first commit

modern-qt/utility/trait/widget.hh

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+#pragma once
+#include <concepts>
+#include <qwidget.h>
+
+namespace creeper {
+
+template <class T>
+concept widget_trait = std::convertible_to<T*, QWidget*>;
+
+template <class T>
+concept widget_pointer_trait = std::convertible_to<T, QWidget*>;
+
+template <class T>
+concept layout_trait = std::convertible_to<T*, QLayout*>;
+
+template <class T>
+concept layout_pointer_trait = std::convertible_to<T, QLayout*>;
+
+template <class T>
+concept item_trait = widget_trait<T> || layout_trait<T>;
+
+template <class T>
+concept linear_trait = requires(T t) {
+    { t.addWidget(std::declval<QWidget*>(), int {}, Qt::AlignCenter) };
+    { t.addLayout(std::declval<QLayout*>(), int {}) };
+};
+
+template <class T>
+concept area_trait = requires(T t) {
+    { t.setWidget(std::declval<QWidget*>()) };
+    { t.setLayout(std::declval<QLayout*>()) };
+};
+
+template <class T>
+concept selectable_trait = requires(T t) {
+    { std::as_const(t).selected() } -> std::convertible_to<bool>;
+    { t.set_selected(bool {}) };
+};
+
+}