api/basic_json/json_base_class_t/index.md

# 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.