json/docs/mkdocs/docs/features/macros.md

Supported Macros

Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the API documentation for macros for examples and more information.

JSON_ASSERT(x)

This macro controls which code is executed for runtime assertions of the library.

See full documentation of JSON_ASSERT(x).

JSON_BRACE_INIT_COPY_SEMANTICS

When defined to 1, single-element brace initialization of a basic_json value (e.g., #!cpp json j{value};) is treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is 0, which preserves the existing behavior.

See full documentation of JSON_BRACE_INIT_COPY_SEMANTICS.

JSON_CATCH_USER(exception)

This macro overrides #!cpp catch calls inside the library.

See full documentation of JSON_CATCH_USER(exception).

JSON_DELETE_DEPRECATED_FUNCTIONS

When defined to 1, all deprecated functions are declared as deleted instead of only being marked as deprecated, so code that still calls them no longer compiles. This way, you can find all calls that need to be replaced before version 4.0.0 removes these functions.

The macro can also be set with the CMake option JSON_DeleteDeprecatedFunctions (OFF by default).

See full documentation of JSON_DELETE_DEPRECATED_FUNCTIONS.

JSON_DIAGNOSTICS

This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable (default).

When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception, see Extended diagnostic messages for an example. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.

The diagnostics messages can also be controlled with the CMake option JSON_Diagnostics (OFF by default) which sets JSON_DIAGNOSTICS accordingly.

See full documentation of JSON_DIAGNOSTICS.

JSON_DIAGNOSTIC_POSITIONS

When enabled, two new member functions start_pos() and end_pos() are added to basic_json values. If the value was created by calling theparse function, then these functions allow querying the byte positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate errors.

The diagnostics positions can also be controlled with the CMake option JSON_Diagnostic_Positions (OFF by default) which sets JSON_DIAGNOSTIC_POSITIONS accordingly.

See full documentation of JSON_DIAGNOSTIC_POSITIONS

JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26

The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.

See full documentation of JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, and JSON_HAS_CPP_26.

JSON_HAS_FILESYSTEM, JSON_HAS_EXPERIMENTAL_FILESYSTEM

When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

See full documentation of JSON_HAS_FILESYSTEM and JSON_HAS_EXPERIMENTAL_FILESYSTEM.

JSON_HAS_RANGES

The library uses std::ranges (and concepts) where available, for example, to parse from C++20 ranges and to construct JSON arrays from range views. The library detects whether the standard library supports ranges and disables the support on toolchains with an incomplete implementation. To override the built-in check, define JSON_HAS_RANGES to 1 or 0.

See full documentation of JSON_HAS_RANGES.

JSON_HAS_RANGE_VIEW_CONVERSION

When JSON_HAS_RANGES is enabled (and the compiler is not MinGW), a JSON array can be constructed directly from a C++20 range view such as std::views::filter(...). To override the built-in check, define JSON_HAS_RANGE_VIEW_CONVERSION to 1 or 0.

See full documentation of JSON_HAS_RANGE_VIEW_CONVERSION.

JSON_HAS_STATIC_RTTI

The library detects whether the compiler supports run time type information (RTTI), which it needs, for instance, to exclude std::any from the candidate types of the implicit conversion on C++17. To override the built-in check, define JSON_HAS_STATIC_RTTI to 1 or 0.

See full documentation of JSON_HAS_STATIC_RTTI.

JSON_HAS_STD_FORMAT

When compiling with C++20 and a standard library that provides <format>, the library provides a std::formatter specialization for JSON values. To override the built-in check, define JSON_HAS_STD_FORMAT to 1 or 0.

See full documentation of JSON_HAS_STD_FORMAT.

JSON_HAS_THREE_WAY_COMPARISON

When the compiler and standard library support 3-way comparison (the spaceship operator <=>), the library provides it for JSON values. To override the built-in check, define JSON_HAS_THREE_WAY_COMPARISON to 1 or 0.

See full documentation of JSON_HAS_THREE_WAY_COMPARISON.

JSON_NOEXCEPTION

Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION.

See full documentation of JSON_NOEXCEPTION.

JSON_DISABLE_ENUM_SERIALIZATION

When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM.

See full documentation of JSON_DISABLE_ENUM_SERIALIZATION.

JSON_DISABLE_TUPLE_REFERENCE_CONVERSION

When defined to 1, a JSON value can no longer be created from a one-element std::tuple holding a reference to a JSON value, such as the result of std::forward_as_tuple(j). This lets std::tuple convert such tuples element-wise. This is planned to become the default in version 4.0.0.

See full documentation of JSON_DISABLE_TUPLE_REFERENCE_CONVERSION.

JSON_NO_AUTOMATIC_UDLS

When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp> with the user-defined string literals operator""_json and operator""_json_pointer. This reduces the compile time of translation units that do not use them, because the literals instantiate the parser in every translation unit that includes them. Include <nlohmann/json_literals.hpp> where the literals are needed.

