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json/docs/mkdocs/docs/api/macros/index.md
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Niels Lohmann 1a48834d4e Scan json_view strings with SIMD and index large objects 
Speed up json_view's parser with SIMD scanning and a hash table
for large objects.

Long runs of string bytes are scanned 16 bytes at a time with NEON
(AArch64, GCC and Clang) and SSE2 (x86-64), both baseline
instruction sets. Keys keep 16 table checks before the vector
loop, because their lengths repeat from record to record; string
values get 8, because their lengths vary more. Non-ASCII text is
validated 16 bytes at a time with simdjson's "lookup4" check
(Keiser and Lemire, 2021), with NEON on AArch64 and, on x86-64,
with SSSE3. SSSE3 is not part of baseline x86-64, so the check is
compiled for SSSE3 with a function attribute and used only where
CPUID reports it, which all x86-64 CPUs since about 2011 do; the
answer is cached in a statically initialized atomic, so there is
no guard of a local static and no global constructor. The same
input is accepted either way. JSON_VIEW_NO_SIMD selects the
portable code.

On x86-64, string runs are now checked vector-first: one SSE2
compare from the first byte finds the end of most keys and short
values, instead of a branch per byte for the first 8-16 bytes.
AArch64 keeps the byte-wise steps, where a NEON mask costs more and
the branches predict well. Entering an object or array no longer
stalls: open() stores the parent's frame field by field instead of
building it on the stack and reading it back with wider loads,
which waited for the narrower stores to retire.

Objects with 128 members or more get an open-addressing hash table
built when the object closes, so operator[], at(), find(),
contains(), count(), value(), and JSON pointers take constant time
on average in such objects; of duplicate keys, the first is kept,
as for the linear search. The idea comes from Boost.JSON.

simdjson is credited in simd.hpp's SPDX block, the README, and
license.md.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2026-10-17 08:31:14 +02:00

