Having created a servant class such as the rudimentary NodeI class, you can instantiate the class to create a concrete servant that can receive invocations from a client. However, merely instantiating a servant class is insufficient to incarnate an object. Specifically, to provide an implementation of an Ice object, you must follow the following steps:

1. Instantiate a servant class.
2. Create an identity for the Ice object incarnated by the servant.
3. Inform the Ice run time of the existence of the servant.
4. Pass a proxy for the object to a client so the client can reach it.

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Instantiating a C++ Servant

Instantiating a servant means to allocate an instance on the heap:

NodePtr servant = new NodeI("Fred");

This code creates a new NodeI instance on the heap and assigns its address to a smart pointer of type NodePtr. This works because NodeI is derived from Node, so a smart pointer of type NodePtr can also look after an instance of type NodeI. However, if we want to invoke a member function of the derived NodeI class at this point, we have a problem: we cannot access member functions of the derived NodeI class through a NodePtr smart pointer, only member functions of Node base class. (The C++ type rules prevent us from accessing a member of a derived class through a pointer to a base class.) To get around this, we can modify the code as follows:

typedef IceUtil::Handle<NodeI> NodeIPtr;
NodeIPtr servant = new NodeI("Fred");

This code makes use of the smart pointer template by defining NodeIPtr as a smart pointer to NodeI instances. Whether you use a smart pointer of type NodePtr or NodeIPtr depends solely on whether you want to invoke a member function of the NodeI derived class; if you only want to invoke member functions that are defined in the Node skeleton base class, it is sufficient to use a NodePtr and you need not define the NodeIPtr type.

Whether you use NodePtr or NodeIPtr, the advantages of using a smart pointer class should be obvious from the smart pointer discussion: they make it impossible to accidentally leak memory.

Creating an Identity in C++

Each Ice object requires an identity. That identity must be unique for all servants using the same object adapter.

An Ice object identity is a structure with the following Slice definition:

module Ice {
    struct Identity {
        string name;
        string category;
    };
    // ...
};

The full identity of an object is the combination of both the name and category fields of the Identity structure. For now, we will leave the category field as the empty string and simply use the name field. (The category field is most often used in conjunction with servant locators.)

To create an identity, we simply assign a key that identifies the servant to the name field of the Identity structure:

Ice::Identity id;
id.name = "Fred"; // Not unique, but good enough for now

Activating a C++ Servant

Merely creating a servant instance does nothing: the Ice run time becomes aware of the existence of a servant only once you explicitly tell the object adapter about the servant. To activate a servant, you invoke the add operation on the object adapter. Assuming that we have access to the object adapter in the _adapter variable, we can write:

_adapter->add(servant, id);

Note the two arguments to add: the smart pointer to the servant and the object identity. Calling add on the object adapter adds the servant pointer and the servant's identity to the adapter's servant map and links the proxy for an Ice object to the correct servant instance in the server's memory as follows:

1. The proxy for an Ice object, apart from addressing information, contains the identity of the Ice object. When a client invokes an operation, the object identity is sent with the request to the server.
2. The object adapter receives the request, retrieves the identity, and uses the identity as an index into the servant map.
3. If a servant with that identity is active, the object adapter retrieves the servant pointer from the servant map and dispatches the incoming request into the correct member function on the servant.

Assuming that the object adapter is in the active state, client requests are dispatched to the servant as soon as you call add.

