MIDI plugins -- partial implemetnation, temporarily disabled.

PiPedalCommon/src/include/json.hpp

@@ -34,6 +34,7 @@
 #include <limits>
 #include <stdexcept>
 #include <chrono>
+#include <optional>
 
 #define DECLARE_JSON_MAP(CLASSNAME) \
     static pipedal::json_map::storage_type<CLASSNAME> jmap
@@ -592,6 +593,17 @@ namespace pipedal
                 write(obj.get());
             }
         }
+        template <typename T>
+        void write(const std::optional<T> &obj)
+        {
+            if (!obj) {
+                write_raw("null");
+            } else {
+                write(obj.get());
+            }
+        }
+
+
         template <typename T>
         void write(const std::weak_ptr<T> &obj)
         {
@@ -883,6 +895,20 @@ namespace pipedal
             }
         }
         template <typename T>
+        void read(std::optional<T> *pOptional)
+        {
+            if (peek() == 'n')
+            {
+                consumeToken("null", "Expecting '{' or 'null'.");
+                *pOptional = std::optional<T>();
+
+            } else {
+                T value;
+                read(&value);
+                *pOptional = std::move(value);
+            }
+        }
+        template <typename T>
         void read(std::weak_ptr<T> *pUniquePtr)
         {
             throw std::domain_error("Can't read std::weak_ptr");