C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  new A v.s. new A()          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Wenjie Guest Posted: Thu Jun 26, 2003 2:34 pm    Post subject: new A v.s. new A() Hello, Is there any difference between: A* p = new A; and A* p = new A(); ? I know it must have been dicussed here, but just not easy to search. BTW, I think A a; is alike new A as far as constructor call concerned (default construtor call?) Right? And A a(); is just a function declaration? Thanks and best regards, Wenjie Back to top Victor Bazarov Guest Posted: Thu Jun 26, 2003 2:50 pm    Post subject: Re: new A v.s. new A() "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... Quote: Is there any difference between: A* p = new A; and A* p = new A(); ? There is no difference for classes, there is a difference for POD: the former leaves the object uninitialised, the latter zero-initialises it. Quote: I know it must have been dicussed here, but just not easy to search. BTW, I think A a; is alike new A as far as constructor call concerned (default construtor call?) Right? It's a declaration (and a definition in certain circumstances) of an object 'a' of type A. If it's a definition, the object is default-initialised, which for a class means that its c-tor is invoked, yes. Quote: And A a(); is just a function declaration? Yes, it is. Victor Back to top Karl Heinz Buchegger Guest Posted: Thu Jun 26, 2003 2:59 pm    Post subject: Re: new A v.s. new A() Wenjie wrote: Quote: Hello, Is there any difference between: A* p = new A; and A* p = new A(); ? Yes, there is a difference in certain circumstances. It has to do with POD's (POD = plain old data structure, basically a class or a struct built from only builtin types and having no member functions, in short: a struct as it was ment to be in C) new A creates a new POD-object of type A and leaves all members uninitialized new A() creates a new POD-object of type A but 0-initializes all members. Quote: A a(); is just a function declaration? Yep. That's a pitfall. -- Karl Heinz Buchegger [email]kbuchegg (AT) gascad (DOT) at[/email] Back to top Ron Natalie Guest Posted: Thu Jun 26, 2003 3:22 pm    Post subject: Re: new A v.s. new A() "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote Quote: There is no difference for classes, non-POD classes (but you knew that). Quote: there is a difference for POD: the former leaves the object uninitialised, the latter zero-initialises it. Back to top Ron Natalie Guest Posted: Thu Jun 26, 2003 3:27 pm    Post subject: Re: new A v.s. new A() "Karl Heinz Buchegger" <kbuchegg (AT) gascad (DOT) at> wrote Quote: Yes, there is a difference in certain circumstances. It has to do with POD's (POD = plain old data structure, basically a class or a struct built from only builtin types and having no member functions, in short: a struct as it was ment to be in C) That's not the definition of POD. POD classes can't have non-static data of non-pod type (other non-POD structs or unions, pointer-to-members or references). user-defined constructors, destructors, or copy-assignment operators base classes protected or private non-static data members virtual functions. They can however have other member functions not specifically mentioned above. In short, a class whose data behaves identically to a C struct. Back to top Victor Bazarov Guest Posted: Thu Jun 26, 2003 10:15 pm    Post subject: Re: new A v.s. new A() "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... Quote: "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... Is there any difference between: A* p = new A; and A* p = new A(); ? There is no difference for classes, there is a difference for POD: the former leaves the object uninitialised, the latter zero-initialises it. The default constructor (whether user defined or not) is called in both cases like for a definition "A a;" right? If it's not defined ("trivial"), how would it be called? Quote: Then "zero-init" is only for simplifying the anwser, since what I know a default constructor could be written as: Zero-initialisation is only performed for POD. As soon as you define a constructor, it's not a POD any longer. Quote: class A { public: A(): a_(12) {} //... private: int a_; }; I know it must have been dicussed here, but just not easy to search. BTW, I think A a; is alike new A as far as constructor call concerned (default construtor call?) Right? It's a