auto_ptr of incomplete type not allowed?
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From: Leigh Johnston on 2 Feb 2010 06:08 Hi, N3000 states that an incomplete type is allowed for shared_ptr and unique_ptr but it doesn't say the same for auto_ptr. Is this omission a mistake or deliberate? An incomplete type auto_ptr member variable should be allowed as long as parent class destructor is not declared inline. This is useful when implemented pimpl idiom: struct opaque_type; struct foo { ~foo(); // dtor defined elsewhere std::auto_ptr<opaque_type> pimpl; }; /Leigh -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Bart van Ingen Schenau on 2 Feb 2010 21:46 On Feb 3, 12:08 am, "Leigh Johnston" <le...(a)i42.co.uk> wrote: > Hi, > > N3000 states that an incomplete type is allowed for shared_ptr and > unique_ptr but it doesn't say the same for auto_ptr. Is this omission a > mistake or deliberate? It is probably deliberate. In the C++03 standard, all types used as template argument for templates in the standard library have to be complete types. As there are possibly/probably some library implementations that rely on the fact that the types must be complete, my guess is that the standards committee did not want to break these implementations by allowing incomplete types for already existing templates. > > /Leigh > Bart v Ingen Schenau -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Walter van der Hee on 2 Feb 2010 21:47 On 3 Feb, 00:08, "Leigh Johnston" <le...(a)i42.co.uk> wrote: > Hi, > > N3000 states that an incomplete type is allowed for shared_ptr and > unique_ptr but it doesn't say the same for auto_ptr. Is this omission a > mistake or deliberate? > > An incomplete type auto_ptr member variable should be allowed as long as > parent class destructor is not declared inline. This is useful when > implemented pimpl idiom: > > struct opaque_type; > > struct foo > { > ~foo(); // dtor defined elsewhere > std::auto_ptr<opaque_type> pimpl; > > }; > > /Leigh > Quote from N3000: "The class template auto_ptr is deprecated. [ Note: The class template unique_ptr (20.8.14) provides a better solution. �end note ]" ciao, Walter -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Daniel Krügler on 3 Feb 2010 00:30
On 3 Feb., 00:08, "Leigh Johnston" <le...(a)i42.co.uk> wrote: > N3000 states that an incomplete type is allowed for shared_ptr and > unique_ptr but it doesn't say the same for auto_ptr. Is this omission a > mistake or deliberate? > > An incomplete type auto_ptr member variable should be allowed as long as > parent class destructor is not declared inline. This is useful when > implemented pimpl idiom: > > struct opaque_type; > > struct foo > { > ~foo(); // dtor defined elsewhere > std::auto_ptr<opaque_type> pimpl; > }; std::auto_ptr is a deprecated component from C++0x on, it's suggested replacement is std::unique_ptr. The design of std::auto_ptr bases essentially on some obscure rule in C++03, and has some inherent defects. It seemed to be the best compromise not to change std::auto_ptr at all - a correct fix would have removed std::auto_ptr_ref but this change would have caused a potential backward-compatibility problem for those who explicitly did depend on this template. The fact that std::auto_ptr_ref was specified as a normal component (and not kept unspecified) was another problem of the original spec. This will be changed for C++0x. Adding support for incomplete types for auto_ptr was considered by the committee but the idea was rejected, because of it's deprecated state. Any change would have required that existing implementations need to be changed and in regard to existing alternatives this seemed not worth the effort, such a correct specification would require. Greetings from Bremen, Daniel Kr�gler -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |