Martchus 35850df15e | ||
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doc | ||
generator | ||
lib | ||
.gitignore | ||
CMakeLists.txt | ||
LICENSE | ||
README.md | ||
TODOs.md |
README.md
Reflective RapidJSON
The main goal of this project is to provide a code generator for serializing/deserializing C++ objects to/from JSON using Clang and RapidJSON.
However, extending the generator to generate code for other applications of reflection or to provide generic reflection would be possible as well.
Open for other reflection approaches
The reflection implementation used behind the scenes of this library is exchangeable:
- This repository already provides a small, additional header to use RapidJSON with Boost.Hana. This allows to
serialize or dezerialize simple data structures declared using the
BOOST_HANA_DEFINE_STRUCT
macro rather than requiring the code generator. - When native reflection becomes standardized, it would be possible to make use of it as well. In this case, the code generator could still act as a fallback.
Current state
The basic functionality is implemented, tested and documented:
- serialization and deserialization of datatypes listed above
- nesting and inheritance is possible
- basic error handling when deserializing
- CMake macro to conveniently include the code generator into the build process
- allow to use Boost.Hana
TODOs
There are still things missing which would likely be very useful in practise. The following list contains the most important TODOs:
- Allow to specify which member variables should be considered.
- This could work similar to Qt's Signals & Slots
- but there should also be a way to do this for 3rdparty types.
- Note that currently, all public member variables are (de)serialized.
- Support getter/setter methods
- Allow to serialize the result of methods.
- Allow to pass a deserialized value to a method.
- Validate enum values when deserializing.
Supported datatypes
The following table shows the mapping of supported C++ types to supported JSON types:
C++ type | JSON type |
---|---|
custom structures/classes | object |
bool |
true/false |
signed and unsigned integral types | number |
float and double |
number |
enum and enum class |
number |
std::string |
string |
const char * |
string |
iteratable lists (std::vector , std::list , ...) |
array |
std::tuple |
array |
std::unique_ptr , std::shared_ptr |
depends/null |
std::map , std::unordered_map |
object |
JsonSerializable |
object |
Remarks
- Raw pointer are not supported. This prevents forgetting to free memoery which would have to be allocated when deserializing.
- For the same reason
const char *
strings are only supported for serialization. - Enums are (de)serialized as their underlying integer value. When deserializing, it is currently not checked whether the present integer value is a valid enumeration item.
- The JSON type for smart pointer depends on the type the pointer refers to. It can also be
null
. - For deserialization
- iteratables must provide an
emplace_back
method. So deserialization of eg.std::forward_list
is currently not supported. - custom types must provide a default constructor.
- constant member variables are skipped.
- iteratables must provide an
- For custom (de)serialization, see the section below.
Usage
This example shows how the library can be used to make a struct
serializable:
#include <reflective-rapidjson/json/serializable.h>
// define structures, eg.
struct TestObject : public JsonSerializable<TestObject> {
int number;
double number2;
vector<int> numbers;
string text;
bool boolean;
};
struct NestingObject : public JsonSerializable<NestingObject> {
string name;
TestObject testObj;
};
struct NestingArray : public JsonSerializable<NestingArray> {
string name;
vector<TestObject> testObjects;
};
// serialize to JSON
NestingArray obj{ ... };
cout << "JSON: " << obj.toJson().GetString();
// deserialize from JSON
const auto obj = NestingArray::fromJson(...);
// in exactly one of the project's translation units
#include "reflection/code-defining-structs.h"
Note that the header included at the bottom must be generated by invoking the code generator appropriately, eg.:
reflective_rapidjson_generator -i "$srcdir/code-defining-structs.cpp" -o "$builddir/reflection/code-defining-structs.h"
Invoking code generator with CMake macro
It is possible to use the provided CMake macro to automate this task:
find_package(reflective-rapidjson REQUIRED)
list(APPEND CMAKE_MODULE_PATH ${REFLECTIVE_RAPIDJSON_MODULE_DIRS})
include(ReflectionGenerator)
add_reflection_generator_invocation(
INPUT_FILES code-defining-structs.cpp
GENERATORS json
OUTPUT_LISTS LIST_OF_GENERATED_HEADERS
)
This will produce the file code-defining-structs.h
in the directory reflection
in the current build directory. So
make sure the current build directory is added to the include directories of your target. The default output directory can
also be overridden by passing OUTPUT_DIRECTORY custom/directory
to the arguments.
