ClickHouse/contrib/libpoco/doc/00100-GuidedTour.page

A Guided Tour Of The POCO C++ Libraries
AAAIntroduction

!!! Introduction

The POCO C++ Libraries are a collection of open source C++
class libraries that simplify and accelerate the development of
network-centric, portable applications in C++. The libraries integrate
perfectly with the C++ Standard Library and fill many of the functional
gaps left open by it. Their modular and efficient design and
implementation makes the POCO C++ Libraries extremely well suited
for embedded development, an area where the C++ programming language is
becoming increasingly popular, due to its suitability for both low-level
(device I/O, interrupt handlers, etc.) and high-level object-oriented
development. Of course, POCO is also ready for enterprise-level
challenges.

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POCO consists of four core libraries, and a number of add-on libraries.
The core libraries are Foundation, XML, Util and Net. Two of the add-on
libraries are NetSSL, providing SSL support for the network classes in
the Net library, and Data, a library for uniformly accessing different
SQL databases.  POCO aims to be for
network-centric, cross-platform C++ software development what Apple's
Cocoa is for Mac development, or Ruby on Rails is for Web development --
a powerful, yet easy and fun to use platform to build your applications
upon. POCO is built strictly using standard ANSI/ISO C++, including the
standard library. The contributors attempt to find a good balance
between using advanced C++ features and keeping the classes
comprehensible and the code clean, consistent and easy to maintain.


!!! The Foundation Library

The Foundation library makes up the heart of POCO. It contains the
underlying platform abstraction layer, as well as frequently used
utility classes and functions. The Foundation library contains types for
fixed-size integers, functions for converting integers between byte
orders, an Poco::Any class (based on <[boost::Any]>), utilities for error handling
and debugging, including various exception classes and support for
assertions. Also available are a number of classes for memory
management, including reference counting based smart pointers, as well
as classes for buffer management and memory pools. For string handling,
POCO contains a number of functions that among other things, trim
strings, perform case insensitive comparisons and case conversions.
Basic support for Unicode text is also available in the form of classes
that convert text between different character encodings, including UTF-8
and UTF-16. Support for formatting and parsing numbers is there,
including a typesafe variant of sprintf. Regular expressions based on
the well-known PCRE library (http://www.pcre.org) are provided as well. 

POCO gives you classes for handling dates and times in various variants. 
For accessing the file system, POCO has Poco::File and Poco::Path classes, as well as a
Poco::DirectoryIterator class. In many applications, some parts of the
application need to tell other parts that something has happened. In
POCO, Poco::NotificationCenter, Poco::NotificationQueue and events (similar to C#
events) make this easy. The following example shows how POCO events can be used. In
this example, class <[Source]> has a public event named <[theEvent]>, having an
argument of type int. Subscribers can subscribe by calling <[operator +=]>
and unsubscribe by calling <[operator -=]>, passing a pointer to an object
and a pointer to a member function. The event can be fired by calling
<[operator ()]>, as its done in <[Source::fireEvent()]>.

    #include "Poco/BasicEvent.h"
    #include "Poco/Delegate.h"
    #include <iostream>
    
    using Poco::BasicEvent;
    using Poco::Delegate;
    
    class Source
    {
    public:
        BasicEvent<int> theEvent;
    
        void fireEvent(int n)
        {
            theEvent(this, n);
        }
    };
    
    class Target
    {
    public:
        void onEvent(const void* pSender, int& arg)
        {
            std::cout << "onEvent: " << arg << std::endl;
        }
    };
    
    int main(int argc, char** argv)
    {
        Source source;
        Target target;
    
        source.theEvent += Delegate<Target, int>(
            &target, &Target::onEvent);
    
        source.fireEvent(42);
    
        source.theEvent -= Delegate<Target, int>(
            &target, &Target::onEvent);
    
        return 0;
    }
----


The stream classes available in POCO have already been mentioned. These are
augmented by Poco::BinaryReader and Poco::BinaryWriter for writing binary data to
streams, automatically and transparently handling byte order issues. 

In complex multithreaded applications, the only way to find problems or
bugs is by writing extensive logging information. POCO provides a
powerful and extensible logging framework that supports filtering,
routing to different channels, and formatting of log messages. Log
messages can be written to the console, a file, the Windows Event Log,
the Unix syslog daemon, or to the network. If the channels provided by
POCO are not sufficient, it is easy to extend the logging framework with
new classes. 

For loading (and unloading) shared libraries at runtime,
POCO has a low-level Poco::SharedLibrary class. Based on it is the Poco::ClassLoader
class template and supporting framework, allowing dynamic loading and
unloading of C++ classes at runtime, similar to what's available to Java
and .NET developers. The class loader framework also makes it a breeze
to implement plug-in support for applications in a platform-independent
way. 

