Re: problems with inheritance and protected attributes
* Lucas Kanebley Tavares:
On Sep 30, 3:23 am, "Alf P. Steinbach" <al...@start.no> wrote:
It would be nice if you posted the actual offending code, but happily
this "problem" is not uncommon, so it is reasonable to assume you have
something like
B::m( B* b )
{
A::m( b );
next->i = 666;
}
That will cause the compiler to complain, because while B can access
protected A features on B objects, and on objects of classes publicly
derived from B, if it could do it could access protected A features on A
objects it could also access protected A features on Z-objects, where Z
is a class I have derived from A.
So in that case "protected" wouldn't be any protection: you could access
protected A features of my Z instances simply by deriving a class from A.
So C++ does not allow that.
--- Compiler error
wwperiod.hpp: In member function 'virtual double
ZWPeriod::getMinimalCost()':
wwperiod.hpp:50: error: 'DATATABLE* WWPeriod::resTable' is protected
zwperiod.cpp:46: error: within this context
(and the same error for a lot of other attributes in the same
method).
--- Classes layout
class WWPeriod {
public:
virtual double getMinimalCost();
protected:
DATATABLE *resTable;
};
class ZWPeriod: public WWPeriod {
public:
virtual double getMinimalCost();
};
--- Line giving the presented error:
zwperiod.cpp:46: resTable[0].tempCost = prev->resTable[0].tempCost +
prev->holdCost;
---
I'm a reasonably good C programmer, but although I know C++ syntax,
I've never really had any serious projects in C++, so I'm
unexperienced.
Thank you for your reply, but my problem remains.
Any more suggestions for a n00b? :D
Only to reconsider the above earlier suggestion, which is now confirmed.
The (or at least one) problem is the attempted access via prev->, as I
surmised it would be. Why that is not permitted is explained above.
How to possibly fix it, explained above -- and perhaps also take a
look at the suggested redesign. In addition to the earlier advice, with
concrete code I can now make the suggestion to introduce public inline
const accessors tempCost() and holdCost() in the base class, and change
the naming convention for data members to e.g. myTempCost or tempCost_.
By the way, please don't top-post (rearranged, see the FAQ please), and
please don't quote signatures.
Cheers, & hth.,
- Alf
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?