reinterpret_cast casts away qualifiers

Question

I add an issue on reinterpreting a variable and I don't know why..

int ProgressBar(const uint64_t data_sent, const uint64_t data_total, void const *const data)
{
    Dialog *dialog = reinterpret_cast<Dialog*> (data);
    dialog->setValue((data_sent *100) / data_total);
}

the reinterpret_cast seems not allowed and say

reinterpret_cast from 'const void *) to Dialog * casts away qualifiers

Any idea

Solution 1

As Nick Strupat stated in comment,

reinterpret_cast can't cast away cv-qualifiers

So you can use reinterpret_cast and const_cast together.

Dialog *dialog = const_cast<Dialog*>(reinterpret_cast<const Dialog *>(data));

Solution 2

You need to also use a const_cast to remove const qualifiers. Also, casting from void * can use static_cast, it does not need to reinterpret. For example:

Dialog const *dialog = static_cast<Dialog const *>(data);
Dialog *d2 = const_cast<Dialog *>(dialog);

However , make sure that the Dialog is actually not a const object; attempting to modify a const object (presumably setValue does this) causes undefined behaviour.

I'd suggest rethinking the interface to ProgressBar to avoid needing this cast.

Comments

[Nick Strupat]

reinterpret_cast can't cast away cv-qualifiers

[Ben Voigt]

@Nick: The last const you put in there has no effect.

[templatetypedef]

Um, why are you trying to get rid of the const here? That sounds unsafe.