Allow custom base class as node customization point (#3110)

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>
Co-authored-by: barcode <barcode@example.com>

include/nlohmann/detail/json_custom_base_class.hpp

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <type_traits> // conditional, is_same
+
+#include <nlohmann/detail/abi_macros.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+namespace detail
+{
+
+/*!
+@brief Default base class of the @ref basic_json class.
+
+So that the correct implementations of the copy / move ctors / assign operators
+of @ref basic_json do not require complex case distinctions
+(no base class / custom base class used as customization point),
+@ref basic_json always has a base class.
+By default, this class is used because it is empty and thus has no effect
+on the behavior of @ref basic_json.
+*/
+struct json_default_base {};
+
+template<class T>
+using json_base_class = typename std::conditional <
+                        std::is_same<T, void>::value,
+                        json_default_base,
+                        T
+                        >::type;
+
+}  // namespace detail
+NLOHMANN_JSON_NAMESPACE_END