Rice is a C++ interface to Ruby's C API. It provides a type-safe and exception-safe interface in order to make embedding Ruby and writing Ruby extensions with C++ easier. It is similar to Boost.Python in many ways, but also attempts to provide an object-oriented interface to all of the Ruby C API.
What Rice gives you:
The source is hosted on github: http://github.com/jasonroelofs/rice
Bug tracking: http://github.com/jasonroelofs/rice/issues
Mailing List: rice@.firstname.lastname@example.org@m.elist.nosp@m..com (your first email will be used as a subscription request and dropped)
Building it locally from a clone of the repository is as follows:
Rice is known to work on *nix and OSX. Windows is not currently supported.
Rice does not work with any Ruby compiled with the Falcon performans patches as they make changes to some internals which Rice relies on.
Also Rice requires a Ruby built with –enable-shared and will not install properly against a Ruby with only static libraries.
Writing an extension with Rice is very similar to writing an extension with the C API.
The first step is to create an extconf.rb file:
Note that we use mkmf-rice instead of mkmf. This will ensure that the extension will be linked with standard C++ library along with the Rice library, and allow access to the Rice header files.
Next we create our extension and save it to test.cpp:
Note the extern "C" line above. This tells the compiler that the function Init_Test should have C linkage and calling convention. This turns off name mangling so that the Ruby interpreter will be able to find the function (remember that Ruby is written in C, not C++).
So far we haven't put anything into the extension, so it isn't particularly useful. The next step is to define a class so we can add methods to it.
Defining a class in Rice is easy:
This will create a class called Test that inherits from Object. If we wanted to inherit from a different class, we could easily do so:
Note the prefix rb_c on the name of the class. This is a convention that the Ruby interpreter and many extensions tend to use. It signifies that this is a class and not some other type of object. Some other naming conventions that are commonly used:
Also note that we don't include "ruby.h" directly. Rice has a wrapper for ruby.h that handles some compatibility issues across platforms and Ruby versions. Always include Rice headers before including anything that might include "ruby.h".
Now let's add a method to our class:
Here we add a method Test::hello that simply returns the string "Hello, World". The method takes self as an implicit parameter, but isn't used, so we comment it out to prevent a compiler warning.
We could also add an #initialize method to our class:
The initialize method sets an instance variable to the value 42. The number is automatically converted to a Fixnum before doing the assignment.
Note that we're chaining calls on the Class object. Most member functions in Module and Class return a reference to self, so we can chain as many calls as we want to define as many methods as we want.
It's useful to be able to define Ruby classes in a C++ style rather than using the Ruby API directly, but the real power Rice is in wrapping already-defined C++ types.
Let's assume we have the following C++ class that we want to wrap:
This is a C++ version of the Ruby class we just created in the previous section. To wrap it:
This example is similar to the one before, but we use Data_Type<> instead of Class and the template version of define_class() instead of the non-template version. This creates a binding in the Rice library between the Ruby class Test and the C++ class Test, so that we pass member function pointers to define_method() and have conversions be done automatically.
It's possible to write the conversion functions ourself (as we'll see below), but Rice does all the dirty work for us.
Let's look again at our example class:
When we wrote our class, we never wrote a single line of code to convert the std::string returned by hello() into a Ruby type. Neverthless, the conversion works, and when we write:
We get the expected result.
Rice has two template conversion functions to convert between C++ and Ruby types:
Rice has included by default specializations for many of the builtin types. To define your own conversion, you can write a specialization:
The implementation of these functions would, of course, depend on the implementation of Foo.
Take another look at the wrapper we wrote for the Test class:
When we called define_class<Test>, it created a Class for us and automatically registered the new Class with the type system, so that the calls:
work as expected.
The Data_Object class is a wrapper for the Data_Wrap_Struct and the Data_Get_Struct macros in C extensions. It can be used to wrap or unwrap any class that has been assigned to a Data_Type. It inherits from Object, so any member functions we can call on an Object we can also call on a Data_Object:
The Data_Object class can be used to wrap a newly-created object:
or to unwrap an already-created object:
A Data_Object functions like a smart pointer:
Like a VALUE or an Object, data stored in a Data_Object will be marked by the garbage collector as long as the Data_Object is on the stack.
