How to find widgets of this type in QLayout?

Question

I can find the QRadioButton as follows:

 for(int i = 0; i < ui->verticalLayout->count(); i++) { QRadioButton* r = qobject_cast<QRadioButton*>(ui->verticalLayout->itemAt(i)->widget()); if(r->isChecked()) //found it! } 

But I do not like this way of iterating over the elements and would like to use the foreach construct. My first attempt failed:

 foreach(QRadioButton* child, ui->verticalLayout->findChildren<QRadioButton*>()) { if(child->isChecked()) //found it! } 

The problem is that ui->verticalLayout->findChildren<QRadioButton*>() returns null elements. It also does not return any elements with findChildren<QObject*>() . Can anyone explain this behavior?

Note: the name of this question is almost identical to mine, but it is related to python Qt and does not contain any useful information for me.

Children versus items?

Experimentally, I realized that ui->verticalLayout->children().count() returns zero, where as ui->verticalLayout->count() returns the number of elements that I have in verticalLayout . This means that itemAt(i) and findChild<QRadioButton*>() do not have access to the same list. Reviewing the Qt documentation on children() did not help me.

Can someone point me to some good stuff on the concept of parent Qt children? I assume that this has nothing to do with access to nested objects, which I am trying to accomplish.

Edit

As suggested by Cuba Obrom, the answers to this question contain valuable information on another topic, while his answer clarifies my question about the layout children. So this is not a duplicate question.

Answer 1

Widgets are not children of the layout in the sense of QObject children - they are children of the parent widget. A QWidget can only be a child of another QWidget - so you cannot expect widgets to be children. As long as new QWidget(new QWidget()) works, new QWidget(new QHBoxLayout()) will not compile.

You can iterate children of widgets of a given type as follows:

 // C++11 for (auto button : findChildren<QRadioButton*>()) if (button->isChecked()) { ... } // C++98 Q_FOREACH (QWidget * button, findChildren<QRadioButton*>()) if (button->isChecked()) { ... } 

If you are using C ++ 11, you should use the range for the loop , not the deprecated foreach or Q_FOREACH .

To repeat child widgets controlled by the layout, you need an iterator adapter for the layout. For instance:

 #include <QLayout> #include <QDebug> #include <QPointer> #include <utility> template<class WT> class IterableLayoutAdapter; template<typename WT> class LayoutIterator { QPointer<QLayout> m_layout; int m_index; friend class IterableLayoutAdapter<WT>; LayoutIterator(QLayout * layout, int dir) : m_layout(layout), m_index(dir>0 ? -1 : m_layout->count()) { if (dir > 0) ++*this; } friend QDebug operator<<(QDebug dbg, const LayoutIterator & it) { return dbg << it.m_layout << it.m_index; } friend void swap(LayoutIterator& a, LayoutIterator& b) { using std::swap; swap(a.m_layout, b.m_layout); swap(a.m_index, b.m_index); } public: LayoutIterator() : m_index(0) {} LayoutIterator(const LayoutIterator & o) : m_layout(o.m_layout), m_index(o.m_index) {} LayoutIterator(LayoutIterator && o) { swap(*this, o); } LayoutIterator & operator=(LayoutIterator o) { swap(*this, o); return *this; } WT * operator*() const { return static_cast<WT*>(m_layout->itemAt(m_index)->widget()); } const LayoutIterator & operator++() { while (++m_index < m_layout->count() && !qobject_cast<WT*>(m_layout->itemAt(m_index)->widget())); return *this; } LayoutIterator operator++(int) { LayoutIterator temp(*this); ++*this; return temp; } const LayoutIterator & operator--() { while (!qobject_cast<WT*>(m_layout->itemAt(--m_index)->widget()) && m_index > 0); return *this; } LayoutIterator operator--(int) { LayoutIterator temp(*this); --*this; return temp; } bool operator==(const LayoutIterator & o) const { return m_index == o.m_index; } bool operator!=(const LayoutIterator & o) const { return m_index != o.m_index; } }; template <class WT = QWidget> class IterableLayoutAdapter { QPointer<QLayout> m_layout; public: typedef LayoutIterator<WT> const_iterator; IterableLayoutAdapter(QLayout * layout) : m_layout(layout) {} const_iterator begin() const { return const_iterator(m_layout, 1); } const_iterator end() const { return const_iterator(m_layout, -1); } const_iterator cbegin() const { return const_iterator(m_layout, 1); } const_iterator cend() const { return const_iterator(m_layout, -1); } }; template <class WT = QWidget> class ConstIterableLayoutAdapter : public IterableLayoutAdapter<const WT> { public: ConstIterableLayoutAdapter(QLayout * layout) : IterableLayoutAdapter<const WT>(layout) {} }; 

Used as follows:

 #include <QApplication> #include <QLabel> #include <QHBoxLayout> int main(int argc, char ** argv) { QApplication app(argc, argv); tests(); QWidget a, b1, b3; QLabel b2; QHBoxLayout l(&a); l.addWidget(&b1); l.addWidget(&b2); l.addWidget(&b3); // Iterate all widget types as constants qDebug() << "all, range-for"; for (auto widget : ConstIterableLayoutAdapter<>(&l)) qDebug() << widget; qDebug() << "all, Q_FOREACH"; Q_FOREACH (const QWidget * widget, ConstIterableLayoutAdapter<>(&l)) qDebug() << widget; // Iterate labels only qDebug() << "labels, range-for"; for (auto label : IterableLayoutAdapter<QLabel>(&l)) qDebug() << label; qDebug() << "labels, Q_FOREACH"; Q_FOREACH (QLabel * label, IterableLayoutAdapter<QLabel>(&l)) qDebug() << label; } 

Some rudimentary tests are as follows:

 void tests() { QWidget a, b1, b3; QLabel b2; QHBoxLayout l(&a); IterableLayoutAdapter<> l0(&l); auto i0 = l0.begin(); qDebug() << i0; Q_ASSERT(i0 == l0.begin() && i0 == l0.end()); l.addWidget(&b1); l.addWidget(&b2); l.addWidget(&b3); IterableLayoutAdapter<> l1(&l); auto i1 = l1.begin(); qDebug() << i1; Q_ASSERT(i1 == l1.begin() && i1 != l1.end()); ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end()); ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end()); ++i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 == l1.end()); --i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end()); --i1; qDebug() << i1; Q_ASSERT(i1 != l1.begin() && i1 != l1.end()); --i1; qDebug() << i1; Q_ASSERT(i1 == l1.begin() && i1 != l1.end()); IterableLayoutAdapter<QLabel> l2(&l); auto i2 = l2.begin(); qDebug() << i2; Q_ASSERT(i2 == l2.begin() && i2 != l2.end()); ++i2; qDebug() << i2; Q_ASSERT(i2 != l2.begin() && i2 == l2.end()); --i2; qDebug() << i2; Q_ASSERT(i2 == l2.begin() && i2 != l2.end()); }