Add RecordComponent::visit

include/openPMD/RecordComponent.hpp

@@ -35,6 +35,7 @@
 #include <stdexcept>
 #include <string>
 #include <type_traits>
+#include <utility>
 #include <vector>
 
 // expose private and protected members for invasive testing
@@ -97,6 +98,29 @@ namespace internal
     };
 } // namespace internal
 
+class RecordComponent;
+
+namespace detail
+{
+    template <typename Functor, typename Res>
+    struct VisitRecordComponent
+    {
+        template <typename T, typename... Args>
+        static Res call(RecordComponent &rc, Args &&...args)
+        {
+            return Functor::template call<T>(rc, std::forward<Args>(args)...);
+        }
+
+        template <int = 0, typename... Args>
+        static Res call(Args &&...)
+        {
+            throw std::runtime_error(
+                "[RecordComponent::visit()] Unknown datatype in "
+                "RecordComponent");
+        }
+    };
+} // namespace detail
+
 class RecordComponent : public BaseRecordComponent
 {
     template <typename T, typename T_key, typename T_container>
@@ -404,6 +428,47 @@ class RecordComponent : public BaseRecordComponent
     template <typename T>
     DynamicMemoryView<T> storeChunk(Offset, Extent);
 
+    /**
+     * @brief Run a template functor on the type of the record component,
+     *        similar to std::visit().
+     *
+     * Note that unlike std::visit(), this template cannot "switch" over
+     * a single existing value, meaning that the interface needs to work
+     * a bit different.
+     * The functor is given as a struct/class with a call() operation.
+     *
+     * (Ideally, this can be harmonized by using template lambdas once we
+     * support C++20)
+     *
+     * @tparam Visitor A struct type that has a static template method:
+     *         Visitor::template call<T>(RecordComponent &, ...)
+     *         In here, T will be instantiated with this RecordComponent's type
+     *         and a reference to this RecordComponent will be passed as first
+     *         argument.
+     *
+     * @tparam Args Types of optional further arguments.
+     * @param args Optional further arguments that will be forwarded to
+     *        Visitor::template call()
+     * @return Whatever Visitor::template call() returned.
+     *         Take special note that the return types must match (i.e. cannot
+     *         be different across instantiations
+     *         of T for Visitor::template call<T>()).
+     *         Formally, the return type is that of
+     *         Visitor::template call<char>(RecordComponent &, Args&&...)
+     *         and returned values from other template instantiations might then
+     *         be implicitly converted.
+     */
+    template <typename Visitor, typename... Args>
+    constexpr auto visit(Args &&...args)
+        -> decltype(Visitor::template call<char>(
+            std::declval<RecordComponent &>(), std::forward<Args>(args)...))
+    {
+        using Res = decltype(Visitor::template call<char>(
+            std::declval<RecordComponent &>(), std::forward<Args>(args)...));
+        return switchNonVectorType<detail::VisitRecordComponent<Visitor, Res>>(
+            getDatatype(), *this, std::forward<Args>(args)...);
+    }
+
     static constexpr char const *const SCALAR = "\vScalar";
 
 private:

test/SerialIOTest.cpp

@@ -1525,6 +1525,15 @@ TEST_CASE("dtype_test", "[serial]")
     }
 }
 
+struct ReadFromAnyType
+{
+    template <typename T>
+    static std::shared_ptr<void> call(RecordComponent &rc)
+    {
+        return std::static_pointer_cast<void>(rc.loadChunk<T>());
+    }
+};
+
 inline void write_test(const std::string &backend)
 {
     Series o = Series("../samples/serial_write." + backend, Access::CREATE);
@@ -1623,6 +1632,12 @@ inline void write_test(const std::string &backend)
     }
 
     o.flush();
+
+    o.close();
+
+    Series read("../samples/serial_write." + backend, Access::READ_ONLY);
+    auto rc = read.iterations[1].particles["e"]["position"]["x"];
+    auto opaqueTypeDataset = rc.visit<ReadFromAnyType>();
 }
 
 TEST_CASE("write_test", "[serial]")