C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Can I avoided a memory problem doing this?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message achava@hotmail.com Guest Posted: Fri Jan 28, 2005 2:04 am    Post subject: Can I avoided a memory problem doing this? Suppose I am overloading an operator for class Zigid and my method looks like this: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); .... return *Sum; } The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How do I get the caller to delete the all memory pointed to by the reference it returns?" In particular an object of the Zigid class must allocate memory dynamically when it is being created, although this does not necessarily happen in the constructor. Any help appreciated. Achava [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Fri Jan 28, 2005 11:21 am    Post subject: Re: Can I avoided a memory problem doing this? [email]achava (AT) hotmail (DOT) com[/email] wrote: Quote: Suppose I am overloading an operator for class Zigid and my method looks like this: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); ... return *Sum; } The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How do I get the caller to delete the all memory pointed to by the reference it returns?" In particular an object of the Zigid class must allocate memory dynamically when it is being created, although this does not necessarily happen in the constructor. Don't do this. When types have things like operator+, these types usually behave like values not like objects. The difference is that values are a mere aggregation of data, with little or no behaviour to them while objects are mostly defined by their behaviour with little or no visible(public or otherwise) data. This is not a strict rule, but a guideline. Now, for values, the typical way to overload operator+ goes like this: class value { value& operator+=( value const& rhs) { ...; return *this; } }; value operator+( value const& v1, value const& v2) { value res(v1); res += v2; return res; } You don't make operator+ a memberfunction because the two involved objects are all equally important, which is better expressed by a free function taking the two objects. Then, operator+ delegates to operator+= which is a member because there is one prominent object involved. One last note: In case you have a Java background, forget about new - in Java you need new to create a new object, in C++ you only need new when you want to manage the lifetime of the object yourself, as opposed to managing its lifetime with the scope. Uli -- FAQ: http://parashift.com/c++-faq-lite/ /* bittersweet C++ */ default: break; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Fri Jan 28, 2005 11:22 am    Post subject: Re: Can I avoided a memory problem doing this? Hello Achava, [email]achava (AT) hotmail (DOT) com[/email] schrieb: Quote: Suppose I am overloading an operator for class Zigid and my method looks like this: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); ... return *Sum; } The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How do I get the caller to delete the all memory pointed to by the reference it returns?" In particular an object of the Zigid class must allocate memory dynamically when it is being created, although this does not necessarily happen in the constructor. Although the code might run fine, but it should be fixed as soon as possible ;-) Honestly, functions returning references to any kind of object should - never return references of local objects of the function (No, you didn't do that) - never return references to non-local objects, where the caller has the responsibility for resource management (Yes, that does happen above!) A signature of something like T& foo(); implies, that the returned reference belongs to an automanaged object in some broader sense: I mean an extended version of self-management, that includes some other possible mechanism of the defining (yours!) module, e.g. a private table of Zigid objects, which are free'd by your module's framework in some internally and invisible manner. Binary arithmetic operators usually return **value** objects and the question arises, why you don't simply write Zigid operator+(const Zigid &A) // No reference return type, const reference argument! { Zigid Sum = Zigid(); .... return Sum; } As you might have noticed, I additionally replaced your mutable argument by an immutable one. This another point of critique of your code: Why the hack should an argument of any binary arithmetic operator be mutable? - Be const-correct! It would be very unusual, if such an operator **really** modifies one of its arguments... Greetings from Bremen, Daniel Krügler [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Efrat Regev Guest Posted: Fri Jan 28, 2005 11:26 am    Post subject: Re: Can I avoided a memory problem doing this? <achava (AT) hotmail (DOT) com> wrote Quote: Suppose I am overloading an operator for class Zigid and my method looks like this: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); ... return *Sum; } The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How do I get the caller to delete the all memory pointed to by the I think that your problem could be solved by some form of garbage collection, which could be done using