18.11 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 represents the remote object. This makes the mapping easy and intuitive to use because, for all intents and purposes (apart from error semantics), making a remote procedure call is no different from making a local procedure call.

18.11.1 Proxy Classes and Proxy Protocols

On the client side, interfaces map to a protocol with member functions that corre­spond to the operations on those interfaces. Consider the following simple interface:

["objc:prefix:EX"]
module Example {
    interface Simple {
        void op();
    }
};

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

@interface EXSimplePrx : ICEObjectPrx
// Mapping‑internal methods here...
@end

@protocol EXSimplePrx <ICEObjectPrx>
‑(void) op;
‑(void) op:(ICEContext *)context;
@end;

As you can see, the compiler generates a proxy protocol EXSimplePrx and a proxy class EXSimplePrx. In general, the generated name for both protocol and class is <module-prefix><interface-name>Prx.

In the client’s address space, an instance of EXSimplePrx is the local ambassador for a remote instance of the Simple interface in a server and is known as a proxy class 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 EXSimplePrx derives from ICEObjectPrx, and that EXSimplePrx adopts the ICEObjectPrx protocol. This reflects the fact that all Slice interfaces implicitly derive from Ice::Object. For each operation in the interface, the proxy protocol has two methods whose name is derived from the operation. For the preceding example, we find that the operation op is mapped to two methods,  op and op:.

The second method has a trailing parameter of type ICEContext. This parameter is for use by the Ice run time to store information about how to deliver a request; normally, you do not need to supply a value here and can pretend that the trailing parameter does not exist. (We examine the ICEContext parameter in detail in Chapter 32. The parameter is also used by IceStorm—see Chapter 44.)

18.11.2 Instantiation and Casting

Client-side application code never manipulates proxy class instances directly. In fact, you are not allowed to instantiate a proxy class directly. Instead, 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.

Proxies are immutable: once the run time has instantiated a proxy, that proxy continues to denote the same remote object and cannot be changed. This means that, if you want to keep a copy of a proxy, it is sufficient to call retain on the proxy. (You can also call copy on a proxy because ICEObjectPrx implements NSCopying. However, calling copy has the same effect as calling retain.)

Proxies are always passed and returned as type id<<module‑prefix><interface-name>Prx>. For example, for the preceding Simple interface, the proxy type is id<EXSimplePrx>.

The ICEObjectPrx base class provides two class methods that allow you cast a proxy from one type to another.

• +(id) checkedCast:(id<ICEObjectPrx>)proxy;

A checkedCast tests whether the object denoted by a proxy implements the specified interface. If so, the cast returns a proxy to the specified interface; otherwise, if the object denoted by the proxy does not implement the specified interface, the cast returns nil. Checked casts are typically used to safely down-cast a proxy to a more derived interface. For example, assuming we have Slice interfaces Base and Derived, you can write the following:

id<EXBasePrx> base = ...;  // Initialize base proxy
id<EXDerivedPrx> derived = [EXDerivedPrx checkedCast:base];
if(derived != nil)
{
    // base implements run‑time type Derived
    // use derived...
} else {
    // Base has some other, unrelated type
}

The expression [EXDerivedPrx checkedCast:base] tests whether base points at an object of type Derived (or an object with a type that is derived from Derived). If so, the cast succeeds and derived is set to point at the same object as base. Otherwise, the cast fails and derived is set to nil. (Proxies that “point nowhere” are represented by nil.)

A checkedCast typically results in a remote message to the server.¹ The message effectively asks the server “is the object denoted by this proxy of type Derived?” The reply from the server is communicated to the application code in form of a successful (non-nil) or unsuccessful (nil) result. Sending a remote message is necessary because, as a rule, there is no way for the client to find out what the actual run-time type of a proxy is without confirmation from the server. (For example, the server may replace the implementation of the object for an existing proxy with a more derived one.) This means that you have to be prepared for a checkedCast to fail. For example, if the server is not running, you will receive an ICEConnectionRefusedException; if the server is running, but the object denoted by the proxy no longer exists, you will receive an ICEObjectNotExistException.

