Qt analog to C struct?

mzimmers

I'm probably being too fussy, but given how easily one can assign/copy QByteArray and QString objects, it's a shame IMO that there's no way to aggregate them as in a C struct. But it's probably their very flexibility that makes such use unfeasible.

It's OK...I'm content to use memcpy_s, but it's just not attractive code.

JonB

@Wieland said in Qt analog to C struct?:

Yes, automatic range checking,

You said this in response to why std::array should be used/preferred. What "automatic range checking" are you saying it provides, under what circumstances?

kshegunov

@mzimmers said in Qt analog to C struct?:

I'm probably being too fussy,

Not really, but it would help to know what you dislike about the above example and why you think you need to use memcpy?

@JNBarchan said in Qt analog to C struct?:

What "automatic range checking" are you saying it provides, under what circumstances?

std::array<int, 3> x;
int z = x[3]; //< Regular arrays allow this (generally)
x[3] = 0;     //< Regular arrays mostly allow this too

e.g. run this through your debugger:

int main(int argc, char ** argv)
{
    int z[2];
    z[2] = 0;
}

JonB

@kshegunov said in Qt analog to C struct?:

@JNBarchan said in Qt analog to C struct?:
What "automatic range checking" are you saying it provides, under what circumstances?
std::array<int, 3> x;
int z = x[3]; //< Regular arrays allow this (generally)
x[3] = 0;     //< Regular arrays mostly allow this too

I'm not understanding you. If you are saying with std::array<int, 3> x; then x[3] will be range checked, then I say it will not (that's why I asked)! Only x.at(3) will be. See docs. If your x[3] is range checked, that is a compiler-specific behaviour and/or you have to pass flags to compiler to ask it to do that.

kshegunov

@JNBarchan said in Qt analog to C struct?:

I say it will not

Indeed, I'm in error.
PS.
My STL (g++ 7.2.x) doesn't even have an assertion there ... :|
.at will throw an exception if out of bounds on the other hand.

JonB

@kshegunov said in Qt analog to C struct?:

PS.
My STL (g++ 7.2.x) doesn't even have an assertion there ... :|
.at will throw an exception if out of bounds on the other hand.

Yup! But some places suggest "Enable the flag _GLIBCXX_DEBUG to do bounds checking on STL containers", though others claim that was still not the case for std::array. Might depend on compiler release. Also see https://stackoverflow.com/a/1290488/489865 for how Visual C++ does have explicit range checking, and https://stackoverflow.com/a/8375312/489865 using __glibcxx_check_subscript(), which you might put into your own code without tampering with the supplied libraries!

kshegunov

Well, as I mentioned in my ~~first~~ third post I'm not big on the new standards. Honestly, if it were me, I'd use the regular old statically sized arrays and leave the compiler to generate the copy constructor. Always has worked for me, and I don't see any good reason to use templates endlessly and unnecessarily, but that's me, the standards committee doesn't agree ...

JonB

@kshegunov
Again, agree with you totally!

mzimmers

@kshegunov I'm using C structs and memcpy_s because in my mind it's simpler to use and understand than the C++ STL for arrays. For example:

struct
{
    ...
    uint8_t iv[16];
    ...
}
...
memcpy_s(msgHeader.iv, sizeof(msgHeader.iv), enc->getIv(), sizeof(msgHeader.iv));

is admittedly unattractive, but it works. Consider the alternative:

struct
{
    ...
    std::array<uint8_t, 16> iv;
}
...
std::copy(enc->getIv().begin(), enc->getIv().end(), msgHeader.iv.begin());

This doesn't work (it causes a memory problem that results in a segmentation fault). The routine getIv is of type QByteArray.

kshegunov

@mzimmers said in Qt analog to C struct?:

is admittedly unattractive, but it works.

What is enc->getIv()? Is it how you read that part of the header from the device?
You could for example do something like this:

struct
{
    ...
    uint8_t iv[16];
    ...
} msgHeader;

QByteArray iface = QByteArray::fromRawData(reinterpret_cast<const char *>(&msgHeader), sizeof(msgHeader));

And then you can write and read directly into/from the byte array (on both sides when serializing/deserializing). This may require you to pack the structure however so you get sensible things from the reinterpret cast and it byte-order should be considered. Another thing, which is what I usually prefer is just to provide streaming from/to a data stream for the struct through
the << and >> operators. Then it's cleaner as you don't have any memcopy stuff outside of the struct/relevant functions. E.g.:

struct X
{
    char a[10], b[5];
};

QDataStream & operator << (QDataStream & out, const X & x)
{
    out.writeRawData(x.a, sizeof(X::a));
    out.writeRawData(x.b, sizeof(X::b));
    return out;
}

QDataStream & operator >> (QDataStream & in, X & x)
{
    if (in.readRawData(x.a, sizeof(X::a)) != sizeof(X::a) || in.readRawData(x.b, sizeof(X::b)) != sizeof(X::b))
        in.setStatus(QDataStream::ReadCorruptData);
    return in;
}

In this case you can stream the whole structure in one go from the binary representation:

QByteArray data; //< However this comes to you, if it's a raw C-array then attach a the byte array with `QByteArray::fromRawData`

QDataStream in(&data, QIODevice::ReadOnly);

X msgHandler;
in >> msgHandler;

if (in.status() != QDataStream::Ok)
    ; // Handle the error

This is one possibility only, there are other options as well.

mzimmers

@kshegunov thanks for the detailed reply. Your post made me aware of two things I hadn't ever noticed before: the fromRawData method, and the reinterpret_cast.

In answer to your question, getIv is just a convenience function to return a QByteArray containing a member object. The actual member is an array of 4 32-bit unsigned integers.

QByteArray Encryption::getIv()
{
    QByteArray a = QByteArray((char *) m_iv, sizeof(m_iv));
    return a;
}

kshegunov

The reinterpret_cast is generally unsafe, so I'd advise against using it, especially if you have not taken into account all relevant peculiarities of the memory layout (byte-order, alignment and padding). The second part is my recommended approach and what I do in my own programs - clean and efficient enough.