C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Unions vs Classes Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message David Guest Posted: Tue Jun 15, 2004 1:19 pm    Post subject: Unions vs Classes Some developers in my group are using UNIONS to define their data types in a C++ program for an embedded system. Are there any pro and cons in doing this when you can define a CLASS to do the same thing? I guess there might be some additional overhead with CLASSES, but is that really an issue with today's computers? Back to top Karl Heinz Buchegger Guest Posted: Tue Jun 15, 2004 1:38 pm    Post subject: Re: Unions vs Classes David wrote: Quote: Some developers in my group are using UNIONS to define their data types in a C++ program for an embedded system. Are there any pro and cons in doing this when you can define a CLASS to do the same thing? 'union' and 'class' do different things. Quote: I guess there might be some additional overhead with CLASSES, but is that really an issue with today's computers? For todays desktop systems the memory savings by using a union is seldome an issue. But embedded system are notorious short on memory. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top JKop Guest Posted: Tue Jun 15, 2004 2:59 pm    Post subject: Re: Unions vs Classes David posted: Quote: Some developers in my group are using UNIONS to define their data types in a C++ program for an embedded system. Are there any pro and cons in doing this when you can define a CLASS to do the same thing? I guess there might be some additional overhead with CLASSES, but is that really an issue with today's computers? Let's say that int = 32 bits char = 8 bits short = 16 bits Take the following: struct MonkeyStruct { int a; char b; short c; }; struct MonkeyUnion { int a; char b; short c; }; sizeof(MonkeyStruct) will be 56 bits. sizeof(MonkeyUnion) will be 32 bits. You can't simply just turn a class into a union or vice-versa! They're two separate types of entity. Why would a union be in any way superior to a struct? I myself can only think of two possible reasons: A) Conserve memory B) Neat memory tricks (like using a 4 char union to determine if a system is LITTLEENDIN or BIGENDIN) -JKop Back to top Siemel Naran Guest Posted: Tue Jun 15, 2004 4:27 pm    Post subject: Re: Unions vs Classes "David" <mail (AT) david-eng (DOT) com> wrote in message Quote: I guess there might be some additional overhead with CLASSES, but is that really an issue with today's computers? Depends on the memory. With a union you could reduce the sizeof by 25% or 50% or more. If you create many such classes, then the savings add up. On the other hand, if you need a type field, you might be better off with polymorphic classes (ie. virtual functions) as the virtual pointer is itself a type field. Back to top Xenos Guest Posted: Tue Jun 15, 2004 7:46 pm    Post subject: Re: Unions vs Classes "JKop" <NULL (AT) NULL (DOT) NULL> wrote Quote: David posted: Let's say that int = 32 bits char = 8 bits short = 16 bits Take the following: struct MonkeyStruct { int a; char b; short c; }; struct MonkeyUnion { int a; char b; short c; }; sizeof(MonkeyStruct) will be 56 bits. Though its implemetation specific, this will most likely be 64, because a 16 short will probably require an alignment of mod 2. Back to top Rolf Magnus Guest Posted: Tue Jun 15, 2004 8:56 pm    Post subject: Re: Unions vs Classes Xenos wrote: Quote: "JKop" <NULL (AT) NULL (DOT) NULL> wrote in message news:99Ezc.2285$Z14.2284 (AT) news (DOT) indigo.ie... David posted: Let's say that int = 32 bits char = 8 bits short = 16 bits Take the following: struct MonkeyStruct { int a; char b; short c; }; struct MonkeyUnion { int a; char b; short c; }; sizeof(MonkeyStruct) will be 56 bits. Though its implemetation specific, this will most likely be 64, because a 16 short will probably require an alignment of mod 2. Even without the char, the size would probably be 64 bits, because the the 4 byte int would need be aligned on a 4 byte boundary, and you have to remember that the struct must also be usable in an array. Back to top puppet_sock@hotmail.com Guest Posted: Tue Jun 15, 2004 9:57 pm    Post subject: Re: Unions vs Classes JKop <NULL (AT) NULL (DOT) NULL> wrote [snip] Quote: Why would a union be in any way superior to a struct? I myself can only think of two possible reasons: A) Conserve memory B) Neat memory tricks (like using a 4 char union to determine if a system is LITTLEENDIN or BIGENDIN) Are you intending to do something like put an integer in and see what order the bytes come back out? I'm doubting this last would be safe. You should not count on how a compiler is going to squash things together in a union. Socks Back to top David Rubin Guest Posted: Wed Jun 16, 2004 2:09 am    Post subject: Re: Unions vs Classes [email]puppet_sock (AT) hotmail (DOT) com[/email] wrote in message news:<c7976c46.0406151357.2b6b8569 (AT) posting (DOT) google.com>... Quote: JKop <NULL (AT) NULL (DOT) NULL> wrote [snip] Why would a union be in any way superior to a struct? I myself can only think of two possible reasons: A) Conserve memory B) Neat memory tricks (like using a 4 char union to determine if a system is LITTLEENDIN or BIGENDIN) Are you intending to do something like put an integer in and see what order the bytes come back out? Given union Int2Bytes{ int i; char b[sizeof(int)]; }; I know you cannot write to .i and read from .b, but is it legal to do this? int i = 0xaabbccdd; char b = ((Int2Bytes*)&i)->b[0]; /david Back to top Jack Klein Guest Posted: Wed Jun 16, 2004 3:34 am    Post subject: Re: Unions vs Classes On 15 Jun 2004 06:19:17 -0700, [email]mail (AT) david-eng (DOT) com[/email] (David) wrote in comp.lang.c++: Quote: Some developers in my group are using UNIONS to define their data types in a C++ program for an embedded system. Are there any pro and cons in doing this when you can define a CLASS to do the same thing? I guess there might be some additional overhead