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IWETHEY Home / IWETHEY Board / Theory and Practice of Programming Forum / Which brings up another question.
Post #265,006
by ChrisR
8/15/06 8:25:15 PM
Reply
New Which brings up another question.
I've seen the "const = 0" in the design patterns book. What does the " = 0" part imply? I'm assuming that since we are talking C++, this is basically setting the function to null?
Post #265,012
by tuberculosis
Picture of tuberculosis
8/15/06 9:01:04 PM
Reply
New Pure virtual functions
The const is a red herring here. You can have it, or not.

This is implementation detail leaking into the syntax.

If you have an abstract base class and you want to specify required protocol but don't happen to have a sensible default implementation in mind, you define a pure virtual function like this:

[code]
class F
{
public:
virtual void f() = 0;
};
[/code]

which essentially tells the compiler to put a null value into the vtable entry (array of function pointers used for dynamic dispatch) for this particular function at this level of abstraction. It also implies that this class is abstract and may not be instantiated because it is illegal to instance a class with a pure virtual function.



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     C++ question on const - (ChrisR) - (24) - Aug. 14, 2006, 10:10:50 AM EDT
         Don't change the object, I believe - (JayMehaffey) - (2) - Aug. 14, 2006, 11:05:16 AM EDT
             Doh. - (ChrisR) - (1) - Aug. 15, 2006, 12:27:00 PM EDT
                 More than that... - (jb4) - Aug. 15, 2006, 04:05:02 PM EDT
         Re: C++ question on const - (tuberculosis) - (20) - Aug. 14, 2006, 11:22:39 PM EDT
             I still don't understand... - (ChrisR) - (19) - Aug. 15, 2006, 10:59:52 AM EDT
                 Const is not always forced - (JayMehaffey) - Aug. 15, 2006, 11:23:16 AM EDT
                 Its counterintuitive - (tuberculosis) - (14) - Aug. 15, 2006, 01:03:04 PM EDT
                     Say WHAT?!? - (jb4) - (13) - Aug. 15, 2006, 04:30:02 PM EDT
                         You can overload on const - (tuberculosis) - (11) - Aug. 15, 2006, 04:47:41 PM EDT
                             Why the hell would it do / allow that? - (drewk) - (1) - Aug. 15, 2006, 04:45:34 PM EDT
                                 Bell labs has an unlimited crack budget - (tuberculosis) - Aug. 15, 2006, 04:57:05 PM EDT
                             The easiness to keep track of is not the issue - (jb4) - (8) - Aug. 15, 2006, 05:49:16 PM EDT
                                 Huh? - (tuberculosis) - (7) - Aug. 15, 2006, 06:51:19 PM EDT
                                     No... - (jb4) - (6) - Aug. 16, 2006, 10:12:37 AM EDT
                                         I think you're talking past each other... - (admin) - Aug. 16, 2006, 11:11:53 AM EDT
                                         And you specify that by.... - (tuberculosis) - (4) - Aug. 16, 2006, 11:48:14 AM EDT
                                             We'll leave it at that. - (jb4) - (3) - Aug. 16, 2006, 12:32:21 PM EDT
                                                 I wasn't trying to be sarcastic - (tuberculosis) - (2) - Aug. 16, 2006, 01:27:58 PM EDT
                                                     No need to specify OO in this case - (JayMehaffey) - Aug. 16, 2006, 01:35:32 PM EDT
                                                     What does the type of the reference have to do with anything - (jb4) - Aug. 16, 2006, 03:06:42 PM EDT
                         Nominating this for LRPD (new thread) - (lincoln) - Aug. 18, 2006, 03:18:45 PM EDT
                 Re: I still don't understand... - (jb4) - (2) - Aug. 15, 2006, 04:16:40 PM EDT
                     Which brings up another question. - (ChrisR) - (1) - Aug. 15, 2006, 08:25:15 PM EDT
                         Pure virtual functions - (tuberculosis) - Aug. 15, 2006, 09:01:04 PM EDT

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