json/doc/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.

## `JSON_ASSERT(x)`

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

!!! info "Default behavior"

    The default value is [`#!cpp assert(x)`](https://en.cppreference.com/w/cpp/error/assert).

    ```cpp
    #define JSON_ASSERT(x) assert(x)
    ```

## `JSON_CATCH_USER(exception)`

This macro overrides [`#!cpp catch`](https://en.cppreference.com/w/cpp/language/try_catch) calls inside the library.
The argument is the type of the exception to catch. As of version 3.8.0, the library only catches `std::out_of_range`
exceptions internally to rethrow them as [`json::out_of_range`](../home/exceptions.md#out-of-range) exceptions. The
macro is always followed by a scope.

See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.

!!! info "Default behavior"

    When exceptions are enabled, the default value is
    [`#!cpp catch(exception)`](https://en.cppreference.com/w/cpp/language/try_catch).

    ```cpp
    #define JSON_CATCH_USER(exception) catch(exception)
    ```

    When exceptions are switched off by the compiler, the default value is `#!cpp if (false)` to make the catch block
    unreachable.

    ```cpp
    #define JSON_CATCH_USER(exception) if (false)
    ```

## `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](json_pointer.md) to the JSON value that triggered the
exception, see [Extended diagnostic messages](../home/exceptions.md#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.

WARNING: As this macro changes the definition of the json object, it MUST be defined in the same way globally, even across different compilation units; do NOT link together code compiled with and without JSON_DIAGNOSTICS defined as this is a violation of the One Definition Rule and will cause undefined behaviour.

!!! info "Default behavior"

    ```cpp
    #define JSON_DIAGNOSTICS 0
    ```

## `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.

!!! info "Default behavior"

    The default value is detected based on the preprocessor macros `#!cpp __cplusplus`, `#!cpp _HAS_CXX17`, or
    `#!cpp _MSVC_LANG`.

## `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`.

!!! info "Default behavior"

    The default value is detected based on the preprocessor macros `#!cpp __cpp_lib_filesystem`,
    `#!cpp __cpp_lib_experimental_filesystem`, `#!cpp __has_include(<filesystem>)`, or
    `#!cpp __has_include(<experimental/filesystem>)`.

Note that older compilers or older versions of libstd++ also require the library `stdc++fs` to be linked to for
filesystem support.

## `JSON_NOEXCEPTION`

Exceptions can be switched off by defining the symbol `JSON_NOEXCEPTION`. When defining `JSON_NOEXCEPTION`, `#!cpp try`
is replaced by `#!cpp if (true)`, `#!cpp catch` is replaced by `#!cpp if (false)`, and `#!cpp throw` is replaced by
`#!cpp std::abort()`.

!!! info "Default behavior"

    By default, the macro is not defined.

    ```cpp
    #undef JSON_NOEXCEPTION
    ```

The same effect is achieved by setting the compiler flag `-fno-exceptions`.

Note the explanatory [`what()`](https://en.cppreference.com/w/cpp/error/exception/what) string of exceptions is not
available for MSVC if exceptions are disabled, see [#2824](https://github.com/nlohmann/json/discussions/2824).

## `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)).

!!! info "Default behavior"

    By default, the macro is not defined.

    ```cpp
    #undef JSON_NO_IO
    ```

## `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.

!!! info "Default behavior"

    By default, the macro is not defined.

    ```cpp
    #undef JSON_SKIP_UNSUPPORTED_COMPILER_CHECK
    ```

## `JSON_THROW_USER(exception)`

This macro overrides `#!cpp throw` calls inside the library. The argument is the exception to be thrown. Note that
`JSON_THROW_USER` should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield
undefined behavior.

!!! info "Default behavior"

    When exceptions are enabled, the default value is
    [`#!cpp throw exception`](https://en.cppreference.com/w/cpp/language/throw).

    ```cpp
    #define JSON_THROW_USER(exception) throw exception
    ```

    When exceptions are switched off by the compiler, the default value is
    [`#!cpp std::abort()`](https://en.cppreference.com/w/cpp/utility/program/abort) to make reaching the throw branch
    abort the process.

    ```cpp
    #define JSON_THROW_USER(exception) std::abort()
    ```

See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.

## `JSON_TRY_USER`

This macro overrides `#!cpp try` calls inside the library. It has no arguments and is always followed by a scope.

!!! info "Default behavior"

    When exceptions are enabled, the default value is
    [`#!cpp try`](https://en.cppreference.com/w/cpp/language/try_catch).

    ```cpp
    #define JSON_TRY_USER try
    ```

    When exceptions are switched off by the compiler, the default value is `#!cpp if (true)` to unconditionally execute
    the following code block.

    ```cpp
    #define JSON_TRY_USER if (true)
    ```

See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.

## `JSON_USE_IMPLICIT_CONVERSIONS`

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

??? example

    This is an example for an implicit conversion:

    ```cpp
    json j = "Hello, world!";
    std::string s = j;
    ```

    When `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`, the code above does no longer compile. Instead, it must be
    written like this:

    ```cpp
    json j = "Hello, world!";
    auto s = j.get<std::string>();
    ```

Implicit conversions can also be controlled with the CMake option `JSON_ImplicitConversions` (`ON` by default) which
sets `JSON_USE_IMPLICIT_CONVERSIONS` accordingly.

!!! info "Default behavior"

    ```cpp
    #define JSON_USE_IMPLICIT_CONVERSIONS 1
    ```

## `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`](#nlohmann_define_type_non_intrusivetype-member), it can access private members.
The first parameter is the name of the class/struct, and all remaining parameters name the members.

See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.

## `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. In order to support that it requires that the type be default constructible. 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`](#nlohmann_define_type_intrusivetype-member) in these scenarios. The
first parameter is the name of the class/struct, and all remaining parameters name the members.

See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.

## `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. In order to support that it requires that the type be default constructible. 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](enum_conversion.md) 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](https://semver.org).