100 lines
5.8 KiB
Markdown
100 lines
5.8 KiB
Markdown
# Supported Macros
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Some aspects of the library can be configured by defining preprocessor macros before including the `json.hpp` header.
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## `JSON_ASSERT(x)`
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The default value is `#!cpp assert(x)`.
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## `JSON_CATCH_USER(exception)`
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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`](../home/exceptions.md#out-of-range) exceptions. The macro is always followed by a scope.
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See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.
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## `JSON_DIAGNOSTICS`
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This macro enables extended diagnostics for exception messages. Possible values are `1` to enable or `0` to disable (default).
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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.
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The diagnostics messages can also be controlled with the CMake option `JSON_Diagnostics` (`OFF` by default) which sets `JSON_DIAGNOSTICS` accordingly.
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## `JSON_HAS_CPP_11`, `JSON_HAS_CPP_14`, `JSON_HAS_CPP_17`, `JSON_HAS_CPP_20`
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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.
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## `JSON_NOEXCEPTION`
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Exceptions can be switched off by defining the symbol `JSON_NOEXCEPTION`.
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When defining `JSON_NOEXCEPTION`, `#!cpp try` is replaced by `#!cpp if (true)`,
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`#!cpp catch` is replaced by `#!cpp if (false)`, and `#!cpp throw` is replaced by `#!cpp std::abort()`.
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The same effect is achieved by setting the compiler flag `-fno-exceptions`.
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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).
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## `JSON_NO_IO`
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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)).
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## `JSON_SKIP_UNSUPPORTED_COMPILER_CHECK`
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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.
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## `JSON_THROW_USER(exception)`
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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.
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See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.
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## `JSON_TRY_USER`
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This macro overrides `#!cpp try` calls inside the library. It has no arguments and is always followed by a scope.
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See [Switch off exceptions](../home/exceptions.md#switch-off-exceptions) for an example.
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## `JSON_USE_IMPLICIT_CONVERSIONS`
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When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on.
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??? example
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This is an example for an implicit conversion:
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```cpp
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json j = "Hello, world!";
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std::string s = j;
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```
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When `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`, the code above does no longer compile. Instead, it must be written like this:
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```cpp
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json j = "Hello, world!";
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auto s = j.get<std::string>();
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```
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Implicit conversions can also be controlled with the CMake option `JSON_ImplicitConversions` (`ON` by default) which sets `JSON_USE_IMPLICIT_CONVERSIONS` accordingly.
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## `NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)`
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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.
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The macro is to be defined inside of the class/struct to create code for. Unlike [`NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`](#nlohmann_define_type_non_intrusivetype-member), it can access private members.
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The first parameter is the name of the class/struct, and all remaining parameters name the members.
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See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.
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## `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)`
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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.
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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`](#nlohmann_define_type_intrusivetype-member) in these scenarios.
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The first parameter is the name of the class/struct, and all remaining parameters name the members.
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See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.
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## `NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)`
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This macro simplifies the serialization/deserialization of enum types. See [Specializing enum conversion](enum_conversion.md) for more information.
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