with CLASSES, but is that really an issue with today's computers? What do "today's computers" have to do with the embedded system? It might make a vast difference if the target platform is an 8051 core with 8K 8-bit bytes of code space and 128 (not K) 8-bit bytes of data space on the one hand, and an ARM or PowerPC with many megabytes of both. Embedded systems vary so widely and by so many orders of magnitude that the difference between common desk top computers and their operating systems _almost_ seems trivial by comparison. What do the developers who prefer unions state is the reason for selecting them over classes? -- 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 Back to top Siemel Naran Guest Posted: Wed Jun 16, 2004 4:09 am    Post subject: Re: Unions vs Classes "Jack Klein" <jackklein (AT) spamcop (DOT) net> wrote in message Quote: What do the developers who prefer unions state is the reason for selecting them over classes? I've heard it's used extensively in low level programming. Also, if you want to write an API and works in both C++ and C, then unions might be right, as in the sockets class. Back to top Chris Hill Guest Posted: Wed Jun 16, 2004 1:03 pm    Post subject: Re: Unions vs Classes On Wed, 16 Jun 2004 04:09:02 GMT, "Siemel Naran" <SiemelNaran (AT) REMOVE (DOT) att.net> wrote: Quote: "Jack Klein" <jackklein (AT) spamcop (DOT) net> wrote in message What do the developers who prefer unions state is the reason for selecting them over classes? I've heard it's used extensively in low level programming. Also, if you want to write an API and works in both C++ and C, then unions might be right, as in the sockets class. I have seen the situation of auto generate interface code that used structs to hold different data and then collected these together using unions and a type field. The client would inspect the type field and then delve into the correct bit of the struct/union combination. This was actually consuming more space than a class hierarchy and proper polymorphism. Since, the unions were as large as the largest member. I suspect that paying the price of VPTRs and using a class hierarchy (even with multiple inheritance) would consume less space. :) Chris. Back to top JKop Guest Posted: Wed Jun 16, 2004 1:25 pm    Post subject: Re: Unions vs Classes posted: Quote: JKop <NULL (AT) NULL (DOT) NULL> wrote in message news:<99Ezc.2285$Z14.2284 (AT) news (DOT) indigo.ie>... [snip] Why would a union be in any way superior to a struct? I myself can only think of two possible reasons: A) Conserve memory B) Neat memory tricks (like using a 4 char union to determine if a system is LITTLEENDIN or BIGENDIN) Are you intending to do something like put an integer in and see what order the bytes come back out? I'm doubting this last would be safe. You should not count on how a compiler is going to squash things together in a union. Socks union { unsigned int FourBytes; struct { unsigned char Byte1; unsigned char Byte2; unsigned char Byte3; unsigned char Byte4; }; } blind; int main(void) { blind.FourBytes = 0x1F1F1F1F; //Now, check each individual byte if (Byte1 == } I did this before in a program I was writing. Can't remember why I did it, or which numbers I used! I'd still have it tucked away in my archive though (Actually, just right now as I'm writing this I thought of the reason, I was encoding digital audio to be sent down to a CD-Writer, and had to know the ENDINESS). -JKop Back to top Siemel Naran Guest Posted: Wed Jun 16, 2004 4:57 pm    Post subject: Re: Unions vs Classes "Chris Hill" <news1 (AT) plantain (DOT) org.uk> wrote in message Quote: This was actually consuming more space than a class hierarchy and proper polymorphism. Since, the unions were as large as the largest member. I suspect that paying the price of VPTRs and using a class hierarchy (even with multiple inheritance) would consume less space. Furthermore the VPTR way is probably faster than a switch-case on the type field. Back to top Bill Seurer Guest Posted: Wed Jun 16, 2004 8:08 pm    Post subject: Re: Unions vs Classes JKop wrote: Quote: union { unsigned int FourBytes; struct { unsigned char Byte1; unsigned char Byte2; unsigned char Byte3; unsigned char Byte4; }; } blind; int main(void) { blind.FourBytes = 0x1F1F1F1F; //Now, check each individual byte if (Byte1 == I did this before in a program I was writing. ... While using unions is generally machine specific anyway the above is especially "bad" because it makes many assumptions (like ints are 4 bytes). Back to top puppet_sock@hotmail.com Guest Posted: Tue Jun 29, 2004 5:53 pm    Post subject: Re: Unions vs Classes [email]davidrubin (AT) warpmail (DOT) net[/email] (David Rubin) wrote in message news:<82b37be.0406151809.13116930 (AT) posting (DOT) google.com>... [puppet sock moaning about in-one-var-out-the-other unions snipped] Quote: Given union Int2Bytes{ int i; char b[sizeof(int)]; }; I know you cannot write to .i and read from .b, You can, it's just that it is not portable, and probably even not reliable. Some things that could bust it: - Different hardware - Different OS (even new version) - Different version of the compiler - Different address of loading the code into memory - Long list of weird things that could happen on specialist platforms Quote: but is it legal to do this? int i = 0xaabbccdd; char b = ((Int2Bytes*)&i)->b[0]; I'm pretty confused by that. What's it supposed to do? Lessee, you've got the address of an int variable, and you've cast that as a pointer to an undeclared union, and you've tried to reference the zeroeth char in the "b" data element of that undeclared union. I'd say, that this *might* compile, but the results won't be good. The best you could hope for would be some kind of memory access violation. The worst would be that char b contains garbage and you don't realize it. Socks 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 Goto page 1, 2  Next Page 1 of 2   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.