confusing error

MScottM

I'm trying to use a function that takes a QByteArray* and returns QInt32. The IDE has an error message: "'this' argument to member function 'byteArrayToUint32' has type 'const MainWindow', but function is not marked const".

Marking the function (or anything else for that matter) as const doesn't change anything, so I'm wondering what the actual issue might be.

Here are my declarations in MainWindow.h:

QByteArray *ba;
qint32 byteArrayToUint32(QByteArray &bytes);

and where I call the function in MainWindow.cpp:

QByteArray fbytes;
ba->resize(query.size());
fbytes = byteArrayToUint32(ba);  // <- error is marked here

and the function in MainWindow.cpp:

quint32 byteArrayToUint32(QByteArray &bytes)
{
    auto count = bytes.size();
    if (count == 0 || count > 4) {
        return 0;
    }
    quint32 number = 0U;
    for (int i = 0; i < count; ++i) {
        auto b = static_cast<quint32>(bytes[count - 1 - i]);
        number += static_cast<quint32>(b << (8 * i));
    }
    return number;
}

Cobra91151

@MScottM

Hello!

I would suggest you use the QByteArray on the stack.

Code:

.h

QByteArray ba;

.cpp

QByteArray fbytes;
ba.resize(query.size());
fbytes.append(byteArrayToUint32(ba));

Test it and reply.

Christian Ehrlicher

@MScottM said in confusing error:

QByteArray fbytes;
ba->resize(query.size());
fbytes = byteArrayToUint32(ba); // <- error is marked here

fbytes is a QByteArray, byteArrayToUint32() returns an quint32
ba is a pointer, byteArrayToUint32 takes a reference
ba gets resized but never initialized, you access the data of this container (when it would compile) inside byteArrayToUint32() so you will get garbage.

MScottM

@Christian-Ehrlicher this is where my lack of formal training is shining through... but, thanks to your reply, I have figured out I am going about what I am trying to do in the wrong way. I'm going to have to redo this bit of code somehow.

Marking this solved - even though it's more like...a lesson learned (maybe).

SimonSchroeder

@MScottM said in confusing error:

The IDE has an error message: "'this' argument to member function 'byteArrayToUint32' has type 'const MainWindow', but function is not marked const".

Here is another lesson to learn: The error tells us that byteArrayToUint32 is a member function of MainWindow. And the error also tells us that you are calling this function from within another function that is marked as const. For member functions everything after the argument list applies to the implicit this pointer in member functions. And even though you are not writing it explicitly you are actually calling this->byteArrayToUint32(ba). If the this pointer is const it can only call member functions that are also marked as const. So, the easy solution is to declare your function const: quint32 byteArrayToUint32(QByteArray &bytes) const;

However, your conversion function actually does not need the this pointer in any way. You can either have it as a free-standing function (declared outside of your class). This might pollute your global namespace unnecessarily. Another approach is to mark it static. The latter would be the general approach I'd prefer.