# Supported Macros - JSON for Modern C++
Author: Niels Lohmann

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# 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_CATCH_USER(exception)`

This macro overrides `catch` calls inside the library.

See full documentation of `JSON_CATCH_USER(exception)`.

## `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 the`parse` 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`

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`, and `JSON_HAS_CPP_20`.

## `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_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_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_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_THROW_USER(exception)`

This macro overrides `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 `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`.

## `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_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`.

## `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`.

November 12, 2025