MATLAB Mapping for Interfaces

The mapping of Slice interfaces revolves around the idea that, to invoke a remote operation, you call a member function on a local class instance that is a proxy for the remote object. This makes the mapping easy and intuitive to use because making a remote procedure call is no different from making a local procedure call (apart from error semantics).

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Proxy Interfaces in MATLAB

A Slice interface maps to a MATLAB class with methods that correspond to the operations on that interface. Consider the following simple interface:

Slice
interface Simple
{
    void op();
}

The Slice compiler generates the following definition for use by the client:

MATLAB
classdef SimplePrx < Ice.ObjectPrx
    methods
        function op(obj, varargin)
            ...
        end
    end
    methods(Static)
        function id = ice_staticId()
            ...
        end
        function r = checkedCast(proxy, varargin)
            ...
        end
        function r = uncheckedCast(proxy, varargin)
            ...
        end
    end
    ...
end

As you can see, the compiler generates a proxy class SimplePrx. In general, the generated name is <interface-name>Prx. If an interface is nested in a module M, the generated class is part of package M, so the fully-qualified name is M.<interface-name>Prx.

In the client's address space, an instance of SimplePrx is the local ambassador for a remote instance of the Simple interface in a server and is known as a proxy instance. All the details about the server-side object, such as its address, what protocol to use, and its object identity are encapsulated in that instance.

Note that SimplePrx inherits from ObjectPrx. This reflects the fact that all Ice interfaces implicitly inherit from Ice::Object.

For each operation in the interface, the proxy class has a method of the same name. For the preceding example, we find that the operation op has been mapped to the method op. Notice that the op method accepts a variable argument list. This allows you to specify an optional request context of type containers.Map. This parameter is for use by the Ice run time to store information about how to deliver a request. You normally do not need to use it.

Proxy instances are always instantiated on behalf of the client by the Ice run time, so client code never has any need to instantiate a proxy directly.

An empty array denotes the null proxy. The null proxy is a dedicated value that indicates that a proxy points "nowhere" (denotes no object).

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Interface Inheritance in MATLAB

Inheritance relationships among Slice interfaces are maintained in the generated MATLAB classes. For example:

Slice
interface A { ... }
interface B { ... }
interface C extends A, B { ... }

The generated code for CPrx reflects the inheritance hierarchy:

MATLAB
classdef CPrx < APrx & BPrx
    ...
end

Given a proxy for C, a client can invoke any operation defined for interface C, as well as any operation inherited from C's base interfaces.

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The ObjectPrx Interface in MATLAB

All Ice objects have Object as the ultimate ancestor type, so all proxies inherit from ObjectPrx. ObjectPrx provides a number of methods:

MATLAB
classdef ObjectPrx < handle
    methods
        function r = eq(obj, other)
        function r = ice_getIdentity(obj)
        function r = ice_isA(obj, id, varargin)
        function r = ice_ids(obj, varargin)
        function r = ice_id(obj, varargin)
        function ice_ping(obj, varargin)
        % ...
    end
end

The methods behave as follows:

  • eq
    This method allows two proxies to be compared for equality using the == operator. Note that all aspects of proxies are compared by this operation, such as the communication endpoints for the proxy. This means that, in general, if two proxies compare unequal, that does not imply that they denote different objects. For example, if two proxies denote the same Ice object via different transport endpoints, equals returns false even though the proxies denote the same object.

  • ice_getIdentity
    This method returns the identity of the object denoted by the proxy. The identity of an Ice object has the following Slice type:

    Slice
    module Ice
{
    struct Identity
    {
        string name;
        string category;
    }
}

    To see whether two proxies denote the same object, first obtain the identity for each object and then compare the identities:

    MATLAB
    proxy1 = ...;
proxy2 = ...;
id1 = proxy1.ice_getIdentity();
id2 = proxy2.ice_getIdentity();

if isequal(id1, id2)
    % proxy1 and proxy2 denote the same object
else
    % proxy1 and proxy2 denote different objects
end

  • ice_isA
    The ice_isA method determines whether the object denoted by the proxy supports a specific interface. The argument to ice_isA is a type ID. For example, to see whether a proxy of type ObjectPrx denotes a Printer object, we can write:

    MATLAB
    proxy = ...;
if ~isempty(proxy) && proxy.ice_isA("::Printer")
    % o denotes a Printer object
else
    % o denotes some other type of object
end

    Note that we are testing whether the proxy is null before attempting to invoke the ice_isA method. This avoids getting an error if the proxy is an empty array.

  • ice_ids
    The ice_ids method returns an array of strings representing all of the type IDs that the object denoted by the proxy supports.
  • ice_id
    The ice_id method returns the type ID of the object denoted by the proxy. Note that the type returned is the type of the actual object, which may be more derived than the static type of the proxy. For example, if we have a proxy of type BasePrx, with a static type ID of ::Base, the return value of ice_id might be ::Base, or it might something more derived, such as ::Derived.
  • ice_ping
    The ice_ping method provides a basic reachability test for the object. If the object can physically be contacted (that is, the object exists and its server is running and reachable), the call completes normally; otherwise, it throws an exception that indicates why the object could not be reached, such as ObjectNotExistException or ConnectTimeoutException.

The ice_isA, ice_ids, ice_id, and ice_ping methods are remote operations and therefore support an optional request context. Also note that there are other methods in ObjectPrx, not shown here. These methods provide different ways to dispatch a call and also provide access to an object's facets.

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Proxy Helper Methods in MATLAB

As we saw earlier, the Slice-to-MATLAB compiler generates static helper methods that support down-casting and type discovery:

MATLAB
classdef SimplePrx < Ice.ObjectPrx
    ...
    methods(Static)
        function id = ice_staticId()
            ...
        end
        function r = checkedCast(proxy, varargin)
            ...
        end
        function r = uncheckedCast(proxy, varargin)
            ...
        end
    end
    ...
end

For checkedCast, if the passed proxy is for an object of type Simple, or a proxy for an object with a type derived from Simple, the cast returns a non-null reference to a proxy of type SimplePrx; otherwise, if the passed proxy denotes an object of a different type (or if the passed proxy is null), the cast returns an empty array. You can optionally supply a facet and a request context, in that order; if you supply a request context, you must also supply a facet.

Given a proxy of any type, you can use a checkedCast to determine whether the corresponding object supports a given type, for example:

MATLAB
proxy = ...;        % Get a proxy from somewhere...

simple = SimplePrx.checkedCast(proxy);
if ~isempty(simple)
    % Object supports the Simple interface...
else
    % Object is not of type Simple...
end

Note that a checkedCast contacts the server. This is necessary because only the implementation of an object in the server has definite knowledge of the type of an object. As a result, a checkedCast may throw a ConnectTimeoutException or an ObjectNotExistException.

In contrast, an uncheckedCast does not contact the server and unconditionally returns a proxy of the requested type. However, if you do use an uncheckedCast, you must be certain that the proxy really does support the type you are casting to; otherwise, if you get it wrong, you will most likely get a run-time exception when you invoke an operation on the proxy. The most likely error for such a type mismatch is OperationNotExistException. However, other exceptions, such as a marshaling exception are possible as well. And, if the object happens to have an operation with the correct name, but different parameter types, no exception may be reported at all and you simply end up sending the invocation to an object of the wrong type; that object may do rather nonsensical things. To illustrate this, consider the following two interfaces:

Slice
interface Process
{
    void launch(int stackSize, int dataSize);
}

// ...

interface Rocket
{
    void launch(float xCoord, float yCoord);
}

Suppose you expect to receive a proxy for a Process object and use an uncheckedCast to down-cast the proxy:

MATLAB
proxy = ...;                                % Get proxy...
process = ProcessPrx.uncheckedCast(proxy);  % No worries...
process.launch(40, 60);                     % Oops...

If the proxy you received actually denotes a Rocket object, the error will go undetected by the Ice run time: because int and float have the same size and because the Ice protocol does not tag data with its type on the wire, the implementation of Rocket::launch will simply misinterpret the passed integers as floating-point numbers.

In fairness, this example is somewhat contrived. For such a mistake to go unnoticed at run time, both objects must have an operation with the same name and, in addition, the run-time arguments passed to the operation must have a total marshaled size that matches the number of bytes that are expected by the unmarshaling code on the server side. In practice, this is extremely rare and an incorrect uncheckedCast typically results in a run-time exception.

You may optionally supply a facet argument to uncheckedCast.

Another method generated for every interface is ice_staticId, which returns the type ID string corresponding to the interface. As an example, for the Slice interface Simple in module M, the string returned by ice_staticId is "::M::Simple".

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Using Proxy Methods in MATLAB

The base proxy class ObjectPrx supports a variety of methods for customizing a proxy. Since proxies are immutable, each of these "factory methods" returns a copy of the original proxy that contains the desired modification. For example, you can obtain a proxy configured with a ten second invocation timeout as shown below:

MATLAB
proxy = communicator.stringToProxy(...);
proxy = proxy.ice_invocationTimeout(10000);

A factory method returns a new proxy object if the requested modification differs from the current proxy, otherwise it returns the current proxy. With few exceptions, factory methods return a proxy of the same type as the current proxy, therefore it is generally not necessary to repeat a checkedCast or uncheckedCast after using a factory method. Furthermore, the mapping generates type-specific factory methods so that no casts are necessary:

MATLAB
base = communicator.stringToProxy(...);
hello = HelloPrx.checkedCast(base);
hello = hello.ice_invocationTimeout(10000); % No cast is necessary
hello.sayHello();

The only exceptions are the factory methods ice_facet and ice_identity. Calls to either of these methods may produce a proxy for an object of an unrelated type, therefore they return a base proxy that you must subsequently down-cast to an appropriate type using checkedCast or uncheckedCast.

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Object Identity and Proxy Comparison in MATLAB

Proxies can be compared for equality using the == operator. Note that proxy comparison with == uses all of the information in a proxy for the comparison. This means that not only the object identity must match for a comparison to succeed, but other details inside the proxy, such as the protocol and endpoint information, must be the same. In other words, comparison with == tests for proxy identity, not object identity. A common mistake is to write code along the following lines:

MATLAB
proxy1 = ...;        % Get a proxy...
proxy2 = ...;        % Get another proxy...

if proxy1 == proxy2
    % proxy1 and proxy2 denote the same object         Correct
else
    % proxy1 and proxy2 denote different objects       WRONG!
end

Even though proxy1 and proxy2 differ, they may still denote the same Ice object. This can happen because, for example, both proxy1 and proxy2 embed the same object identity, but each use a different protocol to contact the target object. Similarly, the protocols may be the same, but denote different endpoints (because a single Ice object can be contacted via several different transport endpoints). In other words, if two proxies compare equal with ==, we know that the two proxies denote the same object (because they are identical in all respects); however, if two proxies compare unequal with ==, the proxies may or may not denote the same object.

To compare the object identities of two proxies, you can use helper functions in the Ice package:

MATLAB
function r = proxyIdentityCompare(lhs, rhs)
function r = proxyIdentityAndFacetCompare(lhs, rhs)

proxyIdentityCompare allows you to correctly compare proxies for identity:

MATLAB
proxy1 = ...;        % Get a proxy...
proxy2 = ...;        % Get another proxy...

if Ice.proxyIdentityCompare(p1, p2) == 0
    % proxy1 and proxy2 denote the same object         Correct
else
    % proxy1 and proxy2 denote different objects       Correct
end

The function returns 0 if the identities are equal, -1 if p1 is less than p2, and 1 if p1 is greater than p2. (The comparison uses name as the major and category as the minor sort key.)

The proxyIdentityAndFacetCompare function behaves similarly, but compares both the identity and the facet name.

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See Also