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# Macros
Some aspects of the library can be configured by defining preprocessor macros **before** including the `json.hpp`
header. See also the [macro overview page](../../features/macros.md).
## Runtime assertions
- [**JSON_ASSERT(x)**](json_assert.md) - control behavior of runtime assertions
## Exceptions
- [**JSON_CATCH_USER(exception)**<br>**JSON_THROW_USER(exception)**<br>**JSON_TRY_USER**](json_throw_user.md) - control exceptions
- [**JSON_DIAGNOSTICS**](json_diagnostics.md) - control extended diagnostics
- [**JSON_DIAGNOSTIC_POSITIONS**](json_diagnostic_positions.md) - access positions of elements
- [**JSON_NOEXCEPTION**](json_noexception.md) - switch off exceptions
## Parsing
- [**JSON_PRECISE_STREAM_POSITION**](json_precise_stream_position.md) - opt in to leaving an input stream positioned
right after a parsed number
- [**JSON_STRICT_BINARY_UTF8**](json_strict_binary_utf8.md) - opt in to checking strings for valid UTF-8 in the CBOR,
UBJSON, BJData, and BSON writers
- [**JSON_STRICT_NUL_HANDLING**](json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input instead of
treating it as end of input
## Language support
- [**JSON_HAS_CPP_11**<br>**JSON_HAS_CPP_14**<br>**JSON_HAS_CPP_17**<br>**JSON_HAS_CPP_20**](json_has_cpp_11.md) - set supported C++ standard
- [**JSON_HAS_FILESYSTEM**<br>**JSON_HAS_EXPERIMENTAL_FILESYSTEM**](json_has_filesystem.md) - control `std::filesystem` support
- [**JSON_HAS_RANGE_VIEW_CONVERSION**](json_has_range_view_conversion.md) - control construction from `std::ranges` views
- [**JSON_HAS_RANGES**](json_has_ranges.md) - control `std::ranges` support
- [**JSON_HAS_STATIC_RTTI**](json_has_static_rtti.md) - control RTTI (run time type information) support
- [**JSON_HAS_STD_FORMAT**](json_has_std_format.md) - control `std::format`/`std::formatter` support
- [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
- [**JSON_NO_AUTOMATIC_UDLS**](json_no_automatic_udls.md) - do not include the user-defined string literals (UDLs) automatically
- [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
- [**JSON_NO_THREAD_LOCAL**](json_no_thread_local.md) - switch off the use of `thread_local` storage
- [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
- [**JSON_USE_GLOBAL_UDLS**](json_use_global_udls.md) - place user-defined string literals (UDLs) into the global namespace
- [**JSON_USE_SIMDUTF**](json_use_simdutf.md) - use the simdutf library to accelerate UTF-8 validation
- [**JSON_VIEW_NO_SIMD**](json_view_no_simd.md) - use only portable code in the parser of `json_view.hpp`
- [**JSON_VIEW_USE_SSSE3**](json_view_use_ssse3.md) - validate non-ASCII strings with SSSE3 in the parser of `json_view.hpp`
## Library version
- [**JSON_SKIP_LIBRARY_VERSION_CHECK**](json_skip_library_version_check.md) - skip library version check
- [**NLOHMANN_JSON_VERSION_MAJOR**<br>**NLOHMANN_JSON_VERSION_MINOR**<br>**NLOHMANN_JSON_VERSION_PATCH**](nlohmann_json_version_major.md)
\- library version information
## Library namespace
- [**NLOHMANN_JSON_NAMESPACE**](nlohmann_json_namespace.md) - full name of the `nlohmann` namespace
- [**NLOHMANN_JSON_NAMESPACE_BEGIN**<br>**NLOHMANN_JSON_NAMESPACE_END**](nlohmann_json_namespace_begin.md) - open and
close the library namespace
- [**NLOHMANN_JSON_NAMESPACE_NO_VERSION**](nlohmann_json_namespace_no_version.md) - disable the version component of
the inline namespace
## Type conversions
- [**JSON_BRACE_INIT_COPY_SEMANTICS**](json_brace_init_copy_semantics.md) - opt in to copy/move semantics for single-element brace initialization
- [**JSON_DISABLE_ENUM_SERIALIZATION**](json_disable_enum_serialization.md) - switch off default serialization/deserialization functions for enums
- [**JSON_DISABLE_TUPLE_REFERENCE_CONVERSION**](json_disable_tuple_reference_conversion.md) - switch off conversion from a one-element tuple of a JSON reference
- [**JSON_USE_IMPLICIT_CONVERSIONS**](json_use_implicit_conversions.md) - control implicit conversions
- [**JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS**](json_use_objects_for_enum_keyed_maps.md) - opt in to storing maps with enum
keys as objects
## Deprecated functions
- [**JSON_DELETE_DEPRECATED_FUNCTIONS**](json_delete_deprecated_functions.md) - opt in to deleting the deprecated
functions ahead of their removal in version 4.0.0
## Comparison behavior
- [**JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON**](json_use_legacy_discarded_value_comparison.md) -
control comparison of discarded values
## Serialization/deserialization macros
### Enums
- [**NLOHMANN_JSON_SERIALIZE_ENUM**](nlohmann_json_serialize_enum.md) - serialize/deserialize an enum
- [**NLOHMANN_JSON_SERIALIZE_ENUM_STRICT**](nlohmann_json_serialize_enum_strict.md) - serialize/deserialize an enum with exceptions
### Classes and structs
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE**](nlohmann_define_type_intrusive.md) - serialize/deserialize a non-derived class
with private members
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT**](nlohmann_define_type_intrusive.md) - serialize/deserialize a
non-derived class with private members; uses default values
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE**](nlohmann_define_type_intrusive.md) - serialize a non-derived class
with private members
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE**](nlohmann_define_type_non_intrusive.md) - serialize/deserialize a non-derived
class
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](nlohmann_define_type_non_intrusive.md) - serialize/deserialize a
non-derived class; uses default values
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](nlohmann_define_type_non_intrusive.md) - serialize a
non-derived class
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE**](nlohmann_define_derived_type.md) - serialize/deserialize a derived class
with private members
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT**](nlohmann_define_derived_type.md) - serialize/deserialize a
derived class with private members; uses default values
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE**](nlohmann_define_derived_type.md) - serialize a derived
class with private members
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE**](nlohmann_define_derived_type.md) - serialize/deserialize a derived
class
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT**](nlohmann_define_derived_type.md) - serialize/deserialize
a derived class; uses default values
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE**](nlohmann_define_derived_type.md) - serialize a derived
class
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a non-derived class
with private members; uses custom names
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a
non-derived class with private members; uses default values; uses custom names
- [**NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize a non-derived class
with private members; uses custom names
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a non-derived
class; uses custom names
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a
non-derived class; uses default values; uses custom names
- [**NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize a
non-derived class; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a derived class
with private members; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a
derived class with private members; uses default values; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize a derived
class with private members; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize a derived
class; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize/deserialize
a derived class; uses default values; uses custom names
- [**NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES**](nlohmann_define_type_with_names.md) - serialize a derived
class; uses custom names
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