QLayout: unify the constructors

Drive-by, make QLayout constructor explicit, as it should have
always been.

[ChangeLog][QtWidgets][QLayout] The QLayout constructor taking
a QWidget pointer is now explicit.

Change-Id: If6accfb2e7e810fa08adacea9e674c99121f6bc6
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/kernel/qgridlayout.cpp

@@ -1077,6 +1077,10 @@ QRect QGridLayoutPrivate::cellRect(int row, int col) const
     There can be only one top-level layout for a widget. It is returned
     by QWidget::layout().
 
+    If \a parent is \nullptr, then you must insert this grid layout
+    into another layout, or set it as a widget's layout using
+    QWidget::setLayout().
+
     \sa QWidget::setLayout()
 */
 QGridLayout::QGridLayout(QWidget *parent)
@@ -1086,23 +1090,6 @@ QGridLayout::QGridLayout(QWidget *parent)
     d->expand(1, 1);
 }
 
-/*!
-    Constructs a new grid layout.
-
-    You must insert this grid into another layout. You can insert
-    widgets and layouts into this layout at any time, but laying out
-    will not be performed before this is inserted into another layout.
-*/
-QGridLayout::QGridLayout()
-    : QLayout(*new QGridLayoutPrivate, nullptr, nullptr)
-{
-    Q_D(QGridLayout);
-    d->expand(1, 1);
-}
-
-
-
-
 /*!
 \internal (mostly)
 

src/widgets/kernel/qgridlayout.h

@@ -61,9 +61,7 @@ class Q_WIDGETS_EXPORT QGridLayout : public QLayout
     QDOC_PROPERTY(int verticalSpacing READ verticalSpacing WRITE setVerticalSpacing)
 
 public:
-    explicit QGridLayout(QWidget *parent);
-    QGridLayout();
-
+    explicit QGridLayout(QWidget *parent = nullptr);
     ~QGridLayout();
 
     QSize sizeHint() const override;

src/widgets/kernel/qlayout.cpp

@@ -106,11 +106,16 @@ static int menuBarHeightForWidth(QWidget *menubar, int w)
 
 /*!
     Constructs a new top-level QLayout, with parent \a parent.
-    \a parent may not be \nullptr.
 
     The layout is set directly as the top-level layout for
     \a parent. There can be only one top-level layout for a
     widget. It is returned by QWidget::layout().
+
+    If \a parent is \nullptr, then you must insert this layout
+    into another layout, or set it as a widget's layout using
+    QWidget::setLayout().
+
+    \sa QWidget::setLayout()
 */
 QLayout::QLayout(QWidget *parent)
     : QObject(*new QLayoutPrivate, parent)
@@ -120,18 +125,6 @@ QLayout::QLayout(QWidget *parent)
     parent->setLayout(this);
 }
 
-/*!
-    Constructs a new child QLayout.
-
-    This layout has to be inserted into another layout before geometry
-    management will work.
-*/
-QLayout::QLayout()
-    : QObject(*new QLayoutPrivate, nullptr)
-{
-}
-
-
 /*! \internal
  */
 QLayout::QLayout(QLayoutPrivate &dd, QLayout *lay, QWidget *w)

src/widgets/kernel/qlayout.h

@@ -77,8 +77,7 @@ public:
     };
     Q_ENUM(SizeConstraint)
 
-    QLayout(QWidget *parent);
-    QLayout();
+    explicit QLayout(QWidget *parent = nullptr);
     ~QLayout();
 
     virtual int spacing() const;