Finally, POCO Foundation contains multithreading abstractions at
different levels. Starting with a Poco::Thread class and the usual
synchronization primitives (Poco::Mutex, Poco::ScopedLock, Poco::Event, 
Poco::Semaphore, Poco::RWLock), a Poco::ThreadPool class and support for 
thread-local storage, also high level abstractions like active objects are 
available. Simply speaking, an active object is an object that has methods executing in
their own thread. This makes asynchronous member function calls possible
-- call a member function, while the function executes, do a bunch of
other things, and, eventually, obtain the function's return value.
The following example shows how this is done in POCO. The <[ActiveAdder]> class in
defines an active method <[add()]>, implemented by the <[addImpl()]>
member function. Invoking the active method in <[main()]> yields an
Poco::ActiveResult (also known as a future), that eventually receives the
function's return value.

    #include "Poco/ActiveMethod.h"
    #include "Poco/ActiveResult.h"
    #include <utility>
    #include <iostream>
    
    using Poco::ActiveMethod;
    using Poco::ActiveResult;
    
    class ActiveAdder
    {
    public:
        ActiveAdder(): add(this, &ActiveAdder::addImpl)
        {
        }
    
        ActiveMethod<int, std::pair<int, int>, ActiveAdder> add;
    
    private:
        int addImpl(const std::pair<int, int>& args)
        {
            return args.first + args.second;
        }
    };
    
    int main(int argc, char** argv)
    {
        ActiveAdder adder;
    
        ActiveResult<int> sum = adder.add(std::make_pair(1, 2));
        // do other things
        sum.wait();
        std::cout << sum.data() << std::endl;
    
        return 0;
    }
----


!!! The XML Library

The POCO XML library provides support for reading, processing and
writing XML. Following one's of POCO's guiding principles -- don't try to
reinvent things that already work -- POCO's XML library supports the
industry-standard SAX (version 2) and DOM interfaces, familiar
to many developers with XML experience. SAX, the Simple API for XML
(http://www.saxproject.org),
defines an event-based interface for reading XML. A SAX-based XML parser
reads through the XML document and notifies the application whenever it
encounters an element, character data, or other XML artifact. A SAX
parser does not need to load the complete XML document into memory, so
it can be used to parse huge XML files efficiently. In contrast, DOM
(Document Object Model, http://www.w3.org/DOM/) gives the application 
complete access to an XML
document, using a tree-style object hierarchy. For this to work, the DOM
parser provided by POCO has to load the entire document into memory. To
reduce the memory footprint of the DOM document, the POCO DOM
implementation uses string pooling, storing frequently occuring strings
such as element and attribute names only once. The XML library is based
on the Expat open source XML parser library (http://www.libexpat.org). 
Built on top of Expat
are the SAX interfaces, and built on top of the SAX interfaces is the
DOM implementation. For strings, the XML library uses <[std::string]>, with
characters encoded in UTF-8. This makes interfacing the XML library to
other parts of the application easy. Support for XPath and XSLT will be
available in a future release.


!!! The Util Library

The Util library has a somewhat misleading name, as it basically
contains a framework for creating command-line and server applications.
Included is support for handling command line arguments (validation,
binding to configuration properties, etc.) and managing configuration
information. Different configuration file formats are supported --
Windows-style INI files, Java-style property files, XML files and the
Windows registry. 

For server applications, the framework provides
transparent support for Windows services and Unix daemons. Every server
application can be registered and run as a Windows service, with no
extra code required. Of course, all server applications can still be
executed from the command line, which makes testing and debugging easier.


!!! The Net Library

POCO's Net library makes it easy to write network-based applications. No
matter whether your application simply needs to send data over a plain
TCP socket, or whether your application needs a full-fledged built-in
HTTP server, you will find something useful in the Net library. 

At the lowest level, the Net library contains socket classes, supporting TCP
stream and server sockets, UDP sockets, multicast sockets, ICMP and raw
sockets. If your application needs secure sockets, these are available
in the NetSSL library, implemented using OpenSSL (http://www.openssl.org). 
Based on the socket classes are two frameworks for building TCP servers -- one for
multithreaded servers (one thread per connection, taken from a thread
pool), one for servers based on the Acceptor-Reactor pattern. The
multithreaded Poco::Net::TCPServer class and its supporting framework are also the
foundation for POCO's HTTP server implementation (Poco::Net::HTTPServer). 
On the client side, the Net library provides classes for talking to HTTP servers, 
for sending and receiving files using the FTP protocol, for sending mail
messages (including attachments) using SMTP and for receiving mail from
a POP3 server.


!!! Putting It All Together

The following example shows the implementation of a simple HTTP server using the
POCO libraries. The server returns a HTML document showing the current
date and time. The application framework is used to build a server
application that can run as a Windows service, or Unix daemon process.
Of course, the same executable can also directly be started from the
shell. For use with the HTTP server framework, a <[TimeRequestHandler]>
class is defined that servers incoming requests by returning a HTML
document containing the current date and time. Also, for each incoming
request, a message is logged using the logging framework. Together with
the <[TimeRequestHandler]> class, a corresponding factory class,
<[TimeRequestHandlerFactory]> is needed; an instance of the factory is
passed to the HTTP server object. The <[HTTPTimeServer]> application class
defines a command line argument help by overriding the <[defineOptions()]>
member function of Poco::Util::ServerApplication. It also reads in the default
application configuration file (in <[initialize()]>) and obtains the value
of some configuration properties in <[main()]>, before starting the HTTP
server.

