xref: /third_party/boost/libs/locale/doc/messages_formatting.txt

//
//  Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//

// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4 filetype=cpp.doxygen
/*!
\page messages_formatting Messages Formatting (Translation)

- \ref messages_formatting_into
- \ref msg_loading_dictionaries
- \ref message_translation
    - \ref indirect_message_translation
    - \ref plural_forms
    - \ref multiple_gettext_domain
    - \ref direct_message_translation
- \ref extracting_messages_from_code
- \ref custom_file_system_support
- \ref msg_non_ascii_keys
- \ref msg_qna

\section messages_formatting_into Introduction

Messages formatting is probably the most important part of
the localization - making your application speak in the user's language.

Boost.Locale uses the <a href="http://www.gnu.org/software/gettext/">GNU Gettext</a> localization model.
We recommend you read the general <a href="http://www.gnu.org/software/gettext/manual/gettext.html">documentation</a>
of GNU Gettext, as it is outside the scope of this document.

The model is following:

-   First, our application \c foo is prepared for localization by calling the \ref boost::locale::translate() "translate" function
    for each message used in user interface.
    \n
    For example:
    \code
    cout << "Hello World" << endl;
    \endcode
    Is changed to
    \n
    \code
    cout << translate("Hello World") << endl;
    \endcode
-   Then all messages are extracted from the source code and a special \c foo.po file is generated that contains all of the
    original English strings.
    \n
    \verbatim
    ...
    msgid "Hello World"
    msgstr ""
    ...
    \endverbatim
-   The \c foo.po file is translated for the supported locales. For example, \c de.po, \c ar.po, \c en_CA.po , and \c he.po.
    \n
    \verbatim
    ...
    msgid "Hello World"
    msgstr "שלום עולם"
    \endverbatim
    And then compiled to the binary \c mo format and stored in the following file structure:
    \n
    \verbatim
    de
    de/LC_MESSAGES
    de/LC_MESSAGES/foo.mo
    en_CA/
    en_CA/LC_MESSAGES
    en_CA/LC_MESSAGES/foo.mo
    ...
    \endverbatim
    \n
    When the application starts, it loads the required dictionaries. Then when the \c translate function is called and the message is written
    to an output stream, a dictionary lookup is performed and the localized message is written out instead.

\section msg_loading_dictionaries Loading dictionaries

All the dictionaries are loaded by the \ref boost::locale::generator "generator" class.
Using localized strings in the application, requires specification
of the following parameters:

-# The search path of the dictionaries
-# The application domain (or name)

This is done by calling the following member functions of the \ref boost::locale::generator "generator" class:

-   \ref boost::locale::generator::add_messages_path() "add_messages_path" - add the root path to the dictionaries.
    \n
    For example: if the dictionary is located at \c /usr/share/locale/ar/LC_MESSAGES/foo.mo, then path should be \c /usr/share/locale.
    \n
-   \ref boost::locale::generator::add_messages_domain() "add_messages_domain" - add the domain (name) of the application. In the above case it would be "foo".

\note At least one domain and one path should be specified in order to load dictionaries.

This is an example of our first fully localized program:

\code
#include <boost/locale.hpp>
#include <iostream>

using namespace std;
using namespace boost::locale;

int main()
{
    generator gen;

    // Specify location of dictionaries
    gen.add_messages_path(".");
    gen.add_messages_domain("hello");

    // Generate locales and imbue them to iostream
    locale::global(gen(""));
    cout.imbue(locale());

    // Display a message using current system locale
    cout << translate("Hello World") << endl;
}
\endcode


\section message_translation Message Translation

There are two ways to translate messages:

-   using \ref boost_locale_translate_family "boost::locale::translate()" family of functions:
    \n
    These functions create a special proxy object \ref boost::locale::basic_message "basic_message"
    that can be converted to string according to given locale or written to \c std::ostream
    formatting the message in the \c std::ostream's locale.
    \n
    It is very convenient for working with \c std::ostream object and for postponing message
    translation
-   Using \ref boost_locale_gettext_family "boost::locale::gettext()" family of functions:
    \n
    These are functions that are used for direct message translation: they receive as a parameter
    an original message or a key and convert it to the \c std::basic_string in given locale.
    \n
    These functions have similar names to thous used in the GNU Gettext library.

\subsection indirect_message_translation Indirect Message Translation

The basic function that allows us to translate a message is \ref boost_locale_translate_family  "boost::locale::translate()" family of functions.

These functions use a character type \c CharType as template parameter and receive either <tt>CharType const *</tt> or <tt>std::basic_string<CharType></tt> as input.

These functions receive an original message and return a special proxy
object - \ref boost::locale::basic_message "basic_message<CharType>".
This object holds all the required information for the message formatting.

When this object is written to an output \c ostream, it performs a dictionary lookup of the message according to the locale
imbued in \c iostream.

If the message is found in the dictionary it is written to the output stream,
otherwise the original string is written to the stream.

For example:

\code
// Translate a simple message "Hello World!"
std::cout << boost::locale::translate("Hello World!") << std::endl;
\endcode

This allows the program to postpone translation of the message until the translation is actually needed, even to different
locale targets.

\code
// Several output stream that we write a message to
// English, Japanese, Hebrew etc.
// Each one them has installed std::locale object that represents
// their specific locale
std::ofstream en,ja,he,de,ar;

// Send single message to multiple streams
void send_to_all(message const &msg)
{
    // in each of the cases below
    // the message is translated to different
    // language
    en << msg;
    ja << msg;
    he << msg;
    de << msg;
    ar << msg;
}

int main()
{
    ...
    send_to_all(translate("Hello World"));
}
\endcode

\note

-   \ref boost::locale::basic_message "basic_message" can be implicitly converted
    to an apopriate std::basic_string using
    the global locale:
    \n
    \code
        std::wstring msg = translate(L"Do you want to open the file?");
    \endcode
-   \ref boost::locale::basic_message "basic_message" can be explicitly converted
    to a string using the \ref boost::locale::basic_message::str() "str()" member function for a specific locale.
    \n
    \code
    std::locale ru_RU = ... ;
    std::string msg = translate("Do you want to open the file?").str(ru_RU);
    \endcode


\subsection plural_forms Plural Forms

GNU Gettext catalogs have simple, robust and yet powerful plural forms support. We recommend to read the
original GNU documentation <a href="http://www.gnu.org/software/gettext/manual/gettext.html#Plural-forms">here</a>.

Let's try to solve a simple problem, displaying a message to the user:

\code
    if(files == 1)
        cout << translate("You have 1 file in the directory") << endl;
    else
        cout << format(translate("You have {1} files in the directory")) % files << endl;
\endcode

This very simple task becomes quite complicated when we deal with languages other than English. Many languages have more
than two plural forms. For example, in Hebrew there are special forms for single, double, plural, and plural above 10.
They can't be distinguished by the simple rule "is n 1 or not"

The correct solution is to give a translator an ability to choose a plural form on its own. Thus the translate
function can receive two additional parameters English plural form a number: <tt>translate(single,plural,count)</tt>

For example:

\code
cout << format(translate( "You have {1} file in the directory",
                          "You have {1} files in the directory",
                          files)) % files << endl;
\endcode

A special entry in the dictionary specifies the rule to choose the correct plural form in the target language.
For example, the Slavic language family has 3 plural forms, that can be chosen using following equation:

\code
    plural=n%10==1 && n%100!=11 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;
\endcode

Such equation is stored in the message catalog itself and it is evaluated during translation to supply the correct form.

So the code above would display 3 different forms in Russian locale for values of 1, 3 and 5:

\verbatim
У вас есть 1 файл в каталоге
У вас есть 3 файла в каталоге
У вас есть 5 файлов в каталоге
\endverbatim

And for Japanese that does not have plural forms at all it would display the same message
for any numeric value.

For more detailed information please refer to GNU Gettext: <a href="http://www.gnu.org/software/gettext/manual/gettext.html#Plural-forms">11.2.6 Additional functions for plural forms</a>


\subsection adding_context_information Adding Context Information

In many cases it is not sufficient to provide only the original English string to get the correct translation.
You sometimes need to provide some context information. In German, for example, a button labeled "open" is translated to
"öffnen" in the context of "opening a file", or to "aufbauen" in the context of opening an internet connection.

In these cases you must add some context information to the original string, by adding a comment.

\code
button->setLabel(translate("File","open"));
\endcode

The context information is provided as the first parameter to the \ref boost::locale::translate() "translate"
function in both singular and plural forms. The translator would see this context information and would be able to translate the
"open" string correctly.

