How to pass Qt::ConnectionType to QObject::connect when connecting a lambda?

Question

I am connecting lambdas to QObject's signals:

    QObject::connect(handle, &BatchHandle::progressMax, [this](const ProcessHandle* const self, const int value) {
        this->maxProgress(value);
    });

The code above compiles with no problems.

However it's absolutely necessary that the Qt::QueuedConnection because the handle object will eventually move to another thread.

I added this to my code:

    QObject::connect(handle, &BatchHandle::finished, [this](const ProcessHandle* const self) {
        this->processIsRunning(false);
    }, (Qt::ConnectionType)Qt::QueuedConnection);

Notice how I added explicit cast to make sure it identifies the value type correctly. Result:

1>src\TechAdminServices\database\techCore\processes\import\ImportManagerDialog.cpp(191): error C2664: 'QMetaObject::Connection QObject::connect<void(__cdecl taservices::ProcessHandle::* )(const taservices::ProcessHandle *),Qt::ConnectionType>(const taservices::ProcessHandle *,Func1,const QObject *,Func2,Qt::ConnectionType)' : cannot convert parameter 3 from 'taservices::`anonymous-namespace'::<lambda58>' to 'const QObject *'
1>          with
1>          [
1>              Func1=void (__cdecl taservices::ProcessHandle::* )(const taservices::ProcessHandle *),
1>              Func2=Qt::ConnectionType
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

How to get a queued connection when connecting a lambda?

I thought for sure this would be a dupe, since the requirement for the target context does make it a bit unintuitive, but apparently not. — Nicolas Holthaus
– Nicolas Holthaus, Commented Feb 10, 2017 at 15:16

G.M. · Accepted Answer · 2017-02-10 14:51:42Z

26

I think you need to use the QObject::connect overload that allows you to specify the context in which the lambda should be invoked...

QObject::connect(
  handle,
  &BatchHandle::progressMax,
  target_context,   /* Target context parameter. */
  [this](const ProcessHandle* const self, const int value)
  {
    this->maxProgress(value);
  },
  Qt::QueuedConnection);

answered Feb 10, 2017 at 14:51

G.M.

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Kuba hasn't forgotten Monica · Accepted Answer · 2017-02-23 14:26:31Z

-1

The queued connection can't work without the target object context, since it's this context that selects the queue the slot call gets inserted into. To be more obtusely, a QMetaCallEvent wrapping the functor is posted to the context object thread()'s event queue.

edited Feb 23, 2017 at 14:26

answered Feb 10, 2017 at 16:37

Kuba hasn't forgotten Monica

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