• +(id) uncheckedCast:(id<ICEObjectPrx>)proxy;

In some cases, it is known that an object supports a more derived interface than the static type of its proxy. For such cases, you can use an unchecked down-cast:

id<EXBasePrx> base;
base = ...;  // Initialize base to point at a Derived
id<EXDerivedPrx> derived = [EXDerivedPrx uncheckedCast:base];
// Use derived...

An uncheckedCast provides a down-cast without consulting the server as to the actual run-time type of the object. You should use an unchecked­Cast only if you are certain that the proxy indeed supports the more derived type: an uncheckedCast, as the name implies, is not checked in any way; it does not contact the object in the server and, if the proxy does not support the specified interface, the cast does not return null. If you use the proxy resulting from an incorrect uncheckedCast to invoke an operation, the behavior is undefined. Most likely, you will receive an ICEOperationNo­tExistException, but, depending on the circumstances, the Ice run time may also report an exception indicating that unmarshaling has failed, or even silently return garbage results.

Despite its dangers, uncheckedCast is still useful because it avoids the cost of sending a message to the server. And, particularly during initialization (see page 639), it is common to receive a proxy of type id<ICEOb­jectPrx>, but with a known run-time type. In such cases, an unchecked­Cast saves the overhead of sending a remote message.

Note that an uncheckedCast is not the same as an ordinary cast. The following is incorrect and has undefined behavior:

id<EXDerivedPrx> derived = (id<EXDerivedPrx>)base; // Wrong!

Both checkedCast and uncheckedCast call autorelease on the proxy they return so, if you want to prevent the proxy from being deallocated once the enclosing autorelease pool is drained, you must call retain on the returned proxy.

18.11.3 Using Proxy Methods

The ICEObjectPrx provides a variety of methods for customizing a proxy (see Section 32.11). 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 timeout as shown below:

id<ICEObjectPrx> proxy = [communicator stringToProxy:...];
proxy = [proxy ice_timeout:10000];

A factory method returns a new (autoreleased) proxy object if the requested modi­fication differs from the current proxy, otherwise it returns the original proxy. The returned proxy is always of the same type as the source proxy.

18.11.4 Object Identity and Proxy Comparison

Apart from the methods discussed in Section 18.11.2, proxy handles also support comparison with isEqual. Note that isEqual 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 as well. In other words, comparison with isEqual tests for proxy identity, not object identity. A common mistake is to write code along the following lines:

id<ICEObjectPrx> p1 = ...;      // Get a proxy...
id<ICEObjectPrx> p2 = ...;      // Get another proxy...

if (![p1 isEqual:p2]) {
    // p1 and p2 denote different objects       // WRONG!
} else {
    // p1 and p2 denote the same object         // Correct
}

Even though p1 and p2 differ, they may denote the same Ice object. This can happen if, for example, p1 and p2 embed the same object identity, but use a different protocol to contact the target object. Similarly, the protocols might be the same, but could 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 isEqual, we know that the two proxies denote the same object (because they are identical in all respects); however, if two proxies compare unequal with isEqual, we know absolutely nothing: the proxies may or may not denote the same object.

To compare the object identities of two proxies, you can use additional methods provided by proxies:

@protocol ICEObjectPrx <NSObject, NSCopying>
// ...
‑(NSComparisonResult) compareIdentity:(id<ICEObjectPrx>)p;
‑(NSComparisonResult) compareIdentityAndFacet:(id<ICEObjectPrx>)p;
@end

The compareIdentity method compares the object identities embedded in two proxies while ignoring other information, such as facet and transport informa­tion. To include the facet name (see Chapter 33) in the comparison, use compa­reIdentityAndFacet instead.

compareIdentity and compareIdentityAndFacet allow you to correctly compare proxies for object identity. The example below demonstrates how to use compareIdentity:

id<ICEObjectPrx> p1 = ...;      // Get a proxy...
id<ICEObjectPrx> p2 = ...;      // Get another proxy...

if ([p1 compareIdentity:p2] != NSOrderedSame) {
    // p1 and p2 denote different objects       // Correct
} else {
    // p1 and p2 denote the same object         // Correct
}

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1

In some cases, the Ice run time can optimize the cast and avoid sending a message. However, the optimization applies only in narrowly-defined circumstances, so you cannot rely on a checkedCast not sending a message.