For example, this is how the \c po file would look:

\code
msgctxt "File"
msgid "open"
msgstr "öffnen"

msgctxt "Internet Connection"
msgid "open"
msgstr "aufbauen"
\endcode

\note Context information requires more recent versions of the gettext tools (>=0.15) for extracting strings and
formatting message catalogs.


\subsection multiple_gettext_domain Working with multiple messages domains

In some cases it is useful to work with multiple message domains.

For example, if an application consists of several independent modules, it may
have several domains - a separate domain for each module.

For example, developing a FooBar office suite we might have:

- a FooBar Word Processor, using the "foobarwriter" domain
- a FooBar Spreadsheet, using the "foobarspreadsheet" domain
- a FooBar Spell Checker, using the "foobarspell" domain
- a FooBar File handler, using the "foobarodt" domain

There are three ways to use non-default domains:

-   When working with \c iostream, you can use the parameterized manipulator \ref
    boost::locale::as::domain "as::domain(std::string const &)", which allows switching domains in a stream:
    \n
    \code
    cout << as::domain("foo") << translate("Hello") << as::domain("bar") << translate("Hello");
    // First translation is taken from dictionary foo and the other from dictionary bar
    \endcode
-   You can specify the domain explicitly when converting a \c message object to a string:
    \code
    std::wstring foo_msg = translate(L"Hello World").str("foo");
    std::wstring bar_msg = translate(L"Hello World").str("bar");
    \endcode
-   You can specify the domain directly using a \ref direct_message_translation "convenience" interface:
    \code
    MessageBox(dgettext("gui","Error Occurred"));
    \endcode

\subsection direct_message_translation Direct translation (Convenience Interface)

Many applications do not write messages directly to an output stream or use only one locale in the process, so
calling <tt>translate("Hello World").str()</tt>  for a single message would be annoying. Thus Boost.Locale provides
GNU Gettext-like localization functions for direct translation of the messages. However, unlike the GNU Gettext functions,
the Boost.Locale translation functions provide an additional optional parameter (locale), and support wide, u16 and u32 strings.

The GNU Gettext like functions prototypes can be found \ref boost_locale_gettext_family "in this section".


All of these functions can have different prefixes for different forms:

-  \c d - translation in specific domain
-  \c n - plural form translation
-  \c p - translation in specific context

\code
    MessageBoxW(0,pgettext(L"File Dialog",L"Open?").c_str(),gettext(L"Question").c_str(),MB_YESNO);
\endcode


\section extracting_messages_from_code Extracting messages from the source code

There are many tools to extract messages from the source code into the \c .po file format. The most
popular and "native" tool is \c xgettext which is installed by default on most Unix systems and freely downloadable
for Windows (see \ref gettext_for_windows).

For example, we have a source file called \c dir.cpp that prints:

\code
    cout << format(translate("Listing of catalog {1}:")) % file_name << endl;
    cout << format(translate("Catalog {1} contains 1 file","Catalog {1} contains {2,num} files",files_no))
            % file_name % files_no << endl;
\endcode

Now we run:

\verbatim
xgettext --keyword=translate:1,1t --keyword=translate:1,2,3t dir.cpp
\endverbatim

And a file called \c messages.po created that looks like this (approximately):

\code
#: dir.cpp:1
msgid "Listing of catalog {1}:"
msgstr ""

#: dir.cpp:2
msgid "Catalog {1} contains 1 file"
msgid_plural "Catalog {1} contains {2,num} files"
msgstr[0] ""
msgstr[1] ""
\endcode

This file can be given to translators to adapt it to specific languages.

We used the \c --keyword  parameter of \c xgettext to make it suitable for extracting messages from
source code localized with Boost.Locale, searching for <tt>translate()</tt> function calls instead of the default <tt>gettext()</tt>
and <tt>ngettext()</tt> ones.
The first parameter <tt>--keyword=translate:1,1t</tt> provides the template for basic messages: a \c translate function that is
called with 1 argument (1t) and the first message is taken as the key. The second one <tt>--keyword=translate:1,2,3t</tt> is used
for plural forms.
It tells \c xgettext to use a <tt>translate()</tt> function call with 3 parameters (3t) and take the 1st and 2nd parameter as keys. An
additional marker \c Nc can be used to mark context information.

