Wrapping connect method in Qt hides information needed by compiler

Question

I want to wrap the Qt connect method (Qt5 syntax), but can't get my code to compile. The simplest example I can come up with is (not tested, just general idea):

// Called like this 
doConnect(&obj1,&obj1:sig,&obj2,&obj2::slot);

// Implemented here
void namespace::doConnect(
  const QObject* senderObject, 
  const QMetaMethod& senderSignal, {
  const QObject* receiverObject, 
  const QMetaMethod& receiverSlot) {
    connect(senderObject, senderSignal, receiverObject, receiverSlot);
}

When I try to compile this I get an error on the line that calls doConnect,

"no matching function for call to doConnect"

and on the connect line

error: static assertion failed: Signal and slot arguments are not compatible.

Which sounds like signal and slot have different/incompatible arguments. However, if I call connect directly by putting the sender object/signal and receiver object/slot in the connect command it works fine. So it's not signature of signal/slot issue.

What is wrong here? Does passing the parameters to a function first hide some information that the connect function (or macro or template) needs?

It would be nice to see the actual call and the exact signature of the functiins passed as signal & slot. — Benjam

n1coc4cola n1coc4cola · Accepted Answer · 2021-02-26T10:04:34

For me the following works well:

void doConnect(const QObject* senderObject, const QMetaMethod& senderSignal, const QObject* receiverObject, const QMetaMethod& receiverSlot)
{
    QObject::connect(senderObject, senderSignal, receiverObject, receiverSlot);
}

You've write "Signal and slot arguments are not compatible" from your compiler. And that's not strange. You could try to do the following: QObject::connect(&obj1,&obj1:sig,&obj2,&obj2::slot);

You'll get the same! Your functions signatures are not compatible. For example: QObject::connect(obj1, SIGNAL("remove(int)"), obj2, SLOT("handleRemove(QString)"); Just that with this connect, you'll get the error at runtime and not at compile time.

It's the same as: QObject::connect(obj1, &C1::remove, obj2, &C2::handleRemove);

You'll get the same error, because yes, an int is not a QString. Simple and fast. The same applies on the number of arguments, if there's more or less, you can go through that.

Moreover, creating a function doConnect that is the same as QObject::connect is useless. Be more careful when you read the docs (if you did).

Wrapping connect method in Qt hides information needed by compiler

1 Answers