deploy: 7c9208bfb3

2026-07-09 12:05:10 +00:00 · 2026-07-08 18:19:46 +00:00
parent 9a231845c2
commit 279d757031
758 changed files with 56489 additions and 503 deletions
@@ -0,0 +1,131 @@
+# nlohmann::basic_json::json_base_class_t
+
+```
+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.
+
+```
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+class visitor_adaptor_with_metadata
+{
+  public:
+    template <class Fnc>
+    void visit(const Fnc& fnc) const;
+
+    int metadata = 42;
+  private:
+    template <class Ptr, class Fnc>
+    void do_visit(const Ptr& ptr, const Fnc& fnc) const;
+};
+
+using json = nlohmann::basic_json <
+             std::map,
+             std::vector,
+             std::string,
+             bool,
+             std::int64_t,
+             std::uint64_t,
+             double,
+             std::allocator,
+             nlohmann::adl_serializer,
+             std::vector<std::uint8_t>,
+             visitor_adaptor_with_metadata
+             >;
+
+template <class Fnc>
+void visitor_adaptor_with_metadata::visit(const Fnc& fnc) const
+{
+    do_visit(json::json_pointer{}, fnc);
+}
+
+template <class Ptr, class Fnc>
+void visitor_adaptor_with_metadata::do_visit(const Ptr& ptr, const Fnc& fnc) const
+{
+    using value_t = nlohmann::detail::value_t;
+    const json& j = *static_cast<const json*>(this);
+    switch (j.type())
+    {
+        case value_t::object:
+            fnc(ptr, j);
+            for (const auto& entry : j.items())
+            {
+                entry.value().do_visit(ptr / entry.key(), fnc);
+            }
+            break;
+        case value_t::array:
+            fnc(ptr, j);
+            for (std::size_t i = 0; i < j.size(); ++i)
+            {
+                j.at(i).do_visit(ptr / std::to_string(i), fnc);
+            }
+            break;
+        case value_t::null:
+        case value_t::string:
+        case value_t::boolean:
+        case value_t::number_integer:
+        case value_t::number_unsigned:
+        case value_t::number_float:
+        case value_t::binary:
+            fnc(ptr, j);
+            break;
+        case value_t::discarded:
+        default:
+            break;
+    }
+}
+
+int main()
+{
+    // create a json object
+    json j;
+    j["null"];
+    j["object"]["uint"] = 1U;
+    j["object"].metadata = 21;
+
+    // visit and output
+    j.visit(
+         [&](const json::json_pointer & p,
+             const json & j)
+    {
+        std::cout << (p.empty() ? std::string{"/"} : p.to_string())
+                  << " - metadata = " << j.metadata << " -> " << j.dump() << '\n';
+    });
+}
+```
+
+Output:
+
+```
+/ - metadata = 42 -> {"null":null,"object":{"uint":1}}
+/null - metadata = 42 -> null
+/object - metadata = 21 -> {"uint":1}
+/object/uint - metadata = 42 -> 1
+```
+
+## Version history
+
+- Added in version 3.12.0.