C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  placement new , is this acceptable?          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message lallous Guest Posted: Tue Feb 10, 2004 10:37 am    Post subject: placement new , is this acceptable? class MyClass { static void *Allocate(int size); static void Free(void *mem); static MyClass *CreateInstance(); void DestroyInstance(); }; MyClass *MyClass::CreateInstance() { void * p = Allocate(sizeof(MyClass)); return new (p) MyClass; } void MyClass::DestroyInstance() { MyClass *i = this; i->~MyClass(); Free(i); } Back to top Jonathan Turkanis Guest Posted: Tue Feb 10, 2004 10:52 am    Post subject: Re: placement new , is this acceptable? "lallous" <lallous (AT) lgwm (DOT) org> wrote Quote: class MyClass { static void *Allocate(int size); static void Free(void *mem); static MyClass *CreateInstance(); void DestroyInstance(); }; MyClass *MyClass::CreateInstance() { void * p = Allocate(sizeof(MyClass)); what if p == 0? Quote: return new (p) MyClass; } void MyClass::DestroyInstance() { MyClass *i = this; i->~MyClass(); Free(i); } Who calls CreateInstace and DestroyInstance? Both are private. This can be made to work, but why? The standard way to provide custom allocation facilities for a class is to provide custom member operators new and delete. Using a factory function instead of a public constructor is appropriate sometimes, but is usually independent of the decision to provide custom allocation. Also, why make the constructor private? Jonathan Back to top lallous Guest Posted: Tue Feb 10, 2004 11:54 am    Post subject: Re: placement new , is this acceptable? "Jonathan Turkanis" <technews (AT) kangaroologic (DOT) com> wrote Quote: "lallous" <lallous (AT) lgwm (DOT) org> wrote in message news:c0ackn$14hqr1$1 (AT) ID-161723 (DOT) news.uni-berlin.de... class MyClass { static void *Allocate(int size); static void Free(void *mem); static MyClass *CreateInstance(); void DestroyInstance(); }; MyClass *MyClass::CreateInstance() { void * p = Allocate(sizeof(MyClass)); what if p == 0? return new (p) MyClass; } void MyClass::DestroyInstance() { MyClass *i = this; i->~MyClass(); Free(i); } Who calls CreateInstace and DestroyInstance? Both are private. This can be made to work, but why? The standard way to provide custom allocation facilities for a class is to provide custom member operators new and delete. Using a factory function instead of a public constructor is appropriate sometimes, but is usually independent of the decision to provide custom allocation. Also, why make the constructor private? Jonathan Hello Consider all method public, and an instance is created as: MyClass *a = MyClass::CreateInstance(); a->DestroyInstance(); What is a factory function? can you provide links? -- Elias Back to top Claudio Puviani Guest Posted: Tue Feb 10, 2004 3:11 pm    Post subject: Re: placement new , is this acceptable? "lallous" <lallous (AT) lgwm (DOT) org> wrote Quote: class MyClass { static void *Allocate(int size); static void Free(void *mem); static MyClass *CreateInstance(); void DestroyInstance(); }; MyClass *MyClass::CreateInstance() { void * p = Allocate(sizeof(MyClass)); return new (p) MyClass; } void MyClass::DestroyInstance() { MyClass *i = this; i->~MyClass(); Free(i); } Aside from the fact that this is an overly convoluted way to do things, your approach doesn't support inheritance. How do you allocate/deallocate an instance of a class derived from MyClass? Claudio Puviani Back to top lallous Guest Posted: Tue Feb 10, 2004 3:23 pm    Post subject: Re: placement new , is this acceptable? "Claudio Puviani" <puviani (AT) hotmail (DOT) com> wrote Quote: "lallous" <lallous (AT) lgwm (DOT) org> wrote class MyClass { public: static void *Allocate(int size); static void Free(void *mem); static MyClass *CreateInstance(); void DestroyInstance(); }; MyClass *MyClass::CreateInstance() { void * p = Allocate(sizeof(MyClass)); return new (p) MyClass; } void MyClass::DestroyInstance() { MyClass *i = this; i->~MyClass(); Free(i); } Aside from the fact that this is an overly convoluted way to do things, your approach doesn't support inheritance. How do you allocate/deallocate an instance of a class derived from MyClass? Claudio Puviani My question is whether the method used to create/destroy an instance is acceptable? For this design, no inheritance is planned. -- Elias Back to top puppet_sock@hotmail.com Guest Posted: Tue Feb 10, 2004 6:26 pm    Post subject: Re: placement new , is this acceptable? "lallous" <lallous (AT) lgwm (DOT) org> wrote [snip] Quote: What is a factory function? can you provide links? A factory function is exactly the kind of function you had in your example, one that returns a pointer to a brand new instance of the class. It gets its name because it is often used in a thing called a factory. The idea is, you put some overhead into each of several classes, then register those classes with the factory. Then when you want an instance of that class, you can request one from the factory. There are many variations on this: For example all the entries