Slice classes are mapped to Python classes with the same name. The generated class contains an attribute for each Slice data member (just as for structures and exceptions). Consider the following class definition:
class TimeOfDay {
short hour; // 0 ‑ 23
short minute; // 0 ‑ 59
short second; // 0 ‑ 59
string format(); // Return time as hh:mm:ss
};
class TimeOfDay(Ice.Object):
def __init__(self, hour=0, minute=0, second=0):
# ...
self.hour = hour
self.minute = minute
self.second = second
def ice_staticId():
return '::M::TimeOfDay'
ice_staticId = staticmethod(ice_staticId)
# ...
#
# Operation signatures.
#
# def format(self, current=None):
1. The generated class TimeOfDay inherits from
Ice.Object. This means that all classes implicitly inherit from
Ice.Object, which is the ultimate ancestor of all classes. Note that
Ice.Object is
not the same as
Ice.ObjectPrx. In other words, you
cannot pass a class where a proxy is expected and vice versa.
Like interfaces, classes implicitly inherit from a common base class, Ice.Object. However, as shown in
Figure 22.2, classes inherit from
Ice.Object instead of
Ice.ObjectPrx (which is at the base of the inheritance hierarchy for proxies). As a result, you cannot pass a class where a proxy is expected (and vice versa) because the base types for classes and proxies are not compatible.
Ice.Object contains a number of member functions:
class Object(object):
def ice_isA(self, id, current=None):
# ...
def ice_ping(self, current=None):
# ...
def ice_ids(self, current=None):
# ...
def ice_id(self, current=None):
# ...
def ice_staticId():
# ...
ice_staticId = staticmethod(ice_staticId)
def ice_preMarshal(self):
# ...
def ice_postUnmarshal(self):
# ...
The member functions of Ice.Object behave as follows:
Note that neither Ice.Object nor the generated class override
__hash__ and
__eq__, so the default implementations apply.
By default, data members of classes are mapped exactly as for structures and exceptions: for each data member in the Slice definition, the generated class contains a corresponding attribute.
Although Python provides no standard mechanism for restricting access to an object’s attributes, by convention an attribute whose name begins with an underscore signals the author’s intent that the attribute should only be accessed by the class itself or by one of its subclasses. You can employ this convention in your Slice classes using the
protected metadata directive. The presence of this directive causes the Slice compiler to prepend an underscore to the mapped name of the data member. For example, the
TimeOfDay class shown below has the
protected metadata directive applied to each of its data members:
class TimeOfDay {
["protected"] short hour; // 0 ‑ 23
["protected"] short minute; // 0 ‑ 59
["protected"] short second; // 0 ‑ 59
string format(); // Return time as hh:mm:ss
};
class TimeOfDay(Ice.Object):
def __init__(self, hour=0, minute=0, second=0):
# ...
self._hour = hour
self._minute = minute
self._second = second
# ...
#
# Operation signatures.
#
# def format(self, current=None):
For a class in which all of the data members are protected, the metadata directive can be applied to the class itself rather than to each member individually. For example, we can rewrite the
TimeOfDay class as follows:
["protected"] class TimeOfDay {
short hour; // 0 ‑ 23
short minute; // 0 ‑ 59
short second; // 0 ‑ 59
string format(); // Return time as hh:mm:ss
};
Classes have a constructor that assigns to each data member a default value appropriate for its type. You can also declare different default values for data members of primitive and enumerated types, as discussed in
Section 4.11.1.
For derived classes, the constructor has one parameter for each of the base class’s data members, plus one parameter for each of the derived class’s data members, in base-to-derived order.
Operations of classes are mapped to methods in the generated class. This means that, if a class contains operations (such as the
format operation of our
TimeOfDay class), you must provide an implementation of the operation in a class that is derived from the generated class. For example:
class TimeOfDayI(TimeOfDay):
def __init__(self, hour=0, minute=0, second=0):
TimeOfDay.__init__(self, hour, minute, second)
def format(self, current=None):
return "%02d:%02d:%02d" %\
(self.hour, self.minute, self.second)
A Slice class such as TimeOfDay that declares or inherits an operation is inherently abstract. Python does not support the notion of abstract classes or abstract methods, therefore the mapping merely summarizes the required method signatures in a comment for your convenience. Furthermore, the mapping generates code in the constructor of an abstract class to prevent it from being instantiated directly; any attempt to do so raises a
RuntimeError exception.
You may notice that the mapping for an operation adds an optional trailing parameter named
current. For now, you can ignore this parameter and pretend it does not exist. (We look at it in more detail in
Section 32.6.)
Having created a class such as this, we have an implementation and we can instantiate the
TimeOfDayI class, but we cannot receive it as the return value or as an out-parameter from an operation invocation. To see why, consider the following simple interface:
When a client invokes the get operation, the Ice run time must instantiate and return an instance of the
TimeOfDay class. However,
TimeOfDay is an abstract class that cannot be instantiated. Unless we tell it, the Ice run time cannot magically know that we have created a
TimeOfDayI class that implements the abstract
format operation of the
TimeOfDay abstract class. In other words, we must provide the Ice run time with a factory that knows that the
TimeOfDay abstract class has a
TimeOfDayI concrete implementation. The
Ice::Communicator interface provides us with the necessary operations:
module Ice {
local interface ObjectFactory {
Object create(string type);
void destroy();
};
local interface Communicator {
void addObjectFactory(ObjectFactory factory, string id);
ObjectFactory findObjectFactory(string id);
// ...
};
};
To supply the Ice run time with a factory for our TimeOfDayI class, we must implement the
ObjectFactory interface:
class ObjectFactory(Ice.ObjectFactory):
def create(self, type):
if type == M.TimeOfDay.ice_staticId():
return TimeOfDayI()
assert(False)
return None
def destroy(self):
# Nothing to do
pass
The object factory’s create method is called by the Ice run time when it needs to instantiate a
TimeOfDay class. The factory’s
destroy method is called by the Ice run time when its communicator is destroyed.
The create method is passed the type ID (see
Section 4.13) of the class to instantiate. For our
TimeOfDay class, the type ID is
"::M::TimeOfDay". Our implementation of
create checks the type ID: if it matches, the method instantiates and returns a
TimeOfDayI object. For other type IDs, the method asserts because it does not know how to instantiate other types of objects.
Note that we used the ice_staticId method to obtain the type ID rather than embedding a literal string. Using a literal type ID string in your code is discouraged because it can lead to errors that are only detected at run time. For example, if a Slice class or one of its enclosing modules is renamed and the literal string is not changed accordingly, a receiver will fail to unmarshal the object and the Ice run time will raise
NoObjectFactoryException. By using
ice_staticId instead, we avoid any risk of a misspelled or obsolete type ID, and we can discover at compile time if a Slice class or module has been renamed.
Given a factory implementation, such as our ObjectFactory, we must inform the Ice run time of the existence of the factory:
ic = ... # Get Communicator...
ic.addObjectFactory(ObjectFactory(),
M.TimeOfDay.ice_staticId())
Now, whenever the Ice run time needs to instantiate a class with the type ID "::M::TimeOfDay", it calls the
create method of the registered
ObjectFactory instance.
The destroy operation of the object factory is invoked by the Ice run time when the communicator is destroyed. This gives you a chance to clean up any resources that may be used by your factory. Do not call
destroy on the factory while it is registered with the communicator—if you do, the Ice run time has no idea that this has happened and, depending on what your
destroy implementation is doing, may cause undefined behavior when the Ice run time tries to next use the factory.
The run time guarantees that destroy will be the last call made on the factory, that is,
create will not be called concurrently with
destroy, and
create will not be called once
destroy has been called. However, calls to
create can be made concurrently.
Note that you cannot register a factory for the same type ID twice: if you call addObjectFactory with a type ID for which a factory is registered, the Ice run time throws an
AlreadyRegisteredException.
Finally, keep in mind that if a class has only data members, but no operations, you need not create and register an object factory to transmit instances of such a class. Only if a class has operations do you have to define and register an object factory.