It is possible to specify multiple input files at once. A separate output file is generated for each input. The output files
will always have the extension ".h
", independently of the extension of the input file.
The full paths of the generated files are also appended to the variable LIST_OF_GENERATED_HEADERS
which then can be added
to the sources of your target. Of course this can be skipped if not required/wanted.
Passing Clang options
It is possible to pass additional options to the Clang tool invocation used by the code generator.
This can be done using the --clang-opt
argument or the CLANG_OPTIONS
argument when using the CMake macro.
It makes most sense to specify the same options as during compilation so the code generator uses the same flags,
defines and include directories as the compiler and hence behaves like the compiler.
When using the CMake macro it is possible to automatically pass all compile flags, compile definitions and include directories
from certain targets to the code generator. The targets can ge specified using the CLANG_OPTIONS_FROM_TARGETS
argument.
Using Boost.Hana instead of the code generator
The same example as above. However, this time Boost.Hana is used - so it doesn't require invoking the generator.
#include "<reflective-rapidjson/json/serializable-boosthana.h>
// define structures using BOOST_HANA_DEFINE_STRUCT, eg.
struct TestObject : public JsonSerializable<TestObject> {
BOOST_HANA_DEFINE_STRUCT(TestObject,
(int, number),
(double, number2),
(vector<int>, numbers),
(string, text),
(bool, boolean)
);
};
struct NestingObject : public JsonSerializable<NestingObject> {
BOOST_HANA_DEFINE_STRUCT(NestingObject,
(string, name),
(TestObject, testObj)
);
};
struct NestingArray : public JsonSerializable<NestingArray> {
BOOST_HANA_DEFINE_STRUCT(NestingArray,
(string, name),
(vector<TestObject>, testObjects)
);
};
// serialize to JSON
NestingArray obj{ ... };
cout << "JSON: " << obj.toJson().GetString();
// deserialize from JSON
const auto obj = NestingArray::fromJson(...);
So beside the BOOST_HANA_DEFINE_STRUCT
macro, the usage remains the same.
Disadvantages
- Use of ugly macro required
- No context information for errors like type-mismatch available
- Inherited members not considered
- Support for enums is unlikely
- Attempt to access private members can not be prevented
Enable reflection for 3rd party classes/structs
It is obvious that the previously shown examples do not work for classes defined in 3rd party header files as it requires adding an additional base class.
To work around this issue, one can use the REFLECTIVE_RAPIDJSON_MAKE_JSON_SERIALIZABLE
macro. It will enable the toJson
and fromJson
methods for the specified class
in the ReflectiveRapidJSON::JsonReflector
namespace:
// somewhere in included header
struct ThridPartyStruct
{ ... };
// somewhere in own header or source file
REFLECTIVE_RAPIDJSON_MAKE_JSON_SERIALIZABLE(ThridPartyStruct)
// (de)serialization
ReflectiveRapidJSON::JsonReflector::toJson(...).GetString();
ReflectiveRapidJSON::JsonReflector::fromJson<ThridPartyStruct>("...");
The code generator will emit the code in the same way as if JsonSerializable
was
used.
By the way, the functions in the ReflectiveRapidJSON::JsonReflector
namespace can also
be used when inheriting from JsonSerializable
(instead of the member functions).
(De)serializing private members
By default, private members are not considered for (de)serialization. However, it is possible
to enable this by adding friend
methods for the helper functions of Reflective RapidJSON.
To make things easier, there's a macro provided:
struct SomeStruct : public JsonSerializable<SomeStruct> {
REFLECTIVE_RAPIDJSON_ENABLE_PRIVATE_MEMBERS(SomeStruct);
public:
std::string publicMember = "will be (de)serialized anyways";
private:
std::string privateMember = "will be (de)serialized with the help of REFLECTIVE_RAPIDJSON_ENABLE_PRIVATE_MEMBERS macro";
};
Caveats
- It will obviously not work for 3rd party structs.
- This way to allow (de)serialization of private members must be applied when using Boost.Hana and there are any private members present. The reason is that accessing the private members can currently not prevented when using Boost.Hana.
Custom (de)serialization
Sometimes it is appropriate to implement custom (de)serialization. For instance, a custom object representing a time value should likey be serialized as a string rather than an object with the internal data members.
An example for such custom (de)serialization can be found in the file
json/reflector-chronoutilities.h
. It provides (de)serialization of DateTime
and
TimeSpan
objects from the C++ utilities library.
