22.12 Mapping for Operations

As we saw in Section 22.11, for each operation on an interface, the proxy class contains a corresponding method with the same name. To invoke an operation, you call it via the proxy. For example, here is part of the definitions for our file system from Section 5.4:

module Filesystem {
    interface Node {
        idempotent string name();
    };
    // ...
};

The name operation returns a value of type string. Given a proxy to an object of type Node, the client can invoke the operation as follows:

node = ...          # Initialize proxy
name = node.name()  # Get name via RPC

22.12.1 Normal and idempotent Operations

You can add an idempotent qualifier to a Slice operation. As far as the signature for the corresponding proxy method is concerned, idempotent has no effect. For example, consider the following interface:

interface Example {
                string op1();
    idempotent  string op2();
};

The proxy class for this is:

class ExamplePrx < Ice::ObjectPrx
    def op1(_ctx=nil)

    def op2(_ctx=nil)
end

Because idempotent affects an aspect of call dispatch, not interface, it makes sense for the two methods to look the same.

22.12.2 Passing Parameters

In Parameters

All parameters are passed by reference in the Ruby mapping; it is guaranteed that the value of a parameter will not be changed by the invocation.

Here is an interface with operations that pass parameters of various types from client to server:

struct NumberAndString {
    int x;
    string str;
};

sequence<string> StringSeq;

dictionary<long, StringSeq> StringTable;

interface ClientToServer {
    void op1(int i, float f, bool b, string s);
    void op2(NumberAndString ns, StringSeq ss, StringTable st);
    void op3(ClientToServer* proxy);
};

The Slice compiler generates the following proxy for this definition:

class ClientToServerPrx < Ice::ObjectPrx
    def op1(i, f, b, s, _ctx=nil)

    def op2(ns, ss, st, _ctx=nil)

    def op3(proxy, _ctx=nil)
end

Given a proxy to a ClientToServer interface, the client code can pass parameters as in the following example:

p = ...                                 # Get proxy...

p.op1(42, 3.14, true, "Hello world!")   # Pass simple literals

i = 42
f = 3.14
b = True
s = "Hello world!"
p.op1(i, f, b, s)                       # Pass simple variables

ns = NumberAndString.new()
ns.x = 42
ns.str = "The Answer"
ss = [ "Hello world!" ]
st = {}
st[0] = ns
p.op2(ns, ss, st)                       # Pass complex variables

p.op3(p)                                # Pass proxy

Out Parameters

As in Java, Ruby functions do not support reference arguments. That is, it is not possible to pass an uninitialized variable to a Ruby function in order to have its value initialized by the function. The Java mapping (see Section 10.12.2) overcomes this limitation with the use of "holder classes" that represent each out parameter. The Ruby mapping takes a different approach, one that is more natural for Ruby users.

The semantics of out parameters in the Ruby mapping depend on whether the operation returns one value or multiple values. An operation returns multiple values when it has declared multiple out parameters, or when it has declared a non-void return type and at least one out parameter.

If an operation returns multiple values, the client receives them in the form of a result array. A non-void return value, if any, is always the first element in the result array, followed by the out parameters in the order of declaration.

If an operation returns only one value, the client receives the value itself.

Here is the same Slice definition we saw on page 596 once more, but this time with all parameters being passed in the out direction:

struct NumberAndString {
    int x;
    string str;
};

sequence<string> StringSeq;

dictionary<long, StringSeq> StringTable;

interface ClientToServer {
    int op1(out float f, out bool b, out string s);
    void op2(out NumberAndString ns,
             out StringSeq ss,
             out StringTable st);
    void op3(out ClientToServer* proxy);
};

The Ruby mapping generates the following code for this definition:

class ClientToServerPrx < Ice::ObjectPrx
    def op1(_ctx=nil)

    def op2(_ctx=nil)

    def op3(_ctx=nil)
end

Given a proxy to a ServerToClient interface, the client code can receive the results as in the following example:

p = ...              # Get proxy...
i, f, b, s = p.op1()
ns, ss, st = p.op2()
stcp = p.op3()

The operations have no in parameters, therefore no arguments are passed to the proxy methods. Since op1 and op2 return multiple values, their result arrays are unpacked into separate values, whereas the return value of op3 requires no unpacking.

Parameter Type Mismatches

Although the Ruby compiler cannot check the types of arguments passed to a method, the Ice run time does perform validation on the arguments to a proxy invocation and reports any type mismatches as a TypeError exception.

Null Parameters

Some Slice types naturally have "empty" or "not there" semantics. Specifically, sequences, dictionaries, and strings all can be nil, but the corresponding Slice types do not have the of a null value. To make life with these types easier, whenever you pass nil as a parameter or return value of type sequence, dictionary, or string, the Ice run time automatically sends an empty sequence, dictionary, or string to the receiver.

This behavior is useful as a convenience feature: especially for deeply-nested data types, members that are sequences, dictionaries, or strings automatically arrive as an empty value at the receiving end. This saves you having to explicitly initialize, for example, every string element in a large sequence before sending the sequence in order to avoid a run-time error. Note that using null parameters in this way does not create null semantics for Slice sequences, dictionaries, or strings. As far as the object model is concerned, these do not exist (only empty sequences, dictionaries, and strings do). For example, it makes no difference to the receiver whether you send a string as nil or as an empty string: either way, the receiver sees an empty string.