C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Call a constructor within a constructor of the same class          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Katja Casutt Guest Posted: Sat Sep 27, 2003 1:08 am    Post subject: Call a constructor within a constructor of the same class Hi I have a class having to constructors: // default constructor MyClass::MyClass() : myMember1(0), myMember2(false) {} // second constructor MyClass::MyClass(int myMember3) : myMember3(0) {} Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} But it did not work. How can I call the default constructor? Thanks in adavnce. BTW: For people knowing Java: I search the C++ call for "this();" Katja [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dirk Feytons Guest Posted: Sat Sep 27, 2003 2:12 pm    Post subject: Re: Call a constructor within a constructor of the same clas Katja Casutt wrote: [snip code] Quote: Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} But it did not work. How can I call the default constructor? Simple answer: you can't. Solution: copy&paste the c'tor initializer list from the default c'tor to the other c'tor. If you really want to avoid the redundancy in the source code you can factor out that code in a private member function and call it in the body of the c'tor. Note that isn't always possible; if myMember1 is const for instance, you can only initialize it by means of the c'tor initializer list. A more knowledgeable person will have to provide the exact technical details on why C++ doesn't allow this syntax. An intuitive explanation could be that a c'tor is a special function with object construction as its sole purpose. It's not like an ordinary function that you can call whenever you want. -- Dirk (PGP keyID: 0x448BC5DD - http://www.gnupg.org - http://www.pgp.com) ..oO° "You got a pea on your head dear." -- Thumb Wars °Oo. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Sat Sep 27, 2003 2:19 pm    Post subject: Re: Call a constructor within a constructor of the same clas Katja Casutt wrote: Quote: Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} But it did not work. How can I call the default constructor? You can't. Anyway, don't treat ctors/dtors as functions, they are something different and special. I personally don't say that they get called but that they get invoked, to make the difference clear, but that is just my personal taste. In case you just want to share the init-code, one common way is to move it to a common init() method that gets called by both ctors. In order to Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Sat Sep 27, 2003 2:26 pm    Post subject: Re: Call a constructor within a constructor of the same clas In article <e343edc3.0309252322.480eb0fe (AT) posting (DOT) google.com>, Katja Casutt <casutt (AT) imt (DOT) ch> writes Quote: Hi I have a class having to constructors: // default constructor MyClass::MyClass() : myMember1(0), myMember2(false) {} // second constructor MyClass::MyClass(int myMember3) : myMember3(0) {} Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} But it did not work. How can I call the default constructor? Thanks in adavnce. BTW: For people knowing Java: I search the C++ call for "this();" In C++ you cannot. This may change in the future as I have a proposal for change for the next release of the C++ Standard. -- Francis Glassborow ACCU If you are not using up-to-date virus protection you should not be reading this. Viruses do not just hurt the infected but the hole community. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dhruv Guest Posted: Sun Sep 28, 2003 1:29 am    Post subject: Re: Call a constructor within a constructor of the same clas On Fri, 26 Sep 2003 21:08:18 -0400, Katja Casutt wrote: Quote: Hi I have a class having to constructors: // default constructor MyClass::MyClass() : myMember1(0), myMember2(false) {} // second constructor MyClass::MyClass(int myMember3) : myMember3(0) {} Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} You can try this, but many ppl will complain about this, because it may not be standard compliant. What you do here is you first destroy the current object, and the reconstruct at the same memory location. MyClass::MyClass(int myMember3) { MyClass *temp = this; this->~MyClass(); //Destroying possibly unconatructed object. new (temp) MyClass; //Any other initialization goes here. } But why would you want to do this anyways? Regards, -Dhruv. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Aaron Bentley Guest Posted: Sun Sep 28, 2003 1:55 am    Post subject: Re: Call a constructor within a constructor of the same clas Dirk Feytons wrote: Quote: Katja Casutt wrote: [snip] How can I call the default constructor? Simple answer: you can't. [snip] A more knowledgeable person will have to provide the exact technical details on why C++ doesn't allow this syntax. An intuitive explanation could be that a c'tor is a special function with object construction as its sole purpose. It's not like an ordinary function that you can call whenever you want. There doesn't seem to be a technical reason not to-- the Standard will likely be revised so you can do this. Aaron -- Aaron Bentley www.aaronbentley.