C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  What is the standard signature for an allocator destructor?          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message apm Guest Posted: Sat Sep 25, 2004 5:42 am    Post subject: What is the standard signature for an allocator destructor? My reading of the std is that the signature of a custom allocator should follow the pattern laid out by the default allocator. Section 20.1.5 (page 354) says that the std library lays down the requirements for allocators. The requirements are in table 32 (page 356). However, the table does not specify signatures for the allocator constructor or destructor. It is reasonable to assume (IMO) that the signatures should follow the form of the default allocator in section 20.4.1. I cannot find anywhere in the std where it actually says this though. This causes a serious problem: if one inherits from a custom allocator that DOES conform to the default allocator dtor signature then the derived class must also have a dtor with an empty throw list. Otherwise a compilation error can result (not all compilers watch out for this but some definitely do). This issue just caused a compilation error in some code I am working on. So it is a real problem. Suppose the derived class had been written by someone else. And that it could not be changed. Then I would have to change the signature of my allocator dtor so as NOT to conform to the pattern laid out in section 20.4.1 to make the compilation error go away. But I feel that this change makes me violate the std. Except that the std seems to be a little grey in this area. Can anyone shed any light please? I would like to tell the author of the derived class that he has to change because I am conformant and he is not. But its hard to do that when the std doesnt seem to cover it completely. Regards, Andrew Marlow [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Birbacher Guest Posted: Mon Sep 27, 2004 10:31 am    Post subject: Re: What is the standard signature for an allocator destruct Hi! apm wrote: Quote: However, the table does not specify signatures for the allocator constructor or destructor. It is reasonable to assume (IMO) that the signatures should follow the form of the default allocator in section 20.4.1. I cannot find anywhere in the std where it actually says this though. For the copy constructor, yes. And for the 'rebinding copy ctor', which contructs a new allocator for a different type. The default ctor however is not required, as e.g. std containers like vector take an instance of an allocator as the last parameter in their ctors. Quote: This causes a serious problem: if one inherits from a custom allocator that DOES conform to the default allocator dtor signature then the derived class must also have a dtor with an empty throw list. Otherwise a compilation error can result (not all compilers watch out for this but some definitely do). This issue just caused a compilation error in some code I am working on. So it is a real problem. Just because a base class has a non-throwing dtor, doesn't mean the derived class has a non-throwing dtor, unless the dtor is virtual. I think dtors of allocators must not be virtual because the std container store a copy of the given allocator and there object slicing must be avoided. Thus you need to specify the exact type of the allocator in use, and thus you don't need a virtual dtor. But I don't know whether the allocator dtor must be non-throwing. Frank [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top apm Guest Posted: Fri Oct 01, 2004 4:39 pm    Post subject: Re: What is the standard signature for an allocator destruct Frank Birbacher <bloodymir.crap (AT) gmx (DOT) net> wrote Quote: Hi! apm wrote: However, the table does not specify signatures for the allocator constructor or destructor. It is reasonable to assume (IMO) that the signatures should follow the form of the default allocator in section 20.4.1. I cannot find anywhere in the std where it actually says this though. For the copy constructor, yes. Well that's what I think also, but where does it say so in the std? Quote: This causes a serious problem: if one inherits from a custom allocator that DOES conform to the default allocator dtor signature then the derived class must also have a dtor with an empty throw list. Otherwise a compilation error can result (not all compilers watch out for this but some definitely do). This issue just caused a compilation error in some code I am working on. So it is a real problem. Just because a base class has a non-throwing dtor, doesn't mean the derived class has a non-throwing dtor, unless the dtor is virtual. So when I got the compilation error, that was a compiler bug? Quote: But I don't know whether the allocator dtor must be non-throwing. I welcome more input concerning where this is covered in the std, please. -apm [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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