# NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END

```cpp
#define NLOHMANN_JSON_NAMESPACE_BEGIN /* value */  // (1)
#define NLOHMANN_JSON_NAMESPACE_END   /* value */  // (2)
```

These macros can be used to open and close the `nlohmann` namespace. See
[`nlohmann` Namespace](../../features/namespace.md#structure) for details.

1. Opens the namespace.
2. Closes the namespace.

## Default definition

The default definitions open and close the `nlohmann` namespace. The precise definition of
`NLOHMANN_JSON_NAMESPACE_BEGIN` varies as described [here](../../features/namespace.md#structure).

1. Default definition of `NLOHMANN_JSON_NAMESPACE_BEGIN`:

    ```cpp
    namespace nlohmann
    {
    inline namespace json_abi_v3_12_0
    {
    ```

2. Default definition of `NLOHMANN_JSON_NAMESPACE_END`:
    ```cpp
    }  // namespace json_abi_v3_12_0
    }  // namespace nlohmann
    ```

When these macros are not defined, the library will define them to their default definitions.

## Examples

??? example

    The example shows how to use `NLOHMANN_JSON_NAMESPACE_BEGIN`/`NLOHMANN_JSON_NAMESPACE_END` from the
    [How do I convert third-party types?](../../features/arbitrary_types.md#how-do-i-convert-third-party-types) page.

    ```cpp
    #include <iostream>
    #include <optional>
    #include <nlohmann/json.hpp>
    
    // partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)
    NLOHMANN_JSON_NAMESPACE_BEGIN
    template <typename T>
    struct adl_serializer<std::optional<T>>
    {
        static void to_json(json& j, const std::optional<T>& opt)
        {
            if (opt == std::nullopt)
            {
                j = nullptr;
            }
            else
            {
                j = *opt;
            }
        }
    };
    NLOHMANN_JSON_NAMESPACE_END
    
    int main()
    {
        std::optional<int> o1 = 1;
        std::optional<int> o2 = std::nullopt;
    
        NLOHMANN_JSON_NAMESPACE::json j;
        j.push_back(o1);
        j.push_back(o2);
        std::cout << j << std::endl;
    }
    
    ```
    
    Output:
    
    ```json
    [1,null]
    
    ```

## See also

- [`nlohmann` Namespace](../../features/namespace.md)
- [NLOHMANN_JSON_NAMESPACE](nlohmann_json_namespace.md)
- [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](nlohmann_json_namespace_no_version.md)

## Version history

- Added in version 3.11.0. Changed inline namespace name in version 3.11.2.