declaration (and a definition in certain circumstances) of an object 'a' of type A. If it's a definition, the object is default-initialised, which for a class means that its c-tor is invoked, yes. And A a(); is just a function declaration? Yes, it is. Victor Thanks, Back to top Yu Cao Guest Posted: Thu Jun 26, 2003 11:28 pm    Post subject: Re: new A v.s. new A() "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> writes: Quote: "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote in message news:<vfm231muclto80 (AT) corp (DOT) supernews.com>... "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... Is there any difference between: A* p = new A; and A* p = new A(); ? There is no difference for classes, there is a difference for POD: the former leaves the object uninitialised, the latter zero-initialises it. This is all fine. I just got into a heated debate with a colleague about operator new[] though: Q1. Is operator new[] allowed to take a non-default constructor for a user-defined type? Example: class A { public: A(int, int, int); }; A* p = new A[100](1, 2, 3); My position is yes. GCC 3.2 apparently agrees. Q2. For a POD type, is operator new[] allowed to take non-default initializers? Example: int* p = new int[100](1); My inclination is to say yes. But GCC is not on my side this time. For int* p = new int(1), *p is initialized to 1. With operator new[] though, it did not. Q3. For a POD type, should the following two statements both perform zero initialization: int *p = new int[100]; int *p = new int[100](); My understanding is the first one doesn't and the second one does, just like for operator new. We've consulted the standard but our interpretations are different. My basic stance is what's quoted for operator new should apply as well to operator new[]. --Yu Back to top Victor Bazarov Guest Posted: Fri Jun 27, 2003 12:21 am    Post subject: Re: new A v.s. new A() "Yu Cao" <yucao (AT) alumnae (DOT) caltech.edu> wrote... Quote: "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> writes: "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote in message news:<vfm231muclto80 (AT) corp (DOT) supernews.com>... "Wenjie" <gokkog (AT) yahoo (DOT) com> wrote... Is there any difference between: A* p = new A; and A* p = new A(); ? There is no difference for classes, there is a difference for POD: the former leaves the object uninitialised, the latter zero-initialises it. This is all fine. I just got into a heated debate with a colleague about operator new[] though: Q1. Is operator new[] allowed to take a non-default constructor for a user-defined type? If the user-defined type does not have a default constructor, the program that tries to dynamically allocate an array of such type is ill-formed. See Standard Quote: Example: class A { public: A(int, int, int); }; A* p = new A[100](1, 2, 3); My position is yes. The Standard doesn't agree with you. See 5.3.4/15. Only default initialisation is allowed for arrays. Quote: GCC 3.2 apparently agrees. Doesn't prove anything. Quote: Q2. For a POD type, is operator new[] allowed to take non-default initializers? Nope. Quote: Example: int* p = new int[100](1); My inclination is to say yes. Oh, well... Quote: But GCC is not on my side this time. For int* p = new int(1), *p is initialized to 1. With operator new[] though, it did not. Right. PODs are left uninitialised. Quote: Q3. For a POD type, should the following two statements both perform zero initialization: int *p = new int[100]; int *p = new int[100](); The first one does not perform any initialisation. Quote: My understanding is the first one doesn't and the second one does, just like for operator new. We've consulted the standard but our interpretations are different. My basic stance is what's quoted for operator new should apply as well to operator new[]. It doesn't. But if you'd like to make a suggestion, you should definitely do so in comp.std.c++. Victor Back to top Ron Natalie Guest Posted: Fri Jun 27, 2003 3:27 pm    Post subject: Re: new A v.s. new A() "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote Quote: The default constructor (whether user defined or not) is called in both cases like for a definition "A a;" right? If it's not defined ("trivial"), how would it be called? As far as the standard is concerend it behaves as if one was automatically inserted by the compiler. Back to top Alexander Terekhov Guest Posted: Fri Jun 27, 2003 6:28 