[Solved]Dynamic cast to void pointer
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Hi
a.jafarabadi i have problem with casting.Hi ambershark
I am NOT PASSING 5 as void *.
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// Get default block from drawing
hBlock = axControl->dynamicCall("DrwGetFirstObject(int)", QVariant(5)).value<void *>();
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In above code actually i am passing 5 as parameter to "DrwGetFirstObject(int)" function which should be a QVariant, and then function suppose to return me a HANDLE (ie void *) of hBlock. It returns me
QVariant(int, 419171632) if i qdebug return value as it is. I want to store that value as a void * in hBlock hence i am using
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QVariant.value<void *>()
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method to cast into void *.
The problem is, after executing this line data in hBlock is 0x0 which is not expected.(expected is something like 0x419171632).http://qt-project.org/doc/qt-4.8/qvariant.html#value
http://qt-project.org/doc/qt-5/qaxbase.html#dynamicCallLet me know if still not clear and thanks for replay....
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For the sake of clarity, try separating the call to QAxBase::dynamicCall() from the call to QVariant::value(). Then verify that the QVariant is valid, at least for development purposes. Finally, cast it to a void pointer, and inspect it again.
QAxBase::dynamicCall() is documented as returning an invalid QVariant if the call fails, or if the called method doesn't return a value. Maybe something is going wrong prior to the cast.
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Hi jeremy_k
I have verified that QVariant is valid. qDebug prints this,
QVariant(int, 419171632).that means dynamic call is working, there is issue with casting.
So, i have a QVariant and i want to convert it to void *.
I am doing
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HANDLE hBlock = QVariant.value<void *>;
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where QVariant is QVariant(int, 419171632)
i get 0x0 in hBlock. -
Try a reinterpret_cast<void *>
@#include <cstdio>
#include <QVariant>int main(int argc, char *argv[])
{
QVariant var(0x12345678);
void * ptr1 = var.value<void *>();
void * ptr2 = reinterpret_cast<void *>(var.value<int>());
printf("%p %p\n", ptr1, ptr2);
return 0;
}@ -
Sorry about the 5 thing, I missed a parenthesis when I was reading the code. :)
Glad the reinterpret_cast worked for you though.
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Hi, ambershark
You should not sorry for that , actually i should sorry for posting some unreadable code.-
reinterpret_cast worked for me, but yesterday i was reading about it and found that its dangerous to use.
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If using QVariant.value<>() is a correct way to convert QVariant to any other type then why it is not working in my case?
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On many forums its recommended not to use reinterpret_cast, then what might be the other way in this case(QVariant to void *)?
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reinterpret_cast tells the compiler "trust me, I know what I'm doing". There is no other options (that I know of) for transforming to or from void *. Nothing inherits from void *, and it doesn't inherit from anything. There is no inherently meaningful way to convert it to a pointer to a valid object without risking converting it into a pointer to an invalid object.
Because of it's lack of meaning, arithmetic and dereferencing void * are also meaningless.
So no, if you don't understand what it is or how it can be used, staying away from both reinterpret_cast and void * is a good idea. The same caveat goes for C-style casting.
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I was slightly hasty dismissing static_cast<void *>. That works from a pointer to void *, and back again.
More detail: http://www.cplusplus.com/doc/tutorial/typecasting/
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I agree with what jeremy_k said. I use reinterpret_cast a bit in code. It can be dangerous because it has no type checking. When dealing with void *s though there is rarely any type checking and using reinterpret_cast is just fine. Like jeremy said it is basically telling the compiler you know what is in that data and want it cast to a certain type.
The "dangerous" part comes in if you are not paying attention as a programmer. If you know for sure your pointer is a valid one of class X then a reinterpret_cast is just fine.
If you think of it in C style it is the same as saying:
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void myVoid = (void)new MyClass;
MyClass *c = (MyClass *)myVoid;
@Both of those above examples are the same as reinterpret_cast's in c++. In fact you can use the C style casting method in C++ just fine.