one of the following: 1. Using "smart pointers", which can deduce when to deallocate memory. You can find discussions on this in (among others) a. "Modern C++" by Alexandrescu b. "C++ Templates: the Complete Guide" by Vadevoorde and Josuttis, c. the boost::smart_ptr library http://www.boost.org/libs/smart_ptr/smart_ptr.htm#Introduction by Dimov et. al. (which you can use directly). Specifically, you might want to check out http://www.boost.org/libs/smart_ptr/sp_techniques.html. 2. Alternatively, you might go for more general garbage collection, e.g., as in http://www.hpl.hp.com/personal/Hans_Boehm/gc/, in which case you wouldn't have to deallocate memory explicitly at all. (There seems to be a wide range of opinions on the merits of using general garbage collection, however.) HTH [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Fri Jan 28, 2005 11:27 am    Post subject: Re: Can I avoided a memory problem doing this? <achava (AT) hotmail (DOT) com> wrote... Quote: Suppose I am overloading an operator for class Zigid and my method looks like this: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); ... return *Sum; } I take it this definition is inside the class, for the addition op, right? I'll assume that. Quote: The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How do I get the caller to delete the all memory pointed to by the reference it returns?" It's ugly, but you can always do Zigid &someZigid = otherZigid + thirdZigid; ... delete &someZigid; // No need in 'someZigid' any longer. // Makes 'someZigid' reference invalid. Quote: In particular an object of the Zigid class must allocate memory dynamically when it is being created, although this does not necessarily happen in the constructor. I have two problems with your approach. First, it is often suboptimal to use freestore where you could simply return a temporary. Second, the addition operator should be able to take a temporary as its argument and for that you need to declare the 'A' argument a reference to const. Third, if operator+ doesn't change the *this object, it should be also declared 'const'. Those are the basics, you can read about them and other very useful things in "Effective C++" by Scott Meyers. Your declaration/definition should be Zigid operator+(Zigid const& A) const { Zigid temp; ... return temp; } Another argument against free store: if you use more than one operator+ in one expression, who's going to clean up? zig = zig1 + zig2 + zig3 + zig4; // where do all the intermediate // results go? how do you catch them? You need to rethink the interface, I guess. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nicola.Musatti@ObjectWay. Guest Posted: Sat Jan 29, 2005 4:28 am    Post subject: Re: Can I avoided a memory problem doing this? The error lies in wanting to return by reference when you should return by value. You should write: // Note the const reference parameter Zigid operator+(const Zigid &A) { Zigid Sum; .... return Sum; } If you feel that returning Zigid's by value is too expensive do not provide operator+ and use operator += instead. Cheers, Nicola Musatti [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top R.F. Pels Guest Posted: Sat Jan 29, 2005 4:34 am    Post subject: Re: Can I avoided a memory problem doing this? [email]achava (AT) hotmail (DOT) com[/email] wrote: Quote: Zigid &operator+(Zigid &A) { Zigid *Sum = new Zigid(); ... return *Sum; } The code runs just fine, but the method allocates memory dynamically and leaves it to the caller to clean up. What I want to know is, "How Don't do that. Just return a new Zigid from operator+, that will circumvent a lot of problems you mention. Zigid operator+(const Zigid& that) { Zigid result(....); ... return result; } -- Ruurd ..o. ...o ooo [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top achava@hotmail.com Guest Posted: Sat Jan 29, 2005 5:35 am    Post subject: Re: Can I avoided a memory problem doing this? Thank you all for your replies. I have rewritten my code to return the object rather than a reference to the object, and no longer do I create the object on the free store. I let the scope do the clean-up. I did in fact work in Java before taking up C++ again after a long lapse, so I got used to using new for everything. Regards, Achava [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Seungbeom Kim Guest Posted: Sun Jan 30, 2005 4:00 am    Post subject: Re: Can I avoided a memory problem doing this? Ulrich Eckhardt wrote: Quote: Don't do this. When types have things like operator+, these types usually behave like values not like objects. The difference is that values are a mere aggregation of data, with little or no behaviour to them while objects are mostly defined by their behaviour with little or no visible(public or otherwise) data. This is not a strict rule, but a guideline. * Even an int variable is an object by definition in C++. * When you use an object, you use it as an l-value or an r-value. Therefore "values" vs "objects" is not a very good way to name them. I understand well your concept of different roles for objects, but you can call them more appropriately as "value objects" and "entity objects". -- Seungbeom Kim [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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