Follow up on the template parameter requirements page: state, for every
template parameter, which concrete types work and where they stop working.
Each entry was verified by compiling and running a common workload (DOM
access, dump, parse, CBOR/MessagePack round-trip, flatten, hash) against that
instantiation.
Findings worth calling out:
- ObjectType no longer needs a key_compare member type, so the std::unordered_map
adapter only has to restore the template argument order. A hash-ordered
ObjectType works everywhere except unflatten(), which reconstructs an array
only when it meets the reference token 0 before the other indices.
- ArrayType: std::deque works when wrapped to add capacity(); std::list does not.
- StringType: std::pmr::string and std::basic_string with a custom allocator
compile for the DOM, dump, and parse, but not for the binary readers, flatten,
or diff, because the library assigns std::string values to string_t and
int_to_string cannot be overloaded for a type in namespace std.
- NumberFloatType: long double works for dump and parse but not for the binary
formats, which have no encoding for it.
- BinaryType: std::vector<std::byte> supports assignment, get, and the binary
formats, but neither dump nor std::hash<basic_json>.
Also record the object_comparator_t fix in its version history.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
The requirements that basic_json places on its eleven template parameters
were only implied by how the library uses the resulting object_t, array_t,
string_t, etc. Consumers had to discover them by trial and error.
Add "Template Parameter Requirements" collecting them, split into what is
always required and what is only required when a particular part of the API
is instantiated. Notable findings that were previously undocumented:
- ObjectType must provide a key_compare member type (actual_object_comparator
names object_t::key_compare in both arms of a std::conditional), and its
third template parameter is used as a comparator, so std::unordered_map
cannot be used without a wrapper.
- ArrayType must provide capacity() -- push_back(), emplace_back(),
operator+=(), and operator[](size_type) call it unconditionally -- and
needs random-access iterators, so std::deque and std::list do not work.
- StringType needs contiguous, null-terminated data(), a one-byte value_type,
and either assignability from std::to_string or an ADL int_to_string().
- NumberFloatType must be float, double, or long double for parsing and
serialization; the integer types must satisfy std::is_integral.
- AllocatorType must be stateless, support incomplete types, and use plain
pointers.
- BooleanType and the number types are union members and must be trivial.
Link the new page from the basic_json overview, the types feature page, and
the individual type alias pages, and correct the container examples given for
ObjectType (std::unordered_map) and ArrayType (std::list), which do not work.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018hxZxz8svM54c6ATEvXp5E
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix discussion #4209: custom BinaryType direct assignment and extraction
When a custom BinaryType is configured (other than the default std::vector<uint8_t>),
users can now:
1. Assign values of that type directly to create binary values (not arrays)
2. Extract binary values back to that type with get<>()
3. Extract arrays to that type (for backward compatibility)
Implementation:
- Add is_compatible_binary_type trait to centralize SFINAE condition
- Update to_json to accept custom BinaryType values directly
- Update from_json to handle both binary and array inputs for custom BinaryType
- Add #include <vector> with IWYU comment to from_json.hpp
- Add comprehensive tests for assignment and array extraction
- Update binary_t documentation with example
This is purely additive and invisible to the default nlohmann::json alias, which
continues to treat std::vector<uint8_t> as arrays.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Fix CI: missing include and const-correctness
- Add #include <vector> to type_traits.hpp for the new
is_compatible_binary_type trait's std::vector<std::uint8_t> reference
(caught by cpplint's include-what-you-use check)
- Mark test-local json variables const where never reassigned
(caught by clang-tidy's misc-const-correctness check)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* 📝 Fix documentation gaps found in a full GitHub Discussions review
Reviewed all 1008 GitHub Discussions (2020-2026) for recurring questions
that better or more visible documentation would have avoided. Adds/expands
documentation for ~26 distinct gaps, including:
- New "Debugging" page collecting natvis, GDB pretty printer, LLDB status,
and JSON_DIAGNOSTICS pointers (previously scattered/undiscoverable)
- Thread-safety and schema-validation FAQ entries
- StringType's char-based requirement (no wstring/u16string/u32string)
- Brace-initialization-yields-arrays warning directly on the constructor
reference page (previously only in the FAQ, missed by users reading
the constructor docs)
- std::any exclusion from get<T>(), with a manual-dispatch example
- Non-string-keyed std::map serializing as an array of pairs
- ordered_json compatibility with NLOHMANN_DEFINE_TYPE_* macros
(already worked, was undocumented)
- std::array truncation on size-mismatched conversion (no exception)
- static_cast vs. get<std::optional<T>>() divergence
- Recipe for omitting a std::optional field instead of emitting null
- No built-in nesting-depth limit during parsing + a callback-based
workaround recipe
- Recipe for streaming a large homogeneous array via parser callbacks
- operator>> stream-position semantics for concatenated JSON values
- JSON Pointer array-vs-object creation rule for non-existing paths
- CMake target name (nlohmann_json_modules) needed to link C++20 modules
- ESP-IDF/PlatformIO: no official package, link to a community fork
- get(key, default) as the Python dict.get() equivalent
- reserve() recipe for pre-allocating array capacity
- JSONC as an alias for the existing ignore_comments/ignore_trailing_commas
combination (distinct from the unsupported JSON5)
- items() dereferenced-element type: decltype() idiom + detail-namespace
stability caveat
- Various macro/type-conversion limitations (MSGPACK_DEFINE_ARRAY
equivalent, char-array round-tripping, ADL serializer macro gap)
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* 🎨 fix format
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels LohmannandClaude Opus 5
Patrick10199