could be in a single inheritance heirarchy so that they all come back as instances of the base type. Or you may need to provide the factory with an initial instance of each registered class. Or you may register the class along with the name to be used to request new instances. Or a factory may be used to create a set of instances of a class, and thus not reallocate the memory, but only recycle it. Lots of fun. Socks Back to top Claudio Puviani Guest Posted: Tue Feb 10, 2004 7:49 pm    Post subject: Re: placement new , is this acceptable? "lallous" <lallous (AT) lgwm (DOT) org> wrote Quote: "Claudio Puviani" <puviani (AT) hotmail (DOT) com> wrote Aside from the fact that this is an overly convoluted way to do things, [...] My question is whether the method used to create/destroy an instance is acceptable? What part of "overly convoluted" doesn't answer your question? I suppose one could get philosophical and ask "acceptable by who's standard?", but in realistic terms, no, it's not acceptable. There are no benefits whatsoever to your approach, but plenty of limitations. It probably served its purpose in giving you a working example of placement new and calling destructors directly, but I seriously recommend never doing that in real code. For some proper ways to construct object factories, I recommend Alexandrescu's book (which someone may have already done): http://search.barnesandnoble.com/textbooks/booksearch/isbnInquiry.asp?isbn=0201704315&TXT=Y&itm=1 Claudio Puviani Back to top Jonathan Turkanis Guest Posted: Wed Feb 11, 2004 1:38 am    Post subject: Re: placement new , is this acceptable? "Jonathan Turkanis" <technews (AT) kangaroologic (DOT) com> wrote Quote: the decision to provide custom allocation. Also, why make the constructor private? I think I meant destructor here. Jonathan Back to top lallous Guest Posted: Wed Feb 11, 2004 12:02 pm    Post subject: Re: placement new , is this acceptable? "Claudio Puviani" <puviani (AT) hotmail (DOT) com> wrote Quote: "lallous" <lallous (AT) lgwm (DOT) org> wrote "Claudio Puviani" <puviani (AT) hotmail (DOT) com> wrote Aside from the fact that this is an overly convoluted way to do things, [...] My question is whether the method used to create/destroy an instance is acceptable? What part of "overly convoluted" doesn't answer your question? I suppose one could get philosophical and ask "acceptable by who's standard?", but in Is the usage of placement new and calling the destructor like that a correct one C++ syntax wise? I heard there is "overhead" or extra bytes that needs to be allocated in addition to the size of the class? [snip ...book recomendation...] -- Elias Back to top Claudio Puviani Guest Posted: Wed Feb 11, 2004 1:51 pm    Post subject: Re: placement new , is this acceptable? "lallous" <lallous (AT) lgwm (DOT) org> wrote Quote: Is the usage of placement new and calling the destructor like that a correct one C++ syntax wise? Syntactically, it works, though your compiler could have told you that much. Quote: I heard there is "overhead" or extra bytes that needs to be allocated in addition to the size of the class? You get overhead -- and typically much more than just one byte -- any time you allocate using new or one of the malloc functions. Some replacement libraries, like Hoard or SmartHeap, reduce this, there's always some amount of overhead. You normally shouldn't have to worry about this, but if you ever have to allocate tens or hundreds of thousands of objects, you might want to consider specialized allocators. Claudio Puviani Back to top Steve Dubak Guest Posted: Sat Mar 06, 2004 3:53 am    Post subject: Re: placement new , is this acceptable? Claudio Puviani wrote: Quote: "lallous" <lallous (AT) lgwm (DOT) org> wrote Is the usage of placement new and calling the destructor like that a correct one C++ syntax wise? Syntactically, it works, though your compiler could have told you that much. I heard there is "overhead" or extra bytes that needs to be allocated in addition to the size of the class? You get overhead -- and typically much more than just one byte -- any time you allocate using new or one of the malloc functions. Some replacement libraries, like Hoard or SmartHeap, reduce this, there's always some amount of overhead. You normally shouldn't have to worry about this, but if you ever have to allocate tens or hundreds of thousands of objects, you might want to consider specialized allocators. Some specialized allocators utilize more memory overhead than others, so if you're allocating many small objects, watch your memory consumption. Hoard is pretty good with memory, much better than smartheap. But alas, you pay for that memory conservation in speed. Smartheap is much faster than hoard. And ESA from cherrystone software labs is much faster than smartheap while using less memory than smartheap. So if you're going to pick a specialized allocator, choose carefully, benchmark, and watch the memory consumption. Quote: Claudio Puviani Steve Dubak [email]steve (AT) barragesystems (DOT) com[/email] 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.