    #include "Poco/Net/HTTPServer.h"
    #include "Poco/Net/HTTPRequestHandler.h"
    #include "Poco/Net/HTTPRequestHandlerFactory.h"
    #include "Poco/Net/HTTPServerParams.h"
    #include "Poco/Net/HTTPServerRequest.h"
    #include "Poco/Net/HTTPServerResponse.h"
    #include "Poco/Net/HTTPServerParams.h"
    #include "Poco/Net/ServerSocket.h"
    #include "Poco/Timestamp.h"
    #include "Poco/DateTimeFormatter.h"
    #include "Poco/DateTimeFormat.h"
    #include "Poco/Exception.h"
    #include "Poco/ThreadPool.h"
    #include "Poco/Util/ServerApplication.h"
    #include "Poco/Util/Option.h"
    #include "Poco/Util/OptionSet.h"
    #include "Poco/Util/HelpFormatter.h"
    #include <iostream>
    
    using Poco::Net::ServerSocket;
    using Poco::Net::HTTPRequestHandler;
    using Poco::Net::HTTPRequestHandlerFactory;
    using Poco::Net::HTTPServer;
    using Poco::Net::HTTPServerRequest;
    using Poco::Net::HTTPServerResponse;
    using Poco::Net::HTTPServerParams;
    using Poco::Timestamp;
    using Poco::DateTimeFormatter;
    using Poco::DateTimeFormat;
    using Poco::ThreadPool;
    using Poco::Util::ServerApplication;
    using Poco::Util::Application;
    using Poco::Util::Option;
    using Poco::Util::OptionSet;
    using Poco::Util::OptionCallback;
    using Poco::Util::HelpFormatter;
    
    class TimeRequestHandler: public HTTPRequestHandler
    {
    public:
        TimeRequestHandler(const std::string& format): _format(format)
        {
        }
    
        void handleRequest(HTTPServerRequest& request,
                           HTTPServerResponse& response)
        {
            Application& app = Application::instance();
            app.logger().information("Request from "
                + request.clientAddress().toString());
    
            Timestamp now;
            std::string dt(DateTimeFormatter::format(now, _format));
    
            response.setChunkedTransferEncoding(true);
            response.setContentType("text/html");
    
            std::ostream& ostr = response.send();
            ostr << "<html><head><title>HTTPTimeServer powered by "
                    "POCO C++ Libraries</title>";
            ostr << "<meta http-equiv=\"refresh\" content=\"1\"></head>";
            ostr << "<body><p style=\"text-align: center; "
                    "font-size: 48px;\">";
            ostr << dt;
            ostr << "</p></body></html>";
        }
    
    private:
        std::string _format;
    };
    
    class TimeRequestHandlerFactory: public HTTPRequestHandlerFactory
    {
    public:
        TimeRequestHandlerFactory(const std::string& format):
            _format(format)
        {
        }
    
        HTTPRequestHandler* createRequestHandler(
            const HTTPServerRequest& request)
        {
            if (request.getURI() == "/")
                return new TimeRequestHandler(_format);
            else
                return 0;
        }
    
    private:
        std::string _format;
    };
    
    class HTTPTimeServer: public Poco::Util::ServerApplication
    {
    public:
        HTTPTimeServer(): _helpRequested(false)
        {
        }
    
        ~HTTPTimeServer()
        {
        }
    
    protected:
        void initialize(Application& self)
        {
            loadConfiguration();
            ServerApplication::initialize(self);
        }
    
        void uninitialize()
        {
            ServerApplication::uninitialize();
        }
    
        void defineOptions(OptionSet& options)
        {
            ServerApplication::defineOptions(options);
    
            options.addOption(
            Option("help", "h", "display argument help information")
                .required(false)
                .repeatable(false)
                .callback(OptionCallback<HTTPTimeServer>(
                    this, &HTTPTimeServer::handleHelp)));
        }
    
        void handleHelp(const std::string& name, 
                        const std::string& value)
        {
            HelpFormatter helpFormatter(options());
            helpFormatter.setCommand(commandName());
            helpFormatter.setUsage("OPTIONS");
            helpFormatter.setHeader(
                "A web server that serves the current date and time.");
            helpFormatter.format(std::cout);
            stopOptionsProcessing();
            _helpRequested = true;
        }
    
        int main(const std::vector<std::string>& args)
        {
            if (!_helpRequested)
            {
                unsigned short port = (unsigned short)
                    config().getInt("HTTPTimeServer.port", 9980);
                std::string format(
                    config().getString("HTTPTimeServer.format", 
                                       DateTimeFormat::SORTABLE_FORMAT));
    
                ServerSocket svs(port);
                HTTPServer srv(new TimeRequestHandlerFactory(format), 
                    svs, new HTTPServerParams);
                srv.start();
                waitForTerminationRequest();
                srv.stop();
            }
            return Application::EXIT_OK;
        }
    
    private:
        bool _helpRequested;
    };
    
    int main(int argc, char** argv)
    {
        HTTPTimeServer app;
        return app.run(argc, argv);
    }
----