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>

docs/mkdocs/docs/api/basic_json/index.md

@@ -13,7 +13,8 @@ template<
     class NumberFloatType = double,
     template<typename U> class AllocatorType = std::allocator,
     template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,
-    class BinaryType = std::vector<std::uint8_t>
+    class BinaryType = std::vector<std::uint8_t,
+    class CustomBaseClass = void>
 >
 class basic_json;
 ```
@@ -32,6 +33,7 @@ class basic_json;
 | `AllocatorType`      | type of the allocator to use                                              |                                             |
 | `JSONSerializer`     | the serializer to resolve internal calls to `to_json()` and `from_json()` | [`json_serializer`](json_serializer.md)     |
 | `BinaryType`         | type for binary arrays                                                    | [`binary_t`](binary_t.md)                   |
+| `CustomBaseClass`    | extension point for user code                                             | [`json_base_class_t`](json_base_class_t.md) |
 
 ## Specializations
 

docs/mkdocs/docs/api/basic_json/json_base_class_t.md

@@ -0,0 +1,44 @@
+# <small>nlohmann::basic_json::</small>json_base_class_t
+
+```cpp
+using json_base_class_t = detail::json_base_class<CustomBaseClass>;
+```
+
+The base class used to inject custom functionality into each instance of `basic_json`.
+Examples of such functionality might be metadata, additional member functions (e.g., visitors), or other application-specific code.
+
+## Template parameters
+
+`CustomBaseClass`
+:   the base class to be added to `basic_json`
+
+## Notes
+
+#### Default type
+
+The default value for `CustomBaseClass` is `void`. In this case an [empty base class](https://en.cppreference.com/w/cpp/language/ebo) is used and no additional functionality is injected.
+
+#### Limitations
+
+The type `CustomBaseClass` has to be a default-constructible class.
+`basic_json` only supports copy/move construction/assignment if `CustomBaseClass` does so as well.
+
+## Examples
+
+??? example
+
+    The following code shows how to inject custom data and methods for each node.
+     
+    ```cpp
+    --8<-- "examples/json_base_class_t.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/json_base_class_t.output"
+    ```
+
+## Version history
+
+- Added in version 3.12.0.

docs/mkdocs/mkdocs.yml

@@ -141,6 +141,7 @@ nav:
         - 'is_string': api/basic_json/is_string.md
         - 'is_structured': api/basic_json/is_structured.md
         - 'items': api/basic_json/items.md
+        - 'json_base_class_t': api/basic_json/json_base_class_t.md
         - 'json_serializer': api/basic_json/json_serializer.md
         - 'max_size': api/basic_json/max_size.md
         - 'meta': api/basic_json/meta.md