Added lazy loading wrapper.

nui/include/nui/utility/lazy.hpp

@@ -0,0 +1,73 @@
+#pragma once
+
+#include <optional>
+#include <functional>
+
+namespace Nui
+{
+    template <typename ValueT>
+    class Lazy
+    {
+      public:
+        Lazy() = default;
+        Lazy(Lazy const&) = default;
+        Lazy(Lazy&&) = default;
+        Lazy& operator=(Lazy const&) = default;
+        Lazy& operator=(Lazy&&) = default;
+
+        template <typename FuncT>
+        Lazy(FuncT&& func)
+            : value_{}
+            , obtainer_{std::forward<FuncT>(func)}
+        {}
+
+        explicit operator bool() const
+        {
+            return value_.has_value();
+        }
+
+        bool hasValue() const
+        {
+            return value_.has_value();
+        }
+
+        bool tryObtainValue() const
+        {
+            if (!value_)
+                value_ = obtainer_();
+            return hasValue();
+        }
+
+        std::optional<ValueT> const& value() const&
+        {
+            if (!value_)
+                value_ = obtainer_();
+            return value_;
+        }
+
+        std::optional<ValueT>&& value() &&
+        {
+            if (!value_)
+                value_ = obtainer_();
+            return std::move(value_);
+        }
+
+        std::optional<ValueT> value() const&&
+        {
+            if (!value_)
+                value_ = obtainer_();
+            return value_;
+        }
+
+        std::optional<ValueT>& value() &
+        {
+            if (!value_)
+                value_ = obtainer_();
+            return value_;
+        }
+
+      private:
+        std::optional<ValueT> value_{};
+        std::function<std::optional<ValueT>()> obtainer_{};
+    };
+}