# 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.

## JSON_CATCH_USER(exception)¶

This macro overrides catch calls inside the library.

## 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.

## JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20¶

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.

## 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.

## JSON_NOEXCEPTION¶

Exceptions can be switched off by defining the symbol 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.

## 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)).

## 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.

## JSON_SKIP_UNSUPPORTED_COMPILER_CHECK¶

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

## JSON_THROW_USER(exception)¶

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

## JSON_TRY_USER¶

This macro overrides try calls inside the library.

## JSON_USE_IMPLICIT_CONVERSIONS¶

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

## NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)¶

This macro can be used to simplify the serialization/deserialization of types if (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object.

The macro is to be defined inside the class/struct to create code for. Unlike NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, it can access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

## NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...)¶

This macro is similar to NLOHMANN_DEFINE_TYPE_INTRUSIVE. It will not throw an exception in from_json() due to a missing value in the JSON object, but can throw due to a mismatched type. The from_json() function default constructs an object and uses its values as the defaults when calling the value function.

## NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)¶

This macro can be used to simplify the serialization/deserialization of types if (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object.

The macro is to be defined inside the namespace of the class/struct to create code for. Private members cannot be accessed. Use NLOHMANN_DEFINE_TYPE_INTRUSIVE in these scenarios. The first parameter is the name of the class/struct, and all remaining parameters name the members.

## NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, member...)¶

This macro is similar to NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE. It will not throw an exception in from_json() due to a missing value in the JSON object, but can throw due to a mismatched type. The from_json() function default constructs an object and uses its values as the defaults when calling the value function.

## NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)¶

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

## 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.

Last update: December 19, 2022