conrad@jupiter.ucsc.edu.UUCP (09/26/87)
Virtual constructors are not allowed. So if I need a a copy of a particular object which has derived classes, how do I do it? For example: rectangle, circle, line, etc. are derived classes of the base class *shape*. Now I want to make a copy of a given shape.
franka@mmintl.UUCP (Frank Adams) (09/30/87)
In article <895@saturn.ucsc.edu> conrad@jupiter.ucsc.edu (Al Conrad) writes: >Virtual constructors are not allowed. So if I need a a copy >of a particular object which has derived classes, how do I >do it? For example: rectangle, circle, line, etc. are derived >classes of the base class *shape*. Now I want to make a copy >of a given shape. class shape { virtual shape * copy(); } class rectangle : public shape { shape * copy() { rectangle * answer = new rectangle; [copy this]; return answer; } } If you really want a constructor, you can now add in class shape: shape(shape * original) {this = original->copy()}; Otherwise, you can just use the copy function. -- Frank Adams ihnp4!philabs!pwa-b!mmintl!franka Ashton-Tate 52 Oakland Ave North E. Hartford, CT 06108
bs@alice.UUCP (10/01/87)
# From: conrad@jupiter.ucsc.edu.UUCP (conrad @ University of California, Santa Cruz; CIS/CE) # Virtual constructors are not allowed. So if I need a a copy # of a particular object which has derived classes, how do I # do it? For example: rectangle, circle, line, etc. are derived # classes of the base class *shape*. Now I want to make a copy # of a given shape. I think you are just thinking about it a little bit wrong. YOU cannot make a copy of something that you don't know the exact type of, but with a little bit forethought you can ask IT to copy itself; after all IT (and nobody else) knows exactly what kind of shape it is (provided it has suitable virtual functions): class shape { // ... virtual shape* copy(); }; class circle : public shape { // ... shape* copy() { return new circle(*this); } }; class rectangle : public shape { // ... shape* copy() { return new rectangle(*this); } }; my_program(shape* p) { shape* p2 = p->copy(); // make a copy of p // ... }
coatta@ubc-csgrads.uucp (Terry Coatta) (01/21/88)
There have been a couple of messages about virtual destructors recently. I have two questions. Can someone post (re-post) some example code which uses virtual destructors and is there such a thing as virtual constructors? Terry Coatta Dept. of Computer Science, UBC, Vancouver BC, Canada coatta@grads.cs.ubc.cdn `What I lack in intelligence, I more than compensate for with stupidity'
mikem@otc.oz (Mike Mowbray) (02/04/88)
In article <1788@ubc-cs.UUCP>, coatta@ubc-csgrads.uucp (Terry Coatta) says: > Can someone post (re-post) some example code which uses virtual destructors Here's a sketch of a library we use internally here. (Apologies if some of the code isn't exactly right - it's just intended to convey the idea.) Consider a linked list class where everything on the list is derived from a "Dlink" class which has a virtual destructor: class Dlink { friend class Dlist; void unlink(); // tidy up on either side protected: Dlink *prev, *next; Dlink(); // set prev and next to null, say virtual ~Dlink(); // call unlink() }; void Dlink::unlink() { if (prev) prev->next = next; if (next) next->prev = prev; next=prev=NULL; } Dlink::~Dlink() { unlink(); } //------------------------------------------ class Item1 : public Dlink { // ..... public: Item1(); ~Item1(); // do some other stuff to clean up class Item2 : public Dlink { // ..... public: Item2(); ~Item2(); // do some other stuff to clean up }; //------------------------------------------- class Dlist { Dlink key; public: Dlist() { key.prev = key.next = &key; } ~Dlist(); // Call the destructor for every Dlink on list void put(Dlink *d); }; void Dlist::put(Dlink *d) { d->unlink(); // for safety d->prev = key->prev; d->next = &key; key->prev->next = d; key->prev = d; } Dlist::~Dlist() { while (key.next != &key) delete key.next; } //---------------------------------------------- Then you can put install Item1's and Item2's on a Dlist, and the correct destructors will be called when the Dlist is deleted. > and is there such a thing as virtual constructors? This would make no sense. A thing can't be created if one doesn't know exactly (at compile time) what one wants to create. Mike Mowbray Systems Development |||| OTC || ACSnet: mikem@otc.oz UUCP: {uunet,mcvax}!otc.oz!mikem Phone: (02) 287-4104 Snail: GPO Box 7000, Sydney 2001, Australia