Qt analog to C struct?

mzimmers

I'm not sure I understand...what should I use std::array for exactly?

kshegunov

std::array is a thin wrapper (template) around a statically sized array of a given type. So you could use it instead of int[], for example:

struct
{
    // ...
    std::array<uint8_t, 23> timestamp;
    std::array<uint8_t, 1> reserved;
} msgHeader;

? Offline

As a replacement for the QByteArrays and the C-style array:

struct MsgHeader
{
    std::array<uint8_t, 32> hmac = {};
    // ...
    std::array<uint8_t, 23> timestamp = {};
};

mzimmers

@Wieland OK. What is the advantage of this -- automatic range checking or some other safeties?

kshegunov

yes, also somewhat more type-safe as it does not decay readily to void *, is a C++ object ... and, well, the cool kids are using it (not that I approve, but that's whole other rant) ... ;)

? Offline

Yes, automatic range checking, increased safety against unintentional pointer mistakes, and compared to QByteArray, increased performance.

mzimmers

OK, thank you both.

? Offline

Lot of cross-posting going on here ^_^

kshegunov

Twice in less than 10 minutes, must be a forum record or something.

mzimmers

Just to pursue this a little further, if I do use the std::array template, can I still do a memcpy, or do I have to assign the elements individually? I couldn't see anything in the cPPreference page on this.

Also, would it be better just to create one large QByteArray and use pointers to access delimited areas within it? (I'm still a little surprised that Qt doesn't have some construct to handle this).

? Offline

@mzimmers said in Qt analog to C struct?:

can I still do a memcpy

The correct way to copy the array is to use std::copy. It's up to the implementation of your standard library to then find the fastest way to copy your data.

kshegunov

@mzimmers said in Qt analog to C struct?:

if I do use the std::array template, can I still do a memcpy

Yes, std::array is an aggregate (means it has only one member that is your regular type[] array). Although(!), you should copy it like any other object - it is an object after all.

Also, would it be better just to create one large QByteArray and use pointers to access delimited areas within it?

And throw away type safety? Why? There's no significant difference between that and having a struct containing fixed sized arrays.

I'm still a little surprised that Qt doesn't have some construct to handle this

To handle what exactly?

? Offline

@mzimmers said in Qt analog to C struct?:

Also, would it be better just to create one large QByteArray and use pointers to access delimited areas within it?

I don't see why you would do that. Just complicates your code for no reason.

mzimmers

I'm just looking for a clean way to handle a data structure. In all candor, both the memcpy_s and the std::array/std::copy approaches are tough on the eyes. I guess, though, that's the price you pay to avoid overrunning boundaries.

kshegunov

I don't follow, perhaps I'm missing something. Suppose you use std::array. It'd look something like this:

struct X
{
    std::array<int, 4> a;
    std::array<double, 2> b;
};

You use it as any POD struct.

X x = {
    { 1, 2, 3, 4 }, // This is an initializer list for std::array<int, 4>
    { 0.1, 12.3 }   // The second member's intialization
};

X y = x; // This is all, copying of the arrays is taken care for you by the STL and the compiler

double z[2];
std::memcpy(z, x.b.data(), x.b.size()); // and voila, we copied the data to `z`

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.