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Supported Macros

Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header.

JSON_ASSERT(x)

The default value is assert(x).

JSON_CATCH_USER(exception)

This macro overrides 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 exceptions. The macro is always followed by a scope.

See Switch off exceptions for an example.

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_NOEXCEPTION

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

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

Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #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)).

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. Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.

See Switch off exceptions for an example.

JSON_TRY_USER

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

See 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:

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:

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.

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 of 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.

See Simplify your life with macros for an example.

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 of 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.

See Simplify your life with macros for an example.

NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)

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