Trying to use QFuture and QFutureWatcher for the first time

Perdrix

OK I've reduced the replication to a relatively simple case:

You can find the source files here:
http://www.perdrix.co.uk/TestQtConcurrent.zip

If you want the entire MSVC solution, say so and I will zip that up and make it available.
David

JonB

@Perdrix
Can't you start by just copy/paste my code and say whether that works or not....?!

Perdrix

I did precisely that and then extended it until I got a fail.
D.

JonB

@Perdrix Ah, well it helps if you say that! I will have a look at your code to see if I can spot something different.....

JonB

@Perdrix said in Trying to use QFuture and QFutureWatcher for the first time:

You can find the source files here:
http://www.perdrix.co.uk/TestQtConcurrent.zip

If I just open it in Chrome nothing happens.
If I force it to download (it seems to start but never finishes) I get about 4k and then it stops.
If you want someone to access this then please verify it works for you from an incognito browsing session....
UPDATE OK, although the download does not finish properly if I rename the temporary .crdownload file to .zip it seems to have content....

Perdrix

The zip file is only 4kB... - worked for me in an incognito Chrome session on Windows, but did bitch that it wasn't https ...

D.

JonB

@Perdrix
So I have copied/pasted the 3 files you supply into their own project.

I don't even see how what you have supplied would compile. In griddata.h there are two errors:

No template named QPromise
Use of undeclared identifier std

These are both visible in Creator. And make sense to me, as the necessary includes are not in griddata.h. That is improved with the addition of #include <QPromise> in griddata.h. (And I had to comment out a couple of other includes.)

Now I think I am down to a couple of compilation error messages like you have been showing. I will investigate further... :)

The zip file is only 4kB...

I know! But for whatever reason my Chrome does not complete the download of this file from you. It seems to get the full content but never sees it finish, so does not rename the temporary download file to the intended .zip. But doing that manually I seem to have the full zip so I am good.

Perdrix

Don't need a template GridData only uses a reference: QPrimose<void>& promise but won't harm to include it.

Don't need to do anything for std:: stuff, so long as you call it out by its full name

JonB

@Perdrix

@Perdrix said in Trying to use QFuture and QFutureWatcher for the first time:

    void
        interpolate(QPromise<void>& promise,
            const std::vector<double>& x, const std::vector<double>& y, const std::vector<double>& z,
            const std::vector<double>& xg, const std::vector<double>& yg, std::vector<double>& zg,
            InterpolationType type, float data = 0.0);

All the other std::vector<double>&s are const, but not the final zg one. Intended or not? This is the error.

If you make that one const too it compiles.

If you test just

void
GridData::interpolate2(QPromise<void>& promise,
                       const std::vector<double> &x)

QFuture<void>future2 = QtConcurrent::run(&GridData::interpolate2, instance.get(), xValues);

That works. But take out the const and it fails.

You could have reduced it down to just this parameter type, nothing to do with all the member methods:

void func1(QPromise<void>& promise, std::vector<double> &xValues)
{
}

QFuture<void> fut1 = QtConcurrent::run(func1, xValues);

std::vector<double> &xValues does not pass. Either (or both) of adding in const or removing the & do pass.

I don't use std::vector and am going leave it to you now. You may need e.g. @Christian-Ehrlicher to explain the deep reason. I am not sure anyway about accessing non-const reference to arrays in the thread(s) run by QtConcurrent. As I wrote, if you can change yours to const std::vector<double>& zg or std::vector<double> zg it won't matter. Though of course we would like to hear from a C++ expert for the definitive explanation!

Phew!

And, yes, no, C++ compiler errors, especially with templates, are not the simplest.

P.S.
Just this fails in same way:

void func1(QPromise<void>& promise, int &test)
{
}

int test;
QtConcurrent::run(func1, test);

And again changing func1() to either const int &test or int test works. So nothing to do with std::vector. You evidently cannot pass a mutable reference parameter here, I am thinking QtConcurrent::run() does not allow this because of threading/need to copy requirements? Now that we knows this is the issue we find e.g.
https://stackoverflow.com/questions/25091518/qt-concurrent-run-pass-value-by-reference-but-the-memory-address-is-different

QtConcurrent::run creates internal copies of all arguments that you pass to it. Your thread function is then given access to these copies, not to the original arguments. Passing something by raw reference does not prevent creation of a copy. In other words, QtConcurrent::run enforces pass-by-value semantics internally.

The address that you see in the ref function is the address of that internal copy.

For this reason, if you specifically want access to the original object, instead of a naked reference you have to use something with pointer semantics or with "copyable reference" semantics.

That was a few years ago. Perhaps the definitions/compiler now show an attempt to pass a non-const reference as a compile-time error where it used to get through but then did not act as expected?

Perdrix

Argh! I need that to be a non const vector reference, as the interpolate code populates that vector.

@JonB Thanks for finding out what was stopping it working! I'd never have guessed that was the problem.

I've updated the files so it should compile OK on Linux, and refreshed the zip file:
http://www.perdrix.co.uk/TestQtConcurrent.zip
@Christian-Ehrlicher Can you advise please?

Meantime I will change the code to use std::ref(zgEccentricity).

UPDATE: using std::ref(zgEccentricity) worked fine. May I suggest that some documentation updates to say you can't pass a non-const thing using a normal reference and that using std::ref might be useful. Either that or?
David

JonB

@Perdrix said in Trying to use QFuture and QFutureWatcher for the first time:

Argh! I need that to be a non const vector reference, as the interpolate code populates that vector.

You have just crossed with my final update to my reply. Have a read through the stackoverflow post (or try to find your own), there may be stuff there which allows you to do something, e.g.

QtConcurrent::run(func1, std::ref(test));

seems to compile against int &test, I don't know anything about std::ref() but it looks good?

P.S.
Being an originally C person, without all this C++ nonsense, references etc. I bet

void func1(QPromise<void>& promise, int *test) {}
QtConcurrent::run(func1, &test);

would work ;-) [But of course don't do this!]

Perdrix

I found the line about the use of std::ref in the docs, so it is there, but it is fairly easy to miss it.

D.

JonB

@Perdrix
It would be good for this thread if you provided the link to whatever you say it says about std::ref() and also pasted the relevant sentence/paragraph here, for others to see in future.

Perdrix

It is buried in the "Additional API Features" section of the doc and says:

It should be right up front where QtConcurrent::run() is described, saying very clearly that you must use std:ref(variable) when passing a reference to a modifiable object.

JonB

@Perdrix
OK, for whatever reason (maybe misplaced) this appears in subsection https://doc.qt.io/qt-6/qtconcurrentrun.html#using-lambda-functions, for anyone reading this.