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Niels Lohmann 40021f38fb Add basic_json::as_base_class and document name conflicts with custom base classes (#5589 )

* Add basic_json::as_base_class and document name conflicts with custom base classes

Members of basic_json hide members of a custom base class with the same
name, and future releases may add members that hide ones accessible
today. Document this in json_base_class_t and add as_base_class() to
reach hidden members without spelling out the cast.

Also make json_base_class_t a public member type. It was documented
since 3.12.0, but declared private, so users could not name it.

Supersedes #3899.

Co-authored-by: Raphael Grimm <1005058+barcode@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Add as_base_class to the docset search index

New public members get an entry in docs/docset/docSet.sql (as done for
to_bon8/from_bon8 in #2998). Without it, the Dash/Zeal docset built from
the documentation cannot find basic_json::as_base_class.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Silence clang-tidy for the hidden type_name() in the base class test

ci_clang_tidy failed with readability-convert-member-functions-to-static
on base_class_with_hidden_members::type_name(). It must stay a
non-static member: the test shows that it is hidden by the non-static
basic_json::type_name() and reachable through as_base_class().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Raphael Grimm <1005058+barcode@users.noreply.github.com>

2026-10-04 12:00:31 +02:00

1.2 KiB

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nlohmann::basic_json::as_base_class

json_base_class_t& as_base_class() noexcept;
const json_base_class_t& as_base_class() const noexcept;

Returns a reference to this object as its custom base class json_base_class_t. No copy is made.

Since basic_json derives from json_base_class_t, a member of basic_json hides any member of the custom base class with the same name. This function makes such hidden members accessible again.

Return value

reference to this object as json_base_class_t

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Constant.

Notes

The function is equivalent to static_cast<json_base_class_t&>(j) (or static_cast<const json_base_class_t&>(j)).

Examples

??? example

The example shows how to use `as_base_class` to access members of the custom base class that are hidden by members
of `basic_json`.

```cpp
--8<-- "examples/as_base_class.cpp"
```

Output:

```json
--8<-- "examples/as_base_class.output"
```

Version history

Added in version 3.13.0.

1.2 KiB Raw Blame History