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nikolai Borissov Guest Posted: Sun Sep 28, 2003 2:02 am    Post subject: Re: Call a constructor within a constructor of the same clas Katja Casutt wrote: Quote: I have a class having to constructors: // default constructor MyClass::MyClass() : myMember1(0), myMember2(false) {} // second constructor MyClass::MyClass(int myMember3) : myMember3(0) {} SNIP How can I call the default constructor? You can use placement new operator to achieve the effect of constructor call, as it is shown in the example below. Note, that I added initialization of myMember3 in the default constructor, otherwise it is left uninitialized when we create an object through the default constructor. #include <new> class MyClass { public: MyClass():myMember1(0),myMember2(false),myMember3(0){} MyClass(int myMember3):myMember3((new(this)MyClass,myMember3)){} private: int myMember1; bool myMember2; int myMember3; }; Nikolai Borissov [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jakob Bieling Guest Posted: Sun Sep 28, 2003 10:08 pm    Post subject: Re: Call a constructor within a constructor of the same clas "Katja Casutt" <casutt (AT) imt (DOT) ch> wrote Quote: Hi I have a class having to constructors: // default constructor MyClass::MyClass() : myMember1(0), myMember2(false) {} // second constructor MyClass::MyClass(int myMember3) : myMember3(0) {} Now, I'd like to call the default constructor in the second constructor. I tried: // second constructor MyClass::MyClass(int myMember3) : MyClass(), myMember3(0) {} But it did not work. How can I call the default constructor? You don't. Quote: Thanks in adavnce. BTW: For people knowing Java: I search the C++ call for "this();" In C++ you cannot call c'tors. In that sense they are no functions. Create an 'init' function that does the construction part that is common to all c'tors and call that from your c'tors. hth -- jb (replace y with x if you want to reply by e-mail) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ingolf Steinbach Guest Posted: Tue Sep 30, 2003 6:43 pm    Post subject: Re: Call a constructor within a constructor of the same clas Nikolai Borissov wrote: Quote: #include class MyClass { public: MyClass():myMember1(0),myMember2(false),myMember3(0){} MyClass(int myMember3):myMember3((new(this)MyClass,myMember3)){} private: int myMember1; bool myMember2; int myMember3; }; IMO, you can easily run into various kinds of trouble with this. For instance: class BigArray { public: BigArray(); // allocates some megabytes on the heap ~BigArray(); // release the heap memory // ... }; class MyClass { public: MyClass() : myMember1(42), myMember2(7) {}; MyClass(int i); // calls MyClass default ctor directly private: BigArray array; int myMember1; int myMember2; }; Here you would probably have a memory leak as array's ctor would have been called twice and the heap memory allocation from the first BigArray ctor call would be lost. Kind regards Ingolf -- Ingolf Steinbach Jena-Optronik GmbH [email]ingolf.steinbach (AT) jena-optronik (DOT) de[/email] ++49 3641 200-147 PGP: 0x7B3B5661 213C 828E 0C92 16B5 05D0 4D5B A324 EC04 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nikolai Borissov Guest Posted: Wed Oct 01, 2003 8:29 am    Post subject: Re: Call a constructor within a constructor of the same clas Ingolf Steinbach wrote: Quote: IMO, you can easily run into various kinds of trouble with this. For instance: class BigArray { public: BigArray(); // allocates some megabytes on the heap ~BigArray(); // release the heap memory // ... }; class MyClass { public: MyClass() : myMember1(42), myMember2(7) {}; MyClass(int i); // calls MyClass default ctor directly private: BigArray array; int myMember1; int myMember2; }; Here you would probably have a memory leak as array's ctor would have been called twice and the heap memory allocation from the first BigArray ctor call would be lost. You are right, though this is not a point here. If constructor call was implemented in C++ explicitly there still would be the same problem. The point here is about a possibility to implement a functionality of a constructor call through the placement new operator. And because this "call" can be made from anywhere, there is no way to take all possible situations into consideration and generalize. It's entirely up to the programmer to watch for data integrity of an object being manipulated. I know that many people don't like this technique even though it is legal and non-ambiguous, because construction process is nothing more than: first step - initialization of members in the order of their declaration; second step - execution of a constructor code on initialized memory; In your example, indeed, there will be a memory leak. It's still possible to avoid a leak by calling BigArray object destructor at some point, but this will result in allocation-deallocation-new_allocation sequence, that may lead to wasted cycles (unless optimized out by compiler). Here, specific design of BigArray is not particularly convenient for a constructor call from another constructor. In another situations, however, it may be necessary to reconstruct the existing object on-the-fly. That's when a constructor call comes handy. There might be a special Reconstruct method for this purpose that destructs the object properly first and then calls one of the constructors. Cheers, Nikolai Borissov [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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