C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  where to read about smart pointers          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Marlene Miller Guest Posted: Wed Jan 26, 2005 10:12 am    Post subject: where to read about smart pointers I request some guidance on how to learn about smart pointers. 1. Modern C++ Design by Andrei Alexandrescu, chapter 7 Smart Pointers. Is that a good place to *start*? For me, MC++D is very advanced, but I think I could cope. Is it essential or *wise* or convenient to read chapters 1-6 before reading chapter 7? 2. More Effective C++ by Scott Meyers. I could lookup "smart pointers" in the index, follow the page references and read every related Item. 3. I don't have Herb Sutter's books (yet). 4. Is there something good that I should read that is not as advanced in preparation for MC++D? I've studied Accelerated C++, that's all I know about reference counting. 5. I'd like to know if I am on the right track. Classes, regarded as an OO concept rather than a language construct, have association relationships. In C++ I suppose an association is implemented as a data member subobject or a data member pointer or reference to an object of class type. If object X has a pointer to object Y, X could delete Y. Not good, if X does not own Y. If X has a reference to Y, and if Y is deleted, X needs to know. In Java, I never worried about this. As long as some object has a reference to Y, Y will not be garbage collected. Class associations are a basic concept. Knowing how to implement associations must also be basic. Knowing how the client object navigates to the service object without the power to delete it must be basic. What is the common/default idiom/technique to manage class associations in C++? Thank you. - Marlene [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Maxim Yegorushkin Guest Posted: Thu Jan 27, 2005 8:58 pm    Post subject: Re: where to read about smart pointers Marlene Miller wrote: Quote: I request some guidance on how to learn about smart pointers. 1. Modern C++ Design by Andrei Alexandrescu, chapter 7 Smart Pointers. Is that a good place to *start*? For me, MC++D is very advanced, but I think I could cope. Is it essential or *wise* or convenient to read chapters 1-6 before reading chapter 7? I think it's beneficial to read the whole book because, IIRC, chapters are based on previous chapters. Anyway you or your friend paid for it, why waste it? ;) Quote: 2. More Effective C++ by Scott Meyers. I could lookup "smart pointers" in the index, follow the page references and read every related Item. This might help, but... Meyers books are pretty simple to novices and concepts described there and understanding gained will pay you back for sure (of course if you are concerned about your CS career rather than passing an exam). Quote: 3. I don't have Herb Sutter's books (yet). Grab it, I mean, if you are really interested in C++, get every mainstream C++ book and read it. The books I would recommend and I would read them myself again in the following order if I had to start again: * The C++ Programming Language by Stroustrup (I had to read it 4 times in 1.5-year period to comprehend and appreciate) * C++ for Real Programmers by Jeff Alger (a little bit outdated by still makes sense) * Effective and More Effective C++ by Scott Meyers (following will get you started about applied OOP before digging deeper) * Design Pattern by Erich Gamma, et al * Object-Oriented Analysis and Design with Applications by Grady Booch (digging deeper in C++) * Exceptional's by Herb Sutter * Modern Design C++ by Andrei Alexandrescu * C++ Gotchas by Stephen C. Dewhurst [] Quote: 5. I'd like to know if I am on the right track. Classes, regarded as an OO concept rather than a language construct, have association relationships. In C++ I suppose an association is implemented as a data member subobject or a data member pointer or reference to an object of class type. The problem is that C++ does not have a direct mapping to OOP(UML) or vice versa. OOP aggregation is most often expressed in C++ as a class having a member of that type (the member is destroyed in the class's destructor) or having a pointer of that type that is deleted whenever the class's designer wishes it. This is the strict ownership concept concept in C++ - who owns the object is he who deletes it, but not vice versa. The shared ownership concept in C++ is - whoever releases its reference to an object last also he who destroys the object, but it neither owns it nor knows that it is the sole referrer at the moment. An association in C++ has no direct representation at all. The OOP concept of association in C++ may be represented as having a reference(pointer) to a class, or by a member function of a class taking a pointer(reference) to a referenced class and using that pointer by calling member functions through that pointer. Most othen, association in C++ means that a class has an access to a declarataion of the referenced class (includes a header file of it) but does not owns an object of it, or uses shared ownership. Quote: If object X has a pointer to object Y, X could delete Y. Not good, if X does not own Y. It depends on ownership. If X ownes Y it can delete it whenever it wants to. Quote: If X has a