const_cast member function pointer

[nick.keighley.zczc]

Yes I read the FAQ! I'm not using a typedef as they don't seem to work withtemplates in C++98.


template<typename WB>
void EditProgramWizard:opulateButton (MParameterButton* paramButton, WB* band,
int (WB::*read)(),
void (WB::*write)(int))
{
int (WB::*const_read)() = const_cast<int (WB::*)() const> (read);
PopulateButton (paramButton, band, const_read, write);
}

the cast fails with:

myprog.cpp: In member function ‘void EditProgramWizard:opulateButton(MParameterButton*, WB*, int (WB::*)(), void (WB::*)(int)) [with WB = WeightBand]’:
myprog.cpp:464:125: instantiated from here
myprog.cpp:1521:68: error: invalid use of const_cast with type ‘int (WeightBand::*)()const’, which is not a pointer, reference, nor a pointer-to-data-member type

I was under the impression ‘int (WeightBand::*)()const' *was* a pointer.

Oh. Are pointers to member functions not const castable?

 

[Alf P. Steinbach]

template<typename WB>
void EditProgramWizard:opulateButton (MParameterButton* paramButton, WB* band,
int (WB::*read)(),
void (WB::*write)(int))
{
int (WB::*const_read)() = const_cast<int (WB::*)() const> (read);
PopulateButton (paramButton, band, const_read, write);
}

the cast fails with:

myprog.cpp: In member function ‘void EditProgramWizard:opulateButton(MParameterButton*, WB*, int (WB::*)(), void (WB::*)(int)) [with WB = WeightBand]’:
myprog.cpp:464:125: instantiated from here
myprog.cpp:1521:68: error: invalid use of const_cast with type ‘int (WeightBand::*)()const’, which is not a pointer, reference, nor a pointer-to-data-member type

I was under the impression ‘int (WeightBand::*)()const' *was* a pointer.

You're adding 'const' to the function signature.

And no, a member pointer is not a pointer in the ordinary sense.

It's more like an offset, which needs to be combined with an object
pointer or reference (using ugly syntax) in order to be usable.

Oh. Are pointers to member functions not const castable?

Pointers yes, functions no.


Cheers & hth.,

- Alf

 

[nick.keighley.zczc]

template<typename WB>

void EditProgramWizard:opulateButton (MParameterButton* paramButton, WB* band,
int (WB::*read)(),
void (WB::*write)(int))
{

int (WB::*const_read)() = const_cast<int (WB::*)() const> (read);
PopulateButton (paramButton, band, const_read, write);
}

the cast fails with:

myprog.cpp: In member function ‘void EditProgramWizard:opulateButton(MParameterButton*, WB*, int (WB::*)(), void (WB::*)(int)) [with WB = WeightBand]’:

myprog.cpp:464:125: instantiated from here

myprog.cpp:1521:68: error: invalid use of const_cast with type ‘int (WeightBand::*)()const’, which is not a pointer, reference, nor a pointer-to-data-member type

as usual gcc is doing its best to tell me what I've done wrong.

so not a pointer...

You're adding 'const' to the function signature.

yes! I wanted to turn a 'int WB::read ()' into a 'int WB::read() const' as there's some const inconsistency in the bowels of the program

And no, a member pointer is not a pointer in the ordinary sense.
It's more like an offset, which needs to be combined with an object
pointer or reference (using ugly syntax) in order to be usable.

ah yes

Pointers yes, functions no.

Cheers & hth.,

yes.

My colleague suggested a reinterpret_cast. I guess this is digging the holeeven deeper?