reflection-traits.md

Reflection traits

jsoncons supports two kinds of reflection traits:

• json variant traits

• streaming traits

json variant traits

The json variant traits define a compile time template based interface for conversion between a basic_json value and a data structure of type T. They are used in basic_json template constructors, basic_json::is<T>, basic_json::as<T>, and basic_json::try_as<T> with the requirement that type T have json_traits defined.

jsoncons::reflect::json_traits

Since 1.4.0, jsoncons defines an interface for traits that support non-throwing conversions and uses-allocator construction. Until 1.9.0, these were named json_conv_traits, since 1.9.0, they have been renamed to json_traits. For backwards compatibility, the old name is aliased to the new name.

json_traits defaults to the legacy json_type_traits if a type conversion is undefined, these traits do not support non-throwing conversions and uses-allocator construction.

See Eigen::Matrix example for an example of specializing json_traits for an Eigen matrix class.

See User-allocator construction example for an example that illustrates uses-allocator construction for types with json_traits defined.

Legacy jsoncons::json_type_traits

json_type_traits defines a compile time template based interface for conversion between a basic_json value and a value of some other type.

See Eigen::Matrix example for an example of specializing json_type_traits for an Eigen matrix class.

Streaming traits

The decode streaming traits define a compile time template based interface for converting a stream of staj_events into a C++ data structure. They are used in the decode_<format> functions such as decode_json.

The encode streaming traits define a compile time template based interface for converting a C++ data structure into a character buffer or an output stream. They are used in the encode_<format> functions such as encode_json.

In the case that the jsoncons decode and encode traits have no specialization for type T, they fall back on the json variant traits.

decode_traits

decode_traits

encode_traits

encode_traits

Convenience macros

jsoncons includes some convenience macros for generating reflection traits classes. Until 1.4.0, these macros generated json_type_traits class templates. Since 1.4.0, they generate json_traits class templates, as well as some additional traits that support streaming.