing a char-array. It's size is that of the largest object you want to store (-> sizeof()). Than you create objects using placement new in this array (and you destroy them manually calleng thir destructor). That is precisely the idea.. but it doesn't work exactly as you described it, because that struct containing a char array might not be correctly aligned. Using the 'alignof()' traits etc seems to enable a solution that *does* work .. however it is still messy for a programmer to read the generated code, which is important. The correct solution is simply to allow unions of constructible types to be declared, to generate appropriate constructors etc where possible, and reject the program where it isn't -- but ONLY if the need arises. The need doesn't arise from merely declaring the union, and here C++ fails to follow a philosophy and practice accepted in many other places in the language. Note that disrciminated unions/variants, or whatever have nothing to do with functional programming. It's just that most FP languages are based on mathematics, which clearly indicates variants are fundamental building blocks for *any* well engineered language. My argument here is that it is bad that they're not a basic part of C++, but it's intolerable that there's no way to even emulate them other than by using pointers: in C, unions might not support safe dispatch on variant tags .. but C doesn't prevent you doing the right thing unsafely .. which could be quite safe if the code was generated. --- [ 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 John Max Skaller Guest Posted: Wed Oct 27, 2004 6:39 pm    Post subject: Re: Union fun On Tue, 26 Oct 2004 22:00:51 +0000, Frank Birbacher wrote: Quote: You use a strcut containing a char-array. It's size is that of the largest object you want to store (-> sizeof()). Than you create objects using placement new in this array (and you destroy them manually calleng thir destructor). After some days of debugging you resort to boost::any, which already does that. I'm not sure exactly what you mean by that. However, please note boost mainly serves the need of people having to hand code solutions. I have a full scale compiler generating code, so I'm treating C++ as a compiler target language. The needs of a target language are different to what you need for human writing, although not unrelated. In my opinion -- C++ offers a number os *significant* advantages as a target language over the traditional choices -- C or assmebler. These include much stronger type checking, which helps debugging the compiler, constructors and friends, which help reduce the amount of work the compiler needs to do by delegating it to the underlying C++ compiler, virtual functions, which make it easy to build an execution model which is fairly simple to maintain and modify .. in general, many of the advantages C++ give a human coder also apply when generating code. Unfortunately, code generators are *greatly* hampered by lack of coherence, orthogonality, generality, etc. When you have this in a target language you have to add lots of special cases to the compiler. The failure of unions to allow constructible types is one such case. Perhaps this protects the human writer and perhaps not, but there's no doubt it creates a significant obstacle to systematic code generation. The need arises in at least two distinct situations, which are universal in the sense all code generators have to handle the problem: implementation of variants and representation of the stack. The latter occurs when you try to build a data structure to model the stack for code like: { int x; } { double y; } where the correct model of the stack is: union { int x; double y; }; This saves storage. Even if x and y here were constructible types it doesn't matter .. the appropriate store will be initialised and destroyed as control flows through the code (when the program counter hits the { and } brackets, more or less .. :) Unions aren't the only problem -- the type system is a pain, the lack of support for recursion in templates makes it much easier to do all generic instantiation in the compiler, rather than try to generate templates, pointers to members are incomplete (you can't add them) etc etc. Still, the union problem is already out of character with the rest of C++, the changes required are fairly clear, and they break nothing. So this obstacle could be fairly easily fixed .. if anyone bothered to consider the idea that C++ might be a useful target language seriously. I personally think that it is important, because C++ is never going to be very good for human coding. Many people have adopted a grossly inferior platform -- Java -- just to get a slighly saner syntax plus garbage collection. Felix tries to provide an alternative which extends C++, rather than requiring you abandon it, so it ought to be worth at least considering the idea that it might just be useful to C++ programmers to make C++ easier to use as a target language. Another example easily fixed: offsetof() macro isn't allowed for non-POD, yet is necessary because pointers to members incomplete and so worthless for a code generator. I happen to use it systematically already .. luckily g++ just gives a warning. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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