Suppose we added a member function to our example class that throws an exception:
If we were to wrap this function:
and call it from inside Ruby:
we would get an exception. Rice will automatically convert any C++ exception it catches into a Ruby exception. But what if we wanted to use a custom eror message when we convert the exception, or what if we wanted to convert to a different type of exception? We can write this:
The handle_my_exception function need only rethrow the exception as a Rice::Exception:
And what if we want to call Ruby code from C++? These exceptions are also converted:
Internally whenever Rice catches a C++ or a Ruby exception, it converts it to an Exception object. This object will later be re-raised as a Ruby exception when control is returned to the Ruby VM.
Rice uses a similar class called Jump_Tag to handle symbols thrown by Ruby's throw/catch or other non-local jumps from inside the Ruby VM.
You've seen this example:
Rice mimics the Ruby class hierarchy as closely as it can given that C++ is statically typed. In fact, the above code also works for Classes:
Rice provides builtin wrappers for many builtin Ruby types, including:
The Array and Hash types can even be iterated over the same way one would iterate over an STL container:
STL algorithms should also work as expected on Array and Hash containers.
Inheritance is a tricky problem to solve in extensions. This is because wrapper functions for base classes typically don't know how to accept pointers to derived classes. It is possible to write this logic, but the code is nontrivial.
Forunately Rice handles this gracefully:
The second template parameter to define_class indicates that Derived inherits from Base.
Rice does not yet support multiple inheritance, but it is believed that this is possible through the use of mixins.
If you try to create a member function pointer to an overloaded function, you will get an error. So how do we wrap classes that have overloaded functions?
Consider a class that uses this idiom for accessors:
We can wrap this class by using typedefs:
A future version of Rice may provide a simplified interface for this.
Rice provides default conversions for many built-in types. Sometimes, however, the default conversion is not their right conversion. For example, consider a function:
Is x a pointer to a single character or a pointer to the first character of a null-terminated string or a pointer to the first character of an array of char?
Because the second case is the most common use case (a pointer to the first character of a C string), Rice provides a default conversion that treats a char * as a C string. But suppose the above function takes a pointer to a char instead?
If we write this:
It will likely have the wrong behavior.
To avoid this problem, it is necessary to write a wrapper function:
Note that the out parameter is returned from wrap_foo, as Ruby does not have pass-by-variable-reference (it uses pass-by-object-reference).
Future versions of Rice will have a cleaner way of dealing with this.
Going back to our initial C++ class example, lets say that hello() now take a few arguments for what to return, one which has a default value:
As default parameter information is not available through templates, it's necessary to define this in Rice explicitly using Rice::Arg:
The syntax here is simply Arg(nameOfParameter)[ = defaultValue]. The name of the parameter is not important (more for readability, and the future for when/if Ruby gets named parameters), but the value set via operator= must match the type of the given parameter.
These Rice::Arg objects must be in the correct order, and if there are more than one of them they must be surrounded in parentheses, as above, or the compilation will fail.
It may be required to explicitly cast the default argument values to their appropriate types:
With this, Ruby will now know about the default arguments, and this wrapper can be used as expected:
This will also work with Constructors:
As polymorphism is the most important tennant of Object Oriented Programming, it's important that Rice supports polymorphic calls travelling between C++ and Ruby seemlessly. Super calls from Ruby subclasses back into C++ already work, but enabling the other direction requires some extra work. While this isn't something Rice can do on it's own, the Rice::Director class, coupled with Rice::Data_Type::define_director exposes this functionality cleanly.
Like SWIG_Director, Rice::Director is a class that is used to build a proxy class to properly send execution up or down the object heiarchy for that class. Take the following class:
Due to the abstract nature of this class, it will not work at all with Rice in it's current form. Any attempt to do so will cause a compilation error due to this class not being constructable. Even without the pure virtual function, any call to VirtualBase::doWork will stop at the C++ level and will not pass down into any Ruby subclasses.
To properly wrap both of these methods, you'll need to build a proxy class that subclasses Rice::Director along with a few methods:
There is a lot going on here, so we'll go through each part.
First, the class needs to subclass both the virtual class and Rice::Director class.