pm    Post subject: Re: new A v.s. new A() Ron Natalie wrote: [...] Quote: int *p = new int[100]; int *p = new int[100](); The latter is ILLEGAL. The former provides no initialization. Another major defect of the C++ language. You just love things like struct IntGarbage { IntGarbage() {} int garbage; }; I guess. ;-) regards, alexander. Back to top Greg Comeau Guest Posted: Sat Jun 28, 2003 1:32 am    Post subject: Re: new A v.s. new A() In article <vfoqcal6bqev1b (AT) corp (DOT) supernews.com>, Victor Bazarov <v.Abazarov (AT) attAbi (DOT) com> wrote: Quote: "Ron Natalie" <ron (AT) sensor (DOT) com> wrote... "Yu Cao" <yucao (AT) alumnae (DOT) caltech.edu> wrote in message news:bdfvj6$pgq$1 (AT) naig (DOT) caltech.edu... Q2. For a POD type, is operator new[] allowed to take non-default initializers? Example: int* p = new int[100](1); // LINE AA What you wrote above is illegal for ANY type. Q3. For a POD type, should the following two statements both perform zero initialization: int *p = new int[100]; // LINE BB int *p = new int[100](); // LINE CC The latter is ILLEGAL. Really? How would you explain that it works in Comeau? Another "fucked-up Comeau extension"? Just so there is no confusion, I labelled the above statements. LINE BB and LINE CC are both valid, legal, and well-defined, and usually mean different things. LINE AA is not allowed. -- Greg Comeau/ 4.3.0.1: FULL CORE LANGUAGE, INCLUDING TC1 Comeau C/C++ ONLINE ==> http://www.comeaucomputing.com/tryitout World Class Compilers: Breathtaking C++, Amazing C99, Fabulous C90. Comeau C/C++ with Dinkumware's Libraries... Have you tried it? Back to top Victor Bazarov Guest Posted: Sat Jun 28, 2003 2:34 pm    Post subject: Re: new A v.s. new A() "Ron Natalie" <ron (AT) sensor (DOT) com> wrote... Quote: "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote The default constructor (whether user defined or not) is called in both cases like for a definition "A a;" right? If it's not defined ("trivial"), how would it be called? As far as the standard is concerend it behaves as if one was automatically inserted by the compiler. How can one verify that "as if"? There is no code to put the breakpoint or a debugging output statement, is there? That's what my question was about. Back to top Victor Bazarov Guest Posted: Sat Jun 28, 2003 2:40 pm    Post subject: Re: new A v.s. new A() "Greg Comeau" <comeau (AT) panix (DOT) com> wrote Quote: In article <vfoqcal6bqev1b (AT) corp (DOT) supernews.com>, Victor Bazarov <v.Abazarov (AT) attAbi (DOT) com> wrote: "Ron Natalie" <ron (AT) sensor (DOT) com> wrote... "Yu Cao" <yucao (AT) alumnae (DOT) caltech.edu> wrote in message news:bdfvj6$pgq$1 (AT) naig (DOT) caltech.edu... Q2. For a POD type, is operator new[] allowed to take non-default initializers? Example: int* p = new int[100](1); // LINE AA What you wrote above is illegal for ANY type. Q3. For a POD type, should the following two statements both perform zero initialization: int *p = new int[100]; // LINE BB int *p = new int[100](); // LINE CC The latter is ILLEGAL. Really? How would you explain that it works in Comeau? Another "fucked-up Comeau extension"? Just so there is no confusion, I labelled the above statements. LINE BB and LINE CC are both valid, legal, and well-defined, and usually mean different things. LINE AA is not allowed. The whole idea for my "Really" was to coerce Ron into looking it up in the Standard in an attempt to make him substantiate his claims. It has become customary here (in response to some newbies' "it works this way here so it's right" statements) to actually quote the Standard (not that it has the desired effect every time), so I would expect both Ron and you not to use "it's legal" or "it's ILLEGAL" without proof. And, mind you, *I* don't need that proof, most of the time. It's the other posters who read the forum I am concerned about. :-) Victor Back to top Ron Natalie Guest Posted: Mon Jun 30, 2003 1:46 pm    Post subject: Re: new A v.s. new A() "Victor Bazarov" <v.Abazarov (AT) attAbi (DOT) com> wrote Quote: How can one verify that "as if"? There is no code to put the breakpoint or a debugging output statement, is there? That's what my question was about. Debugging is not part of the language. The constructor is treated AS IF it exists. Whether the compiler emits code or calls it is an implementation detail. Read 12.1/5 and 12.1/7 Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.