Servant Life Time and Reference Counts

Putting the preceding points together, we can write a simple function that instantiates and activates one of our NodeI servants. For this example, we use a simple helper function called activateServant that creates and activates a servant with a given identity:

void
activateServant(const string& name)
{
    NodePtr servant = new NodeI(name);          // Refcount == 1
    Ice::Identity id;
    id.name = name;
    _adapter->add(servant, id);                 // Refcount == 2
}                                               // Refcount == 1

Note that we create the servant on the heap and that, once activateServant returns, we lose the last remaining handle to the servant (because the servant variable goes out of scope). The question is, what happens to the heap-allocated servant instance? The answer lies in the smart pointer semantics:

• When the new servant is instantiated, its reference count is initialized to 0.
• Assigning the servant's address to the servant smart pointer increments the servant's reference count to 1.
• Calling add passes the servant smart pointer to the object adapter which keeps a copy of the handle internally. This increments the reference count of the servant to 2.
• When activateServant returns, the destructor of the servant variable decrements the reference count of the servant to 1.

The net effect is that the servant is retained on the heap with a reference count of 1 for as long as the servant is in the servant map of its object adapter. (If we deactivate the servant, that is, remove it from the servant map, the reference count drops to zero and the memory occupied by the servant is reclaimed; we discuss these life cycle issues in Object Life Cycle.)

UUIDs as Identities in C++

The Ice object model assumes that object identities are globally unique. One way of ensuring that uniqueness is to use UUIDs (Universally Unique Identifiers) as identities. The IceUtil namespace contains a helper function to create such identities:

#include <IceUtil/UUID.h>
#include <iostream>

using namespace std;

int
main()
{
    cout << IceUtil::generateUUID() << endl;
}

When executed, this program prints a unique string such as 5029a22c-e333-4f87-86b1-cd5e0fcce509. Each call to generateUUID creates a string that differs from all previous ones.

You can use a UUID such as this to create object identities. For convenience, the object adapter has an operation addWithUUID that generates a UUID and adds a servant to the servant map in a single step. Using this operation, we can rewrite the code shown earlier like this:

void
activateServant(const string& name)
{
    NodePtr servant = new NodeI(name);
    _adapter->addWithUUID(servant);
}

Creating Proxies in C++

Once we have activated a servant for an Ice object, the server can process incoming client requests for that object. However, clients can only access the object once they hold a proxy for the object. If a client knows the server's address details and the object identity, it can create a proxy from a string, as we saw in our first example in Hello World Application. However, creation of proxies by the client in this manner is usually only done to allow the client access to initial objects for bootstrapping. Once the client has an initial proxy, it typically obtains further proxies by invoking operations.

The object adapter contains all the details that make up the information in a proxy: the addressing and protocol information, and the object identity. The Ice run time offers a number of ways to create proxies. Once created, you can pass a proxy to the client as the return value or as an out-parameter of an operation invocation.

Proxies and Servant Activation in C++

The add and addWithUUID servant activation operations on the object adapter return a proxy for the corresponding Ice object. This means we can write:

typedef IceUtil::Handle<NodeI> NodeIPtr;
NodeIPtr servant = new NodeI(name);
NodePrx proxy = NodePrx::uncheckedCast(_adapter->addWithUUID(servant));

// Pass proxy to client...

Here, addWithUUID both activates the servant and returns a proxy for the Ice object incarnated by that servant in a single step.

Note that we need to use an uncheckedCast here because addWithUUID returns a proxy of type Ice::ObjectPrx.

Direct Proxy Creation in C++

The object adapter offers an operation to create a proxy for a given identity:

module Ice {
    local interface ObjectAdapter {
        Object* createProxy(Identity id);
        // ...
    };
};

Note that createProxy creates a proxy for a given identity whether a servant is activated with that identity or not. In other words, proxies have a life cycle that is quite independent from the life cycle of servants:

Ice::Identity id;
id.name = IceUtil::generateUUID();
ObjectPrx o = _adapter->createProxy(id);

This creates a proxy for an Ice object with the identity returned by generateUUID. Obviously, no servant yet exists for that object so, if we return the proxy to a client and the client invokes an operation on the proxy, the client will receive an ObjectNotExistException. (We examine these life cycle issues in more detail in Object Life Cycle.)

See Also

• Hello World Application
• Object Adapter States
• Servant Locators
• Object Life Cycle
• The C++ generateUUID Function