Remarks
- Static member variables are currently ignored by the generator.
- It is currently not possible to ignore a specific member.
Further examples
- Checkout the test cases for further examples. Relevant files are in
the directories
lib/tests
andgenerator/tests
. - There's also my tag editor, which uses Reflective RapidJSON to provide a JSON export. See json.h and mainfeatures.cpp#exportToJson.
Architecture
The following diagram gives an overview about the architecture of the code generator and wrapper library around RapidJSON:
- blue: classes from LibTooling/Clang
- grey: conceivable extension or use
Install instructions
Dependencies
The following dependencies are required at build time. Note that Reflective RapidJSON itself and none of these dependencies are required at runtime by an application which makes use of Reflective RapidJSON.
- C++ compiler and C++ standard library supporting at least C++14
- the CMake build system
- LibTooling from Clang for the code generator (optional when using Boost.Hana)
- RapidJSON for JSON (de)serialization
- C++ utilities for various helper functions
Optional
- Boost.Hana for using
BOOST_HANA_DEFINE_STRUCT
instead of code generator - CppUnit for building and running the tests
- Doxygen for generating API documentation
- Graphviz for diagrams in the API documentation
Remarks
- It is not required to use CMake as build system for your own project. However, when using a different build system, there is no helper for adding the code generator to the build process provided (so far).
- I usually develop using the latest version of those dependencies. So it is recommend to get the
the latest versions as well. I tested the following versions so far:
- GCC 7.2.1 or Clang 5.0 as compiler
- libstdc++ from GCC 7.2.1
- CMake 3.10.1
- Clang 5.0.0/5.0.1 for LibTooling
- RapidJSON 1.1.0
- C++ utilities 4.12
- Boost.Hana 1.65.1 and 1.66.0
- CppUnit 1.14.0
- Doxygen 1.8.13
- Graphviz 2.40.1
How to build
1. Install dependencies
Install all required dependencies. Under a typical GNU/Linux system most of these dependencies can be installed via the package manager. Otherwise follow the links in the "Dependencies" section above.
C++ utilities is likely not available as package. However, it is possible to build C++ utilities
together with reflective-rapidjson
to simplify the build process. The following build script makes
use of this. (To use system C++ utilities, just skip any lines with "c++utilities
" in the following
examples.)
2. Make dependencies available
When installing (some) of the dependencies at custom locations, it is likely neccassary to tell CMake where to find them. If you installed everything using packages provided by the system, you can skip this step of course.
To specify custom locations, just set some environment variables before invoking CMake. This can likely be done in your IDE settings and of course at command line. Here is a Bash example:
export PATH=$CUSTOM_INSTALL_PREFIX/bin:$PATH
export CMAKE_PREFIX_PATH=$CUSTOM_INSTALL_PREFIX:$CMAKE_PREFIX_PATH
export CMAKE_LIBRARY_PATH=$CUSTOM_INSTALL_PREFIX/lib:$CMAKE_LIBRARY_PATH
export CMAKE_INCLUDE_PATH=$CUSTOM_INSTALL_PREFIX/include:$CMAKE_INCLUDE_PATH
There are also a lot of useful variables that can be specified as CMake arguments. It is also possible to create a toolchain file.
3. Get sources, eg. using Git:
cd $SOURCES
git clone https://github.com/Martchus/cpp-utilities.git c++utilities
git clone https://github.com/Martchus/reflective-rapidjson.git
If you don't want to build the development version, just checkout the desired version tag.
4. Run the build script
Here is an example for building with GNU Make:
cd $BUILD_DIR
# generate Makefile
cmake \
-DCMAKE_BUILD_TYPE:STRING=Release \
-DCMAKE_INSTALL_PREFIX:PATH="/final/install/prefix" \
-DBUNDLED_CPP_UTILITIES_PATH:PATH="$SOURCES/c++utilities" \
"$SOURCES/reflective-rapidjson"
# build library and generators
make
# build and run tests (optional, requires CppUnit)
make check
# build tests but do not run them (optional, requires CppUnit)
make tests
# generate API documentation (optional, reqquires Doxygen)
make apidoc
# install header files, libraries and generator
make install DESTDIR="/temporary/install/location"
Add eg. -j$(nproc)
to make
arguments for using all cores.
Packages
I currently only provide an Arch Linux package for the current Git version. This package shows the required dependencies and commands to build in a plain way. So it might be useful for making Reflective RapidJSON available under other platforms, too.