reference to Y, and if Y is deleted, X needs to know. If it happens in C++ - it's a mess. In such cases they use shared ownership - somebody releases its reference to Y but you don't care, because as long as you use Y you hold a reference to it, and as long as Y has a reference it's not deleted. Quote: In Java, I never worried about this.As long as some object has a reference to Y, Y will not be garbage collected. Class associations are a basic concept. Java hides the ownership and memory management concepts from developerz. -- Maxim Yegorushkin [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Patrick Leslie Polzer Guest Posted: Thu Jan 27, 2005 9:14 pm    Post subject: Re: where to read about smart pointers Marlene Miller wrote: Quote: In C++ I suppose an association is implemented as a data member subobject or a data member pointer or reference to an object of class type. If object X has a pointer to object Y, X could delete Y. Not good, if X does not own Y. Make it const then. Pointers to const data cannot be deleted. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Fri Jan 28, 2005 12:37 am    Post subject: Re: where to read about smart pointers "Marlene Miller" <marlenemiller (AT) worldnet (DOT) att.net> writes: Quote: I request some guidance on how to learn about smart pointers. 1. Modern C++ Design by Andrei Alexandrescu, chapter 7 Smart Pointers. Is that a good place to *start*? For me, MC++D is very advanced, but I think I could cope. Is it essential or *wise* or convenient to read chapters 1-6 before reading chapter 7? Well, I think MC++D is a very instructive book, but it's not particularly well-focused on smart pointers. There's a lot of material about how to write fancy libraries that probably doesn't directly address what you're interested in, and since much of that material is used in the discussion of Andrei's smart pointer design, you will probably have to invest quite a lot just to get the smart-pointer-specific information out of it. You could probably learn quite a lot very quickly about smart pointers by reading http://www.boost.org/libs/smart_ptr/smart_ptr.htm and following the links there. Be sure not to miss http://www.boost.org/libs/smart_ptr/sp_techniques.html; it's a gold mine! It's not quite comprehensive but it's a very good start. You might follow that with http://home.comcast.net/~jturkanis/move_ptr (if can stand slow websites ;->). Quote: 2. More Effective C++ by Scott Meyers. I could lookup "smart pointers" in the index, follow the page references and read every related Item. You can learn quite a lot that way. However, be sure you check his errata pages, e.g. http://www.aristeia.com/BookErrata/M29Source_frames.html Quote: 3. I don't have Herb Sutter's books (yet). 4. Is there something good that I should read that is not as advanced in preparation for MC++D? I've studied Accelerated C++, that's all I know about reference counting. I think Meyers is a good way to go. Quote: 5. I'd like to know if I am on the right track. Classes, regarded as an OO concept rather than a language construct, have association relationships. In C++ I suppose an association is implemented as a data member subobject or a data member pointer or reference to an object of class type. If object X has a pointer to object Y, X could delete Y. Not good, if X does not own Y. If X has a reference to Y, and if Y is deleted, X needs to know. I think the only place mentioned so far that you'll learn how to handle this in C++ is in the boost docs, particularly looking at weak_ptr. Quote: In Java, I never worried about this. As long as some object has a reference to Y, Y will not be garbage collected. Class associations are a basic concept. Knowing how to implement associations must also be basic. Knowing how the client object navigates to the service object without the power to delete it must be basic. What is the common/default idiom/technique to manage class associations in C++? I think most people tend to use raw pointers, which simply dangle when the target object is deleted. That's more error-prone than using weak_ptr, but, of course, it's easier and sometimes more efficient. HTH, -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Fri Jan 28, 2005 12:39 am    Post subject: Re: where to read about smart pointers Patrick Leslie Polzer wrote: Quote: Pointers to const data cannot be deleted. Why do you say this? Try it. It's perfectly legal. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marlene Miller Guest Posted: Fri Jan 28, 2005 11:23 am    Post subject: Re: where to read about smart pointers Thank you David. Quote: I think most people tend to use raw pointers, which simply dangle when the target object is deleted. That's more error-prone than using weak_ptr, but, of course, it's easier and sometimes more efficient. Suppose you have an application that is composed of layers. Layer 1 manages some resources. Layer 2 requests access to a resource. Could it happen that L1 has to give L2 a pointer to one of its objects, then L2 deletes the object? Do you ever need to protect your objects? