dynamic_cast help

144

Author

September 12, 2004 05:31 AM

Object* a = new Object(40, 'a'); delete a; void* b = new Object(40, 'b'); a=dynamic_cast<Object*>b;//why can't i do this? delete a; Thank you!

Anonymous

September 12, 2004 05:49 AM

to do a dynamic cast from b to a
a has to be inherited from b and b has to have a virtual destructor

joanusdmentia

1,060

September 12, 2004 05:55 AM

Quote:Original post by Hermes
Object* a = new Object(40, 'a');
delete a;
void* b = new Object(40, 'b');
a=dynamic_cast<Object*>b;//why can't i do this?
delete a;

Thank you!

Firstly,
a=dynamic_cast<Object*>b
should be
a=dynamic_cast<Object*>(b)

Secondly, the compiler can't determine what type the object is because your providing it with a void*, which provides absolutely no information to the compiler about the type of the object. The compiler at least needs to know what class heirachy the object is part of. A better example of dynamic_cast would be

ObjectBase* b = new Object(40,'b');Object* a = dynamic_cast<Object*>(b);

"Voilà! In view, a humble vaudevillian veteran, cast vicariously as both victim and villain by the vicissitudes of Fate. This visage, no mere veneer of vanity, is a vestige of the vox populi, now vacant, vanished. However, this valorous visitation of a bygone vexation stands vivified, and has vowed to vanquish these venal and virulent vermin vanguarding vice and vouchsafing the violently vicious and voracious violation of volition. The only verdict is vengeance; a vendetta held as a votive, not in vain, for the value and veracity of such shall one day vindicate the vigilant and the virtuous. Verily, this vichyssoise of verbiage veers most verbose, so let me simply add that it's my very good honor to meet you and you may call me V.".....V

joanusdmentia

1,060

September 12, 2004 05:58 AM

Quote:Original post by Anonymous Poster
to do a dynamic cast from b to a
a has to be inherited from b and b has to have a virtual destructor

A virtual destructor isn't necessary for dynamic_cast, but it's a good idea none-the-less.

"Voilà! In view, a humble vaudevillian veteran, cast vicariously as both victim and villain by the vicissitudes of Fate. This visage, no mere veneer of vanity, is a vestige of the vox populi, now vacant, vanished. However, this valorous visitation of a bygone vexation stands vivified, and has vowed to vanquish these venal and virulent vermin vanguarding vice and vouchsafing the violently vicious and voracious violation of volition. The only verdict is vengeance; a vendetta held as a votive, not in vain, for the value and veracity of such shall one day vindicate the vigilant and the virtuous. Verily, this vichyssoise of verbiage veers most verbose, so let me simply add that it's my very good honor to meet you and you may call me V.".....V

snk_kid

1,312

September 12, 2004 07:11 AM

Quote:Original post by Hermes
Object* a = new Object(40, 'a');
delete a;
void* b = new Object(40, 'b');
a=dynamic_cast<Object*>b;//why can't i do this?
delete a;

i can't see why you really must need to do this but if you really do then dynamic_cast is not the one for you, dynamic_cast is used to navigate type hierarchies of related types, you wont a cast that lets you cast between un-related types which is reinterpret_cast e.g

Object* a = new Object(40, 'a');delete a;void* b = new Object(40, 'b');a= reinterpret_cast<Object*>(b);delete a;

Jolle

178

September 12, 2004 07:35 AM

Quote:Original post by joanusdmentia
Quote:Original post by Anonymous Poster
to do a dynamic cast from b to a
a has to be inherited from b and b has to have a virtual destructor

A virtual destructor isn't necessary for dynamic_cast, but it's a good idea none-the-less.

From what I've heard, lots of implementations uses the v-table to check on types and when using dynamic_cast, so you'll need at least one virtual function.. and as a general rule, when at least one function is virtual, the destructor should be virtual, too.

joanusdmentia

1,060

September 12, 2004 04:48 PM

Quote:Original post by Jolle
From what I've heard, lots of implementations uses the v-table to check on types and when using dynamic_cast, so you'll need at least one virtual function.

That could very well be true, it does sound familiar.

Quote:and as a general rule, when at least one function is virtual, the destructor should be virtual, too.

Yep, hence me mentioning that the virtual destructor was a good idea anyway. I was really just nit-picking [smile]

"Voilà! In view, a humble vaudevillian veteran, cast vicariously as both victim and villain by the vicissitudes of Fate. This visage, no mere veneer of vanity, is a vestige of the vox populi, now vacant, vanished. However, this valorous visitation of a bygone vexation stands vivified, and has vowed to vanquish these venal and virulent vermin vanguarding vice and vouchsafing the violently vicious and voracious violation of volition. The only verdict is vengeance; a vendetta held as a votive, not in vain, for the value and veracity of such shall one day vindicate the vigilant and the virtuous. Verily, this vichyssoise of verbiage veers most verbose, so let me simply add that it's my very good honor to meet you and you may call me V.".....V

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