Re: curve Symbol

[Hendrik V. <HVe...@gm...>]

Hi Gianni,

my 0.02$ worth:  If I remember correctly, the idea was that a) QwtPlotCurve 
does not call any non-const functions (hence it gets a pointer to const 
QwtSymbol in setSymbol() -- oops, edit:  just saw that in 6.1.0, the const was 
dropped apparently?) and b) it has no way of knowing if the symbol changed, 
just like in your case:  you change your one property but the curve does not 
get noticed and hence cannot react (like trigger an auto-replot and such 
stuff).  But since the curve only gets a pointer to const, it also can only 
give you a pointer to const back (again, after writing, I just saw that 
setSymbol() was changed in 6.1.0)...

So, yes, I guess, as it is implemented now, you need some sort of a "clone" 
function.  And I would guess that's the reason why the copy constructor is 
private:  you are encouraged to subclass QwtSymbol, but in the case of a 
subclass, the copy constructor just won't suffice (assignment is still 
available, though, but that would need to be called explicitly -- probably, 
that's the intention:  preventing you from "accidentally" calling the copy 
constructor) and you would really need some clone function, such that the 
following would work (MySymbol::clone :

MySymbol *symbol = MySymbol::clone(curve->symbol()) ;
symbol->doMyChanges(...) ;
curve->setSymbol(symbol) ;

Ok, this may be cumbersome if you only use the default implementation and 
don't actually have a custom symbol class.  In that case, using the assignment 
operator is probably your best choice...
Again, just my idea of what should be done; Uwe will certainly have the better 
answer ;)

Best,

Hendrik


On Monday, August 05, 2013 15:16:51 Gianni Ambrosio wrote:
> Dear Uwe,
> I moved from Qwt 6.0.1 to Qwt 6.1.0 and I fonud some changes about plot
> symbol.
> Basically I need to change just one parameter of a plotcurve symbol but
> can just get a "const" QwtSymbol* from a QwtPlotCurve. I could barely
> const cast it, anyway, since setSymbol() implementation of QwtPlotCurve
> would delete the previos symbol I think the only way to change just one
> feature of an existing symbol is to create a new one and "manually" copy
> style, brush, pen and size. In fact also the QwtSymbol copy constructor
> is now private.
> Maybe I'm worng or there's probably a reason, but it seems, in this
> case, Qwt API is not so simple.
> 
> Regards
> Gianni
> 
> 
> 
> 
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