Bitwise copy contructor
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From: pfultz2 on 19 Mar 2010 17:26 How would I go about writing a bitwise copy constructor for a class? I would like to call memcpy() before any constructors are called on the members. I cant seem to find any information on this. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Daniel Krügler on 20 Mar 2010 04:18 On 20 Mrz., 09:26, pfultz2 <pful...(a)yahoo.com> wrote: > How would I go about writing a bitwise copy constructor for a class? I > would like to call memcpy() before any constructors are called on the > members. I cant seem to find any information on this. It is not really clear to me, what you question is - a more specific use-case would help. Are you asking for the syntax to realize that? Or are you asking how you can do that without provoking undefined behavior (UB)? The life-time and trivial type rules of C++ put rather severe restrictions on class types (I assume your class contains members of class-type, because otherwise your remark about constructors doesn't make sense to me), so simply performing memcpy on a std::string within or without a constructor will rather easy lead to UB land. What are you going to achieve and why isn't a normal copy constructor sufficient? 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! ]
From: pfultz2 on 20 Mar 2010 11:08 On Mar 20, 3:18 pm, Daniel Kr�gler <daniel.krueg...(a)googlemail.com> wrote: > On 20 Mrz., 09:26, pfultz2 <pful...(a)yahoo.com> wrote: > > > How would I go about writing a bitwise copy constructor for a class? I > > would like to call memcpy() before any constructors are called on the > > members. I cant seem to find any information on this. > > It is not really clear to me, what you > question is - a more specific use-case > would help. Are you asking for the > syntax to realize that? Or are you asking > how you can do that without provoking > undefined behavior (UB)? > > The life-time and trivial type rules of > C++ put rather severe restrictions on > class types (I assume your class contains > members of class-type, because otherwise > your remark about constructors doesn't > make sense to me), so simply performing > memcpy on a std::string within or without > a constructor will rather easy lead > to UB land. > > What are you going to achieve and why > isn't a normal copy constructor > sufficient? > > Greetings from Bremen, > > Daniel Kr�gler Well if i call a copy constructor like this: class foo { bar b; public: foo(cont foo& rhs) { memcpy(this, &rhs, sizeof(foo)); } }; When it calls the copy constructor the constructor for the fields(such as bar b) are called first, then memcpy() is called. I want to know how to call memcpy and not call the constructors for the fields. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Mathias Gaunard on 20 Mar 2010 11:06 On 20 mar, 08:26, pfultz2 <pful...(a)yahoo.com> wrote: > How would I go about writing a bitwise copy constructor for a class? I > would like to call memcpy() before any constructors are called on the > members. I cant seem to find any information on this. That's because you can't do it. If you want to bitwise copy a class with non-POD members, you must do that outside of the copy constructor, and still, that's obviously highly UB and something you shouldn't do. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Nevin :-] Liber on 20 Mar 2010 22:14
In article <5f856b43-6878-4a3a-b2eb-8de6ef7572d8(a)r1g2000yqj.googlegroups.com>, pfultz2 <pfultz2(a)yahoo.com> wrote: > > What are you going to achieve and why > > isn't a normal copy constructor > > sufficient? > > Well if i call a copy constructor like this: > class foo > { > bar b; > public: > foo(cont foo& rhs) > { > memcpy(this, &rhs, sizeof(foo)); > } > }; > When it calls the copy constructor the constructor for the fields(such > as bar b) are called first, then memcpy() is called. Again, what are you trying to achieve (the goal, not the mechanism)? > I want to know > how to call memcpy and not call the constructors for the fields. You can't, and that is a good thing. You can pick which constructors are called (via the member initialization list), but you cannot circumvent calling constructors. -- Nevin ":-)" Liber <mailto:nevin(a)eviloverlord.com> 773 961-1620 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |