Is there a possibility to set a QBitArray using a long int?

[Kevin470]

Hello,
I have a long int variable and I have a QBitArray of size(32).
Is it possible to write the long int value to the QBitArray and read individual bits?
Or is there any other better option in Qt to read bits of an integer?

Thanks again!

[Christian Ehrlicher]

Qt5 or Qt6?

https://doc.qt.io/qt-6/qbitarray.html#toUInt32

[Kevin470]

@Christian-Ehrlicher Qt6.
But I am looking for fromUInt32 and not toUInt32 unfortunately.

[Christian Ehrlicher]

Then use https://doc.qt.io/qt-6/qbitarray.html#fromBits

[Kevin470]

@Christian-Ehrlicher
Could you help me out with an example please?
I tried

    QBitArray bitArray;
    quint32 intvalue = 9999999;

    bitArray.fromBits(intvalue,32);
    qInfo() << "Size:" << bitArray.size();

and it throws an error saying main.cpp:12:23: Cannot initialize a parameter of type 'const char *' with an lvalue of type 'quint32' (aka 'unsigned int')

If I try the one below, the program crashes.

    QBitArray bitArray;
    quint32 intvalue = 9999999;

    bitArray.fromBits((char*)(intvalue),32);
    qInfo() << "Size:" << bitArray.size();

[jsulm]

@Kevin470 said in Is there a possibility to set a QBitArray using a long int?:

bitArray.fromBits((char*)(intvalue),32);

Shouldn't it be

bitArray.fromBits((char*)(&intvalue), 32);

?

[Kevin470]

@jsulm Oops sorry. Yes it should be bitArray.fromBits((char*)(&intvalue), 32);

But it still prints Size: 0

[jsulm]

@Kevin470 said in Is there a possibility to set a QBitArray using a long int?:

But it still prints Size: 0

Yes, because https://doc.qt.io/qt-6/qbitarray.html#fromBits is a static method...

[Kevin470]

@jsulm Thanks a lot. Works Perfectly!

QBitArray bitArray;
    quint32 intvalue = 9999999;

    bitArray = QBitArray::fromBits((char*)(&intvalue),32);
    qInfo() << "Size:" << bitArray.size();

[Christian Ehrlicher]

@Kevin470 Then please mark this topic as solved, thx.

[J.Hilk]

where's the downvote button!?!?! c-style casts 🫢

[Pl45m4]

@J-Hilk said in Is there a possibility to set a QBitArray using a long int?:

where's the downvote button

Casted to (void)downvoteBtn;

:)

[JonB]

@J-Hilk
I always have to go look it up, should this one be reinterpret_cast<>, or has C++ introduced yet another new one of these casting templates to use for this case?

Not to mention that the other day @Chris-Kawa wrote in https://forum.qt.io/topic/139176/qgraphicsitem-conversion-into-long-using-reinterpret_cast/2

I'll just mention that on some platforms pointers to different types can have different sizes too i.e. sizeof(char*) != sizeof(int*), so, although unlikely that it affects you, what you're doing is a risky business.

which seems to me to make a nonsense of half of these castings anyway...!

[J.Hilk]

@JonB

• static_cast for basic types
• dynamic_cast for pointers or references to polymorphic class types;
• const_cast const or volatile qualifiers;
• reinterpret_cast is for special circumstances and you think you know better than the compiler. Converting between pointers or references and completely unrelated types.

[Christian Ehrlicher]

@JonB said in Is there a possibility to set a QBitArray using a long int?:

which seems to me to make a nonsense of half of these castings anyway...!

They add validation if the cast is valid which is not available with the c-style cast.

[JonB]

@J-Hilk said in Is there a possibility to set a QBitArray using a long int?:

reinterpret_cast is for special circumstances and you think you know better than the compiler. Converting between pointers or references and completely unrelated types.

And that (reinterpret_cast<>) is what should be used in the case we are discussing (int * -> char *), right?

[Separate: did I see recently that C++20 or whatever has introduced yet another new somethingelse_cast<> construct beyond these 4, or do you think I'm imagining this?]

@Christian-Ehrlicher said in Is there a possibility to set a QBitArray using a long int?:

They add validation if the cast is valid which is not available with the c-style cast.

But not in the reinterpret_cast<> case which (I believe) we are going to use here, right?

[J.Hilk]

@JonB said in Is there a possibility to set a QBitArray using a long int?:

[Separate: did I see recently that C++20 or whatever has introduced yet another new somethingelse_cast<> construct beyond these 4, or do you think I'm imagining this?]

std::bit_cast

but, it's a library feature not a language feature.

Technically, if the OP uses c++20 it would be the "saver" option, otherwise reinterpret_cast