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Sat Jan 29, 2005 4:24 am    Post subject: Re: where to read about smart pointers "Marlene Miller" <marlenemiller (AT) worldnet (DOT) att.net> wrote Quote: I think most people tend to use raw pointers, which simply dangle when the target object is deleted. That's more error-prone than using weak_ptr, but, of course, it's easier and sometimes more efficient. Suppose you have an application that is composed of layers. Layer 1 manages some resources. Layer 2 requests access to a resource. Could it happen that L1 has to give L2 a pointer to one of its objects, then L2 deletes the object? Do you ever need to protect your objects? You can't totally protect you objects in C++. But you can go a long way towards preventing programming errors. First of all, L1 should not pass an object pointer to L2, but a reference (or some smart pointer like weak or shared_ptr). This will strongly hint that L2 is not to delete the object. Next, instead of providing access to the full object implementation to L2, L1 could only provide access to a base class that has a protected destructor, which also helps prevent accidental destruction. Example design of L1: /////////////////////////////////// // L1.h class L1Resource { public: virtual void doSomething()=0; //... protected: ~L1Resource() {} }; L1Resource& accessObject(); // returns an object /////////////////////////////////// // L1.cpp class L1Resource_Impl : public L1Resource { public: virtual void doSomething() {....implementation....} }; L1Resource& accessObject() { static L1Resource_Impl obj; // (simplification) return obj; } I hope this helps, Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Sat Jan 29, 2005 4:47 am    Post subject: Re: where to read about smart pointers "Marlene Miller" <marlenemiller (AT) worldnet (DOT) att.net> writes: Quote: Thank you David. I think most people tend to use raw pointers, which simply dangle when the target object is deleted. That's more error-prone than using weak_ptr, but, of course, it's easier and sometimes more efficient. Suppose you have an application that is composed of layers. Layer 1 manages some resources. Layer 2 requests access to a resource. Could it happen that L1 has to give L2 a pointer to one of its objects, then L2 deletes the object? It could, but that's why we have smart pointers and tend to eschew delete. Use auto_ptr when you want to transfer ownership from L1 to L2. Use boost::shared_ptr when you essentially want garbage collection -- the resource only disappears when everyone is done with it. Quote: Do you ever need to protect your objects? What do you mean when you say "protect?" -- Dave Abrahams Boost Consulting www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marlene Miller Guest Posted: Sat Jan 29, 2005 9:35 am    Post subject: Re: where to read about smart pointers Thank you Ivan. Quote: First of all, L1 should not pass an object pointer to L2, but a reference (or some smart pointer like weak or shared_ptr). This will strongly hint that L2 is not to delete the object. If the resource provider L1 gives out a reference to one of its objects, L1 should not delete that object unless it informs all the clients. Maybe L1 could give out objects that know where the real resource object is. Then L1 and L2 can delete their objects without concern for the other. (I am trying to learn the common ways C++ is used to implement associations.) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Sat Jan 29, 2005 3:26 pm    Post subject: Re: where to read about smart pointers "Marlene Miller" <marlenemiller (AT) worldnet (DOT) att.net> wrote Quote: If the resource provider L1 gives out a reference to one of its objects, L1 should not delete that object unless it informs all the clients. Maybe L1 could give out objects that know where the real resource object is. Then L1 and L2 can delete their objects without concern for the other. (I am trying to learn the common ways C++ is used to implement associations.) It is a common idiom is to expose only a Handle (or Handle-like class) that maintains a reference to the actual resource, and will gracefully fail if the resource was released by the provider. std::weak_ptr could be used as such a 'weak handle'. The interface of boost::weak_ptr is even designed to be compatible with a multi-threaded environment, as it requires 'locking' the object (with a shared_ptr) before being able to access it. See the usage notes in the Boost library docs: http://www.boost.org/libs/smart_ptr/weak_ptr.htm Essentially, the strategy will depend on which module, by design, has the ownership of the resources. Other specific requirements, such as the need to serialize access to the objects themselves, may also be determinant. This said, using shared_ptr may be the easiest solution if you want the object to only be deleted when both the provided (L1) and the user (L2) are done with it. Cheers, Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marlene Miller Guest Posted: Sun Jan 30, 2005 4:09 am    Post subject: Re: where to read about smart pointers Quote: Do you ever need to protect your objects? What do you mean when you say "protect?" Your question might be an answer to my question. Protecting objects from being deleted by objects in other parts of an application must not be an ordinary concern. Even when the application is distributed. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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