This page shows how to create an Ice application with C++.
On this page:
Compiling a Slice Definition for C++
The first step in creating our C++ application is to compile our Slice definition to generate C++ proxies and skeletons. You can compile the definition as follows:
slice2cpp compiler produces two C++ source files from this definition,
Printer.hheader file contains C++ type definitions that correspond to the Slice definitions for our
Printerinterface. This header file must be included in both the client and the server source code.
Printer.cppfile contains the source code for our
Printerinterface. The generated source contains type-specific run-time support for both clients and servers. For example, it contains code that marshals parameter data (the string passed to the
printStringoperation) on the client side and unmarshals that data on the server side.
Printer.cppfile must be compiled and linked into both client and server.
Writing and Compiling a Server in C++
The source code for the server takes only a few lines and is shown in full here:
There appears to be a lot of code here for something as simple as a server that just prints a string. Do not be concerned by this: most of the preceding code is boiler plate that never changes. For this very simple server, the code is dominated by this boiler plate.
Every Ice source file starts with an include directive for
Ice.h, which contains the definitions for the Ice run time. We also include
Printer.h, which was generated by the Slice compiler and contains the C++ definitions for our printer interface, and we import the contents of the
Demo namespaces for brevity in the code that follows:
Our server implements a single printer servant, of type
PrinterI. Looking at the generated code in
Printer.h, we find the following (tidied up a little to get rid of irrelevant detail):
Printer skeleton class definition is generated by the Slice compiler. (Note that the
printString method is pure virtual so the skeleton class cannot be instantiated.) Our servant class inherits from the skeleton class to provide an implementation of the pure virtual
printString method. (By convention, we use an
I-suffix to indicate that the class implements an interface.)
The implementation of the
printString method is trivial: it simply writes its string argument to
printString has a second parameter of type
Ice::Current. As you can see from the definition of
Printer::printString, the Slice compiler generates a default argument for this parameter, so we can leave it unused in our implementation. (We will examine the purpose of the
Ice::Current parameter later.)
What follows is the server main program. Note the general structure of the code:
The body of
main contains the declaration of two variables,
status variable contains the exit status of the program and the
ic variable, of type
Ice::CommunicatorPtr, contains the main handle to the Ice run time.
Following these declarations is a
try block in which we place all the server code, followed by two
catch handlers. The first handler catches all exceptions that may be thrown by the Ice run time; the intent is that, if the code encounters an unexpected Ice run-time exception anywhere, the stack is unwound all the way back to
main, which prints the exception and then returns failure to the operating system. The second handler catches string constants; the intent is that, if we encounter a fatal error condition somewhere in our code, we can simply throw a string literal with an error message. Again, this unwinds the stack all the way back to
main, which prints the error message and then returns failure to the operating system.
try block, we see a bit of cleanup code that calls the
destroy method on the communicator (provided that the communicator was initialized). The cleanup call is outside the first
try block for a reason: we must ensure that the Ice run time is finalized whether the code terminates normally or terminates due to an exception.
Failure to call
destroy on the communicator before the program exits results in undefined behavior.
The body of the first
try block contains the actual server code:
The code goes through the following steps:
- We initialize the Ice run time by calling
Ice::initialize. (We pass
argvto this call because the server may have command-line arguments that are of interest to the run time; for this example, the server does not require any command-line arguments.) The call to
initializereturns a smart pointer to an
Ice::Communicatorobject, which is the main object in the Ice run time.
- We create an object adapter by calling
Communicatorinstance. The arguments we pass are
"SimplePrinterAdapter"(which is the name of the adapter) and
"default -p 10000", which instructs the adapter to listen for incoming requests using the default protocol (TCP/IP) at port number 10000.
- At this point, the server-side run time is initialized and we create a servant for our
Printerinterface by instantiating a
- We inform the object adapter of the presence of a new servant by calling
addon the adapter; the arguments to
addare the servant we have just instantiated, plus an identifier. In this case, the string
"SimplePrinter"is the name of the Ice object. (If we had multiple printers, each would have a different name or, more correctly, a different object identity.)
- Next, we activate the adapter by calling its
activatemethod. (The adapter is initially created in a holding state; this is useful if we have many servants that share the same adapter and do not want requests to be processed until after all the servants have been instantiated.) The server starts to process incoming requests from clients as soon as the adapter is activated.
- Finally, we call
waitForShutdown. This call suspends the calling thread until the server implementation terminates, either by making a call to shut down the run time, or in response to a signal. (For now, we will simply interrupt the server on the command line when we no longer need it.)
Note that, even though there is quite a bit of code here, that code is essentially the same for all servers. You can put that code into a helper class and, thereafter, will not have to bother with it again. (Ice provides such a helper class, called
Ice::Application.) As far as actual application code is concerned, the server contains only a few lines: six lines for the definition of the
PrinterI class, plus three lines to instantiate a
PrinterI object and register it with the object adapter.
Assuming that we have the server code in a file called
Server.cpp, we can compile it as follows:
This compiles both our application code and the code that was generated by the Slice compiler. Depending on your platform, you may have to add additional include directives or other options to the compiler; please see the demo programs that ship with Ice for the details.
Finally, we need to link the server into an executable:
Again, depending on the platform, the actual list of libraries you need to link against may be longer. The demo programs that ship with Ice contain all the detail. The important point to note here is that the Ice run time is shipped in two libraries,
Writing and Compiling a Client in C++
The client code looks very similar to the server. Here it is in full:
Note that the overall code layout is the same as for the server: we include the headers for the Ice run time and the header generated by the Slice compiler, and we use the same
try block and
catch handlers to deal with errors.
The code in the
try block does the following:
- As for the server, we initialize the Ice run time by calling
- The next step is to obtain a proxy for the remote printer. We create a proxy by calling
stringToProxyon the communicator, with the string
"SimplePrinter:default -p 10000". Note that the string contains the object identity and the port number that were used by the server. (Obviously, hard-coding object identities and port numbers into our applications is a bad idea, but it will do for now; we will see more architecturally sound ways of doing this when we discuss IceGrid.)
- The proxy returned by
stringToProxyis of type
Ice::ObjectPrx, which is at the root of the inheritance tree for interfaces and classes. But to actually talk to our printer, we need a proxy for a
Printerinterface, not an
Objectinterface. To do this, we need to do a down-cast by calling
PrinterPrx::checkedCast. A checked cast sends a message to the server, effectively asking "is this a proxy for a
Printerinterface?" If so, the call returns a proxy to a
Printer; otherwise, if the proxy denotes an interface of some other type, the call returns a null proxy.
- We test that the down-cast succeeded and, if not, throw an error message that terminates the client.
- We now have a live proxy in our address space and can call the
printStringmethod, passing it the time-honored
"Hello World!"string. The server prints that string on its terminal.
Compiling and linking the client looks much the same as for the server:
Running Client and Server in C++
To run client and server, we first start the server in a separate window:
At this point, we won't see anything because the server simply waits for a client to connect to it. We run the client in a different window:
The client runs and exits without producing any output; however, in the server window, we see the
"Hello World!" that is produced by the printer. To get rid of the server, we interrupt it on the command line for now. (We will see cleaner ways to terminate a server in our discussion of
If anything goes wrong, the client will print an error message. For example, if we run the client without having first started the server, we get: