C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  lazy method for overloading operator=?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Jesse Guest Posted: Sat Feb 21, 2004 11:02 am    Post subject: lazy method for overloading operator=? Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Any help is appreciated. Jesse [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sat Feb 21, 2004 6:11 pm    Post subject: Re: lazy method for overloading operator=? In message <Xns9495ADCEAD240nonenonecom (AT) 199 (DOT) 45.49.11>, Jesse <none (AT) none (DOT) com> writes Quote: Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Encapsulate all the shallow data in either a nested class or a base class and use the generated assignment for that to largely automate the shallow copying. But assuming memcpy() will actually work correctly for what you are doing -- an assumption that needs careful checking -- does the speed really matter? Unless you are doing a vast amount of assignment in relation to everything else in your program I very much doubt it. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jonathan Turkanis Guest Posted: Sat Feb 21, 2004 7:22 pm    Post subject: Re: lazy method for overloading operator=? "Jesse" <none (AT) none (DOT) com> wrote Quote: Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? You might group the members for which the compiler generated assignment would be okay into a helper class, and replace those members with an instance of the helper. Then you can assign the helper in one operation. Or you could put them in a base class. Jonathan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dhruv Matani Guest Posted: Sat Feb 21, 2004 7:25 pm    Post subject: Re: lazy method for overloading operator=? On Sat, 21 Feb 2004 06:02:20 -0500, Jesse wrote: Quote: Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Just create a Implementation class say called Impl. Simple run off the mill String class to demonstarate the uses: struct string { struct Impl { int Length; int/float/double/bool Other_Pod_Data; }; Impl impl; char *data; //... //Copy Ctor: string (string const& _s) : impl(_s.impl) { //Do a deep copy for the data part. } }; -- Regards, -Dhruv. Proud to be a Vegetarian. http://www.vegetarianstarterkit.com/ http://www.vegkids.com/vegkids/index.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Steve Haigh Guest Posted: Sun Feb 22, 2004 1:42 am    Post subject: Re: lazy method for overloading operator=? Jesse wrote: Quote: Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Not quite an answer as such, but have you considered autogenerating the code? It sounds like you have a lot of members (or you are very very lazy), depending on the situation it may be worth the effort to write a script or another program to generate your code for you. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan Odgaard Guest Posted: Sun Feb 22, 2004 10:57 am    Post subject: Re: lazy method for overloading operator=? Jesse <none (AT) none (DOT) com> wrote Quote: Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Rather than wrap all but your member which needs deep copy (as most have suggested), you can also just wrap the member which need deep copy, and overload operator= for the wrapper. // assuming strdup() and free() works for NULL-pointers struct StringWrapper { typedef StringWrapper self; char* str; StringWrapper (char* aStr = NULL) { str = strdup(aStr)); } StringWrapper (self const& rhs) { str = strdup(rhs.str)); } ~StringWrapper () { free(str); } self& operator= (self const& rhs) { if(this != &rhs) { free(str); str = strdup(rhs.str); } } operator char* () { return str; } }; Now you can just wrap your strings with the above: struct BigAndHeavy { int i, j, k, l, m, n; T obj1, obj2, obj3, obj4, obj5; StringWrapper str; BigAndHeavy (char* someStr) : str(someStr) { ... } ... }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dylan Nicholson Guest Posted: Mon Feb 23, 2004 11:40 am    Post subject: Re: lazy method for overloading operator=? [email]Duff (AT) DIKU (DOT) DK[/email] (Allan Odgaard) wrote in message news:<689e217.0402212128.7db3dbb4 (AT) posting (DOT) google.com>... Quote: Jesse <none (AT) none (DOT) com> wrote Hello, I have a quick little question. I want to overload the = op for a class of mine, but I only need to do a deep copy for a single variable, and I'm too lazy to assign each memeber by hand to get an initial shallow copy. While I can just use memcpy to do the shallow copy and then do the deep copy myself, apparently memcpy is slower for small operations than doing them by hand, and I figure maybe there is a nice built in solution? Rather than wrap all but your member which needs deep copy (as most have suggested), you can also just wrap the member which need deep copy, and overload operator= for the wrapper. I'm glad someone suggested this - it's certainly the only route I would recommend. I'm a strong believer in using compiler-generated code where-ver possible: it both cuts down on maintenance and increases code readability. The real reason for allowing the compiler to generate operator= for you is not laziness, but to avoid programmer error. One of the worse bugs I ever found was because someone had forgotten to update operator= when a new member was added. Likewise, when implementing swap(), you have to be extra diligent that every member is swapped (and swapped exactly once!). Compiler-generated swap is a must, IMO. Which is not to say there should not be a way of turning it off (or even that maybe explicit syntax is needed to specific that you want it). Dylan [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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