11 KiB
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 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.
!!! 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 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.
!!! warning
As this macro changes the definition of the `basic_json` object, it MUST be defined in the same way globally, even
across different compilation units; DO NOT link together code compiled with different definitions of
`JSON_DIAGNOSTICS` 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() 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)).
!!! info "Default behavior"
By default, the macro is not defined.
```cpp
#undef 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.
!!! info "Default behavior"
By default, the macro is not defined.
```cpp
#undef 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 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 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 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, 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_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 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_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 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.