Why does Qt use reinterpret_cast rather than static_cast for void*?

Question

You can cast to/from any pointer to T to/from void* with a static_cast, why does Qt use reinterpret_cast?

int SOME_OBJECT::qt_metacall(QMetaObject::Call _c, int _id, void **_a)
{
    _id = QMainWindow::qt_metacall(_c, _id, _a);
    if (_id < 0)
        return _id;
    if (_c == QMetaObject::InvokeMetaMethod) {
        switch (_id) {
        // Why reinterpret_cast here??
        case 0: on_tabWidget_tabCloseRequested((*reinterpret_cast< int(*)>(_a[1]))); break;
        default: ;
        }
        _id -= 1;
    }
    return _id;
}

Answer 1

Frankly, I've never been able to figure it out either. The void ** structure is created the same way, simply casts an int* to void* and then performs this weird cast on the other side. As far as I can tell, static_cast would not only be fine, it would be better.

You'll find that there's a lot of questionable code in large projects like Qt. Sometimes stuff slips through review or just sticks around because nobody wants to go through the hassle of changing it.

Answer 2

This is a bit old, but I'd disagree with the consensus. You should not be able to use static_cast to cast from any type to void* , you do that only with reinterpret_cast . static_cast is reserved for types that are supposedly compatible, some compile time checking is performed (ie derived classes, numeric types between them).

From this link MSDN :

dynamic_cast      Used for conversion of polymorphic types.
static_cast       Used for conversion of nonpolymorphic types.
const_cast        Used to remove the const, volatile, and __unaligned attributes.
reinterpret_cast  Used for simple reinterpretation of bits.