# SampleBinding Example

The example showcases how you can generate CPython-based binding code for a C++ library using Shiboken. The C++ library is called `Universe`, with two classes: `Icecream` and `Truck`. Ice creams are characterized by their flavor, and `Truck` serves as a vehicle of `Icecream` distribution for kids in a neighborhood.

First, let’s look at the definition of the two classes:

icecream.h

     class Icecream
     {
     public:
         Icecream(const std::string &flavor);
         virtual Icecream *clone();
         virtual ~Icecream();
         virtual const std::string getFlavor();

     private:
         std::string m_flavor;
     };

truck.h

     class Truck {
     public:
         Truck(bool leaveOnDestruction = false);
         Truck(const Truck &other);
         Truck& operator=(const Truck &other);
         ~Truck();

         void addIcecreamFlavor(Icecream *icecream);
         void printAvailableFlavors() const;

         bool deliver() const;
         void arrive() const;
         void leave() const;

         void setLeaveOnDestruction(bool value);
         void setArrivalMessage(const std::string &message);

     private:
         void clearFlavors();

         bool m_leaveOnDestruction = false;
         std::string m_arrivalMessage = "A new icecream truck has arrived!\n";
         std::vector m_flavors;
     };

Here is a summary of what the `Universe` library includes:

- The `Icecream` polymorphic type, which is intended to be overridden.

- The `Icecream::getFlavor()` method returns the flavor depending on the actual derived type.

- The `Truck` value type that contains pointers, hence the copy constructor.

- `Truck` stores the `Icecream` objects in a vector, which can be modified via `Truck::addIcecreamFlavor()`.

- The `Truck’s` arrival message can be customized using its `setArrivalMessage()` method.

- The `Truck::deliver()` method tells us if the ice cream delivery was successful.

## Shiboken typesystem

Now that the library definitions are in place, Shiboken generator needs a header file that includes the types we are interested in:

bindings.h

     #ifndef BINDINGS_H
     #define BINDINGS_H
     #include "icecream.h"
     #include "truck.h"
     #endif // BINDINGS_H

In addition, Shiboken also requires an XML-based typesystem file that defines the relationship between C++ and Python types:

bindings.xml

     <?xml version="1.0"?>
     <typesystem package="Universe">
         <primitive-type name="bool"/>
         <primitive-type name="std::string"/>
         <object-type name="Icecream">
             <modify-function signature="clone()">
                 <modify-argument index="0">
                     <define-ownership owner="c++"/>
                 </modify-argument>
             </modify-function>
         </object-type>
         <value-type name="Truck">
             <modify-function signature="addIcecreamFlavor(Icecream*)">
                 <modify-argument index="1">
                     <define-ownership owner="c++"/>
                 </modify-argument>
             </modify-function>
         </value-type>
     </typesystem>

The first important thing to notice here is that we declare `"bool"` and `"std::string"` as primitive types. These types are used by some of the C++ methods as parameters or return types, so Shiboken must know about them. It can then generate relevant conversion code between C++ and Python, although most C++ primitive types are handled by Shiboken without additional code.

Next, we declare the two aforementioned classes. One of them as an “object-type” and the other as a “value-type”. The main difference is that object-types are passed around in generated code as pointers, whereas value-types are copied (value semantics).

By specifying the names of these classes in the typesystem file, Shiboken automatically tries to generate bindings for all methods of those classes. You need not mention all the methods manually in the XML file, unless you want to modify them.

## Object ownership rules

Shiboken cannot magically know who is responsible for freeing the C++ objects allocated in the Python code. It can guess, but it’s not always correct. There can be cases where Python should release the C++ memory when the ref count of the Python object becomes zero. It should never delete the C++ object assuming that it will not be deleted by the C++ library or maybe it’s parented to another object (like QWidgets).

In our case, the `clone()` method is only called inside the C++ library, and we assume that the C++ code takes care of releasing the cloned object.

As for `addIcecreamFlavor()`, we know that a `Truck` owns the `Icecream` object, and will remove it once the `Truck` is destroyed. That’s why the ownership is set to “c++” in the typesystem file, so that the C++ objects are not deleted when the corresponding Python names go out of scope.

## Building

To build the `Universe` custom library and then generate bindings for it, use the `CMakeLists.txt` file provided with the example. You can reuse the file for your own libraries with minor changes.

Now, run the command "cmake ." from the prompt to configure the project and build with the toolchain of your choice (we recommend the ‘(N)Makefiles’ generator though).

As a result, you end up with two shared libraries: `libuniverse.(so/dylib/dll)` and `Universe.(so/pyd)`. The former is the custom C++ library, and the latter is the Python module that can be imported in your Python script.

Refer to the `README.md` file for more details about the Windows-specific build instructions.

## Using the Python module

The following script uses the `Universe` module, derives a few types from `Icecream`, implements virtual methods, instantiates objects, and much more:

main.py

     from Universe import Icecream, Truck

     class VanillaChocolateIcecream(Icecream):
         def __init__(self, flavor=""):
             super(VanillaChocolateIcecream, self).__init__(flavor)

         def clone(self):
             return VanillaChocolateIcecream(self.getFlavor())

         def getFlavor(self):
             return "vanilla sprinked with chocolate"

     class VanillaChocolateCherryIcecream(VanillaChocolateIcecream):
         def __init__(self, flavor=""):
             super(VanillaChocolateIcecream, self).__init__(flavor)

         def clone(self):
             return VanillaChocolateCherryIcecream(self.getFlavor())

         def getFlavor(self):
             base_flavor = super(VanillaChocolateCherryIcecream, self).getFlavor()
             return base_flavor + " and a cherry"

     if __name__ == '__main__':
         leave_on_destruction = True
         truck = Truck(leave_on_destruction)

         flavors = ["vanilla", "chocolate", "strawberry"]
         for f in flavors:
             icecream = Icecream(f)
             truck.addIcecreamFlavor(icecream)

         truck.addIcecreamFlavor(VanillaChocolateIcecream())
         truck.addIcecreamFlavor(VanillaChocolateCherryIcecream())

         truck.arrive()
         truck.printAvailableFlavors()
         result = truck.deliver()

         if result:
             print("All the kids got some icecream!")
         else:
             print("Aww, someone didn't get the flavor they wanted...")

         if not result:
             special_truck = Truck(truck)
             del truck

             print("")
             special_truck.setArrivalMessage("A new SPECIAL icecream truck has arrived!\n")
             special_truck.arrive()
             special_truck.addIcecreamFlavor(Icecream("SPECIAL *magical* icecream"))
             special_truck.printAvailableFlavors()
             special_truck.deliver()
             print("Now everyone got the flavor they wanted!")
             special_truck.leave()

After importing the classes from the `Universe` module, it derives two types from `Icecream` for different “flavors”. It then creates a `truck` to deliver some regular flavored Icecreams and two special ones.

If the delivery fails, a new `truck` is created with the old flavors copied over, and a new magical flavor that will surely satisfy all customers.

The script above shows how to derive from C++ types, override virtual methods, create and destroy objects, and more. Try running it to see if the ice creams are delivered.

Note

You can find the sources for this example under `<PYTHON_ENV_ROOT>/site-packages/lib/PySide2/examples/samplebinding`.

Refer to the following topics for detailed information about using Shiboken:

  - Shiboken module

  - Type System Variables

  - User Defined Type Conversion

  - Object ownership

  - Frequently Asked Questions