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

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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>
This commit is contained in:
Niels Lohmann
2026-09-27 14:53:50 +02:00
co-authored by Raphael Grimm
parent 6bd106893a
commit 1cd636bd61
9 changed files with 219 additions and 2 deletions
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include/nlohmann/json.hpp
+17 -1
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@@ -153,7 +153,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
/// workaround type for MSVC
using basic_json_t = NLOHMANN_BASIC_JSON_TPL;
using json_base_class_t = ::nlohmann::detail::json_base_class<CustomBaseClass>;
JSON_PRIVATE_UNLESS_TESTED:
// convenience aliases for types residing in namespace detail;
@@ -213,6 +212,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
using cbor_tag_handler_t = detail::cbor_tag_handler_t;
/// how to encode BJData
using bjdata_version_t = detail::bjdata_version_t;
/// base class used to inject custom functionality into each instance of basic_json
/// @sa https://json.nlohmann.me/api/basic_json/json_base_class_t/
using json_base_class_t = ::nlohmann::detail::json_base_class<CustomBaseClass>;
/// helper type for initializer lists of basic_json values
using initializer_list_t = std::initializer_list<detail::json_ref<basic_json>>;
@@ -2690,6 +2692,20 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
return *get_ptr<const binary_t*>();
}
/// @brief access the custom base class
/// @sa https://json.nlohmann.me/api/basic_json/as_base_class/
json_base_class_t& as_base_class() noexcept
{
return static_cast<json_base_class_t&>(*this);
}
/// @brief access the custom base class
/// @sa https://json.nlohmann.me/api/basic_json/as_base_class/
const json_base_class_t& as_base_class() const noexcept
{
return static_cast<const json_base_class_t&>(*this);
}
/// @}
////////////////////