For Rice::Director to work its magic, every instance of this class needs to have a handle to the Ruby instance of this class as well. The constructor must take a Rice::Object as the first argument, then any other arguments follow and should be passed back to the superclass as needed. The code here is the minimum required for a Rice::Director proxy.
The two methods seen here directly correspond to the two code directions this class opens up. The virtual method is this class's hook into C++'s polymorphism. Any calls that need to be forwarded into Ruby are done as specified here: get the Ruby object for the instance of this class, call Rice::Object::call, and if necessary convert the return value from Ruby back into C++ types.
The default_doWork method will be used as Rice's hookup of calling back up the heirarchy (wrapping is below). This method needs to do one of two things: call up the class heirarchy, as seen here, or call raisePureVirtual() as seen in the processWorker example:
The method raisePureVirtual() exists to allow wrapping a pure virtual method into Ruby but making sure any users of this extension are informed quickly that there's nothing in the C++ to call for the given method.
Once the proxy class is built, it's time to wrap it into Ruby:
The wrapping is the same as is described earlier in this document. Expose the class VirtualBase, and register VirtualBaseProxy as a director proxy of VirtualBase with Rice::Data_Type::define_director, then define methods pointing to the proxy object as necessary.
You must use the Rice::Director proxy class in the Constructor line, this allows proper object construction / destruction of the types in question.
There are times when a library exposes classes that while unrelated are built to be interchangeable across the library. One example of this, taken from the Open Source 3d rendering engine OGRE, are the Degree and Radian classes. When a given method takes a Radian, you're free to pass in a Degree, and vice versa.
Rice cannot automatically figure out if this kind of functionality is possible in a given library but it does have a simple API for defining these relationships: Rice::define_implicit_cast<From, To>().
This support is still being fleshed out and has a few requirements for proper use:
To see a full example of this feature, please check out test/test_Data_Type.cpp.
There are a number of common problems when writing C or C++ extensions for Ruby:
Rice addresses these issues in many ways:
There are a number projects which server similar functions to Rice. Two such popular projects are SWIG and Boost.Python. Rice has some distinct features which set it apart from both of these projects.
Rice is not trying to replace SWIG. Rice is not a generic wrapper interface generator. Rice is a C++ library for interfacing with the Ruby C API. This provides a very natural way for C++ programmers to wrap their C++ code, without having to learn a new domain-specific language. However, there is no reason why SWIG and Rice could not work together; a SWIG module could be written to generate Rice code. Such a module would combine the portability of SWIG with the maintainability of Rice (I have written extensions using both, and I have found Rice extensions to be more maintainable when the interface is constantly changing. Your mileage may vary).
Rice is also not trying to simply be a Ruby version of Boost.Python. Rice does use some of the same template tricks that Boost.Python uses, however there are some important distinctions. First of all, Boost.Python attempts to create a declarative DSL in C++ using templates. Rice is a wrapper around the Ruby C API and attempts to make its interface look like an OO version of the API; this means that class declarations look procedural rather than declarative. Secondly, the Ruby object model is different from the python object model. This is reflected in the interface to Rice; it mimics the Ruby object model at the C++ level. Thirdly, Rice uses Ruby as a code generator; I find this to be much more readable than using the Boost preprocessor library.
Rice originated as a project to interface with C++-based trading software at Automated Trading Desk in Mount Pleasant, South Carolina. The Ruby bindings for Swig were at the time less mature than they are today, and did not suit the needs of the project.
Excruby was written not as a wrapper for the Ruby API, but rather as a set of helper functions and classes for interfacing with the Ruby interpreter in an exception-safe manner. Over the course of five years, the project grew into wrappers for pieces of the API, but the original helper functions remained as part of the public interface.
This created confusion for the users of the library, because there were multiple ways of accomplishing most tasks – directly through the C API, through a low-level wrapper around the C API, and through a high-level abstraction of the lower-level interfaces.
Rice was then born in an attempt to clean up the interface. Rice keeps the lower-level wrappers, but as an implementation detail; the public interface is truly a high-level abstraction around the Ruby C API.
You can embed the Ruby interpter in your application by using the VM class:
If the VM is not initialized from main() – from a callback, for example – then you may need to initialize the stack whenever you use Rice or the Ruby API:
Be aware that initializing the Ruby VM can cause a call to exit() if certain command-line options are specified. This has two implications: