Question

I want to invoke a slot asynchronously from a different thread using QMetaObject::invokeMethod()

The class containing the slot is declared as:

class Paintable : public QObject {
  Q_OBJECT
  [...]
  public slots:
    void drawString(uint16_t x, uint16_t y, uint16_t size, const char* str, color c);
}

and the method that calls invokeMethod is defined as:

void drawStringAsynchronously(uint16_t x, uint16_t y, uint16_t size, const char* str, color c) {
  QMetaObject::invokeMethod(paintable,
               "drawString",
               Qt::QueuedConnection,
               Q_ARG(uint16_t, x), Q_ARG(uint16_t, y), Q_ARG(uint16_t, size),
               Q_ARG(const char*, str), Q_ARG(color, c));
}

(where paintable is of type Paintable*)

But Qt seems to be unable to use uint16_t or char* in invokeMethod because at runtime I get the following message:

QMetaMethod::invoke: Unable to handle unregistered datatype 'const char*'

and

QMetaMethod::invoke: Unable to handle unregistered datatype 'uint16_t'

respectively.

I was able to successfully register my custom struct color using qRegisterMetaType() but since uint16_t and char* are no structs or classes this won’t work.

I’d be very glad if somebody could show me how to do it or show a good alternative.

Answer 1

The problem with registering uint16_t is this: It’s a typedef, and Qt has already registered this type, but it’s under a different name. Since the QMetaType system is based on determining types by their name this causes problems.

You can get around this with:

Q_DECLARE_METATYPE(uint16_t)

then:

qRegisterMetaType<uint16_t>("uint16_t");

This creates an alias so that metatypes can be created with that name.