See full documentation of JSON_NO_AUTOMATIC_UDLS.

JSON_NO_IO

When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).

See full documentation of JSON_NO_IO.

JSON_NO_THREAD_LOCAL

When defined, the library does not use #!cpp thread_local storage. Copying a value and comparing two values then always avoid the call stack rather than descending into a bounded number of levels first, which is slower but yields the same values and the same comparisons.

See full documentation of JSON_NO_THREAD_LOCAL.

JSON_PRECISE_STREAM_POSITION

When defined to 1, operator>> and non-strict sax_parse leave an input stream positioned right after the parsed value, instead of also consuming the character that terminates a number. The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

See full documentation of JSON_PRECISE_STREAM_POSITION.

JSON_SKIP_LIBRARY_VERSION_CHECK

When defined, the library will not create a compiler warning when a different version of the library was already included.

See full documentation of JSON_SKIP_LIBRARY_VERSION_CHECK.

JSON_SKIP_UNSUPPORTED_COMPILER_CHECK

When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.

See full documentation of JSON_SKIP_UNSUPPORTED_COMPILER_CHECK.

JSON_STRICT_BINARY_UTF8

When defined to 1, to_cbor, to_ubjson, to_bjdata, and to_bson throw type_error.316 for a string value or object key that is not valid UTF-8. The default value is 0, which writes the bytes unchanged as before version 3.13.0; this is planned to become the default in version 4.0.0.

The check can also be enabled with the CMake option JSON_StrictBinaryUTF8 (OFF by default) which sets JSON_STRICT_BINARY_UTF8 accordingly.

See full documentation of JSON_STRICT_BINARY_UTF8.

JSON_STRICT_NUL_HANDLING

When defined to 1, a '\0' (NUL) byte anywhere in the input is rejected with parse_error.101, like any other unexpected byte, instead of being silently treated as end of input (see the FAQ entry for background). The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

The strict handling can also be enabled with the CMake option JSON_StrictNulHandling (OFF by default) which sets JSON_STRICT_NUL_HANDLING accordingly.

See full documentation of JSON_STRICT_NUL_HANDLING.

JSON_THROW_USER(exception)

This macro overrides #!cpp throw calls inside the library. The argument is the exception to be thrown.

See full documentation of JSON_THROW_USER(exception).

JSON_TRY_USER

This macro overrides #!cpp try calls inside the library.

See full documentation of JSON_TRY_USER.

JSON_USE_IMPLICIT_CONVERSIONS

When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on.

See full documentation of JSON_USE_IMPLICIT_CONVERSIONS.

JSON_USE_GLOBAL_UDLS

When defined to 1 (default), the user-defined string literals operator""_json and operator""_json_pointer are placed into the global namespace instead of nlohmann::literals::json_literals.

See full documentation of JSON_USE_GLOBAL_UDLS.

JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON

When defined to 1, the library restores the legacy behavior in which a discarded value compared equal to itself. This behavior is deprecated and switched off (0) by default.

See full documentation of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.

JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS

When defined to 1, maps with enum keys (e.g., std::map<E, T>) are stored as objects, using the enum's conversion for the keys, instead of arrays of [key, value] pairs. It is switched off (0) by default.

See full documentation of JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS.

JSON_USE_SIMDUTF

When defined, UTF-8 validation of JSON strings read from contiguous byte input is delegated to the simdutf library instead of the built-in scalar validator. This is an opt-in external dependency and is not defined by default.

See full documentation of JSON_USE_SIMDUTF.

NLOHMANN_DEFINE_TYPE_*(...), NLOHMANN_DEFINE_DERIVED_TYPE_*(...)

The library defines 12 macros to simplify the serialization/deserialization of types. See the page on arbitrary type conversion for a detailed discussion.

NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END, NLOHMANN_JSON_NAMESPACE_NO_VERSION

These macros relate to the versioned, inline nlohmann namespace:

• NLOHMANN_JSON_NAMESPACE evaluates to the full name of the nlohmann namespace (including the inline ABI namespace).
• NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END open and close the namespace (for example, to add specializations).
• NLOHMANN_JSON_NAMESPACE_NO_VERSION, when defined to 1, omits the version component from the inline namespace.

See the nlohmann Namespace page, and the full documentation of NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END, and NLOHMANN_JSON_NAMESPACE_NO_VERSION.

NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)

This macro simplifies the serialization/deserialization of enum types. See Specializing enum conversion for more information.

See full documentation of NLOHMANN_JSON_SERIALIZE_ENUM.

A strict variant NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throws an exception on undefined input instead of falling back to the first mapping.

NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH

These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.

See full documentation of NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, and NLOHMANN_JSON_VERSION_PATCH.