Can I swap QObject child keeping signals and slots connected - c++

Question

Suppose I have class:
class Foo : public QObject{
Q_OBJECT
public:
...
private:
Bar* m_bar;
}
And for whatever reason, I'd like to replace m_bar with a pointer to an object of DerivedBar. The use case I most often find is when I want to replace it with an object of MockBar (using the google mock framework) for unit testing.
Now I know in most cases, I could just extend the constructor to be something like:
Foo(Bar* bar, QObject* parent)
and just set m_bar accordingly.
But the problem I have is when the classes are QWidgets and I'm assembling them via Qt Designer. Imagine Foo and Bar are widgets, and Bar is placed inside of Foo from designer; their connections are also set via designer (ie, stored in the qml file). I've tried something like:
Bar* bar = foo.findChild<Bar*>();
bar = new MockBar(&foo);
(neglecting any memory leaks here, just trying to get functionality) But when I go to set expectations, it does not seem to be connected like the existing one was. Slots aren't called in response to signals. I don't think this is entirely unexpected due to the way connections are made, but I'm wondering if there's a way to get the effect I'm looking for.
(nb: for now, using qt 4.8 and gcc 4.6, which does limit some options for c++

Answer 1

Here's a better answer I've found: The issue for me is ultimately to do with the QWidgets generated from ui files. But since the Ui::FooWidget class is functionally just a pimpl pattern, I can create a pointer to a local copy of one, and modify Foo's constructor to allow me to optionally inject the ui implementation. If Foo is making a direct function call to BarWidget, I can replace that ui's 'barWidget' object with my MockBarWidget, and set expectations and such and perform testing. The previous answer will still satisfy to some degree when the interaction between the two widgets is one of signal/slot connections.
Previous Answer:
Here's one answer I've found so far, but I don't much like it:
Bar* bar = foo.findChild<Bar*>();
bool connectTest = QObject::connect(&foo, SIGNAL(setValue(int)), bar, SLOT(setValue(int)), Qt::UniqueConnection);
ASSERT_FALSE(connectTest);
bar = new MockBar(&foo);
connectTest = QObject::connect(&foo, SIGNAL(setValue(int)), bar, SLOT(setValue(int)), Qt::UniqueConnection);
ASSERT_TRUE(connectTest);
//... rest of test
I could probably wrap the whole thing up in some kind of helper function, and that function may be better with c++11/Qt5 where signals and slots can be method pointers. But I wonder if I'm missing an easier way to do it.
Edit: It turns out this pattern was giving me a false positive, only the first 'connectTest' part of the test was giving a valid test of behaviour. This flaw is more obvious when Foo doesn't emit a signal to bar , but merely calls a method on bar. The mock is not injected through this pattern.