The full set of xgettext parameters suitable for Boost.Locale is:

\code
xgettext --keyword=translate:1,1t --keyword=translate:1c,2,2t       \
         --keyword=translate:1,2,3t --keyword=translate:1c,2,3,4t   \
         --keyword=gettext:1 --keyword=pgettext:1c,2                \
         --keyword=ngettext:1,2 --keyword=npgettext:1c,2,3          \
         source_file_1.cpp ... source_file_N.cpp
\endcode

Of course, if you do not use "gettext" like translation you
may ignore some of these parameters.

\subsection custom_file_system_support Custom Filesystem Support

When the access to actual file system is limited like in ActiveX controls or
when the developer wants to ship all-in-one executable file,
it is useful to be able to load \c gettext  catalogs from a custom location -
a custom file system.

Boost.Locale provides an option to install boost::locale::message_format facet
with customized options provided in boost::locale::gnu_gettext::messages_info structure.

This structure contains \c boost::function based
\ref boost::locale::gnu_gettext::messages_info::callback_type "callback"
that allows user to provide custom functionality to load message catalog files.

For example:

\code
// Configure all options for message catalog
namespace blg = boost::locale::gnu_gettext;
blg::messages_info info;
info.language = "he";
info.country = "IL";
info.encoding="UTF-8";
info.paths.push_back(""); // You need some even empty path
info.domains.push_back(blg::messages_info::domain("my_app"));
info.callback = some_file_loader; // Provide a callback

// Create a basic locale without messages support
boost::locale::generator gen;
std::locale base_locale = gen("he_IL.UTF-8");

// Install messages catalogs for "char" support to the final locale
// we are going to use
std::locale real_locale(base_locale,blg::create_messages_facet<char>(info));
\endcode

In order to setup \ref boost::locale::gnu_gettext::messages_info::language "language", \ref boost::locale::gnu_gettext::messages_info::country "country" and other members you may use \ref boost::locale::info facet for convenience,

\code
// Configure all options for message catalog
namespace blg = boost::locale::gnu_gettext;
blg::messages_info info;

info.paths.push_back(""); // You need some even empty path
info.domains.push_back(blg::messages_info::domain("my_app"));
info.callback = some_file_loader; // Provide a callback

// Create an object with default locale
std::locale base_locale = gen("");

// Use boost::locale::info to configure all parameters

boost::locale::info const &properties = std::use_facet<boost::locale::info>(base_locale);
info.language = properties.language();
info.country  = properties.country();
info.encoding = properties.encoding();
info.variant  = properties.variant();

// Install messages catalogs to the final locale
std::locale real_locale(base_locale,blg::create_messages_facet<char>(info));
\endcode

\section msg_non_ascii_keys Non US-ASCII Keys

Boost.Locale assumes that you use English for original text messages. And the best
practice is to use US-ASCII characters for original keys.

However in some cases it us useful in insert some Unicode characters in text like
for example Copyright "©" character.

As long as your narrow character string encoding is UTF-8 nothing further should be done.

Boost.Locale assumes that your sources are encoded in UTF-8 and the input narrow
string use UTF-8 - which is the default for most compilers around (with notable
exception of Microsoft Visual C++).

However if your narrow strings encoding in the source file is not UTF-8 but some other
encoding like windows-1252, the string would be misinterpreted.

You can specify the character set of the original strings when you specify the
domain name for the application.

\code
#include <boost/locale.hpp>
#include <iostream>

using namespace std;
using namespace boost::locale;

int main()
{
    generator gen;

    // Specify location of dictionaries
    gen.add_messages_path(".");
    // Specify the encoding of the source string
    gen.add_messages_domain("copyrighted/windows-1255");

    // Generate locales and imbue them to iostream
    locale::global(gen(""));
    cout.imbue(locale());

    // In Windows 1255 (C) symbol is encoded as 0xA9
    cout << translate("© 2001 All Rights Reserved") << endl;
}
\endcode

Thus if the programs runs in UTF-8 locale the copyright symbol would
be automatically converted to an appropriate UTF-8 sequence if the
key is missing in the dictionary.


\subsection msg_qna Questions and Answers

-   Do I need GNU Gettext to use Boost.Locale?
    \n
    Boost.Locale provides a run-time environment to load and use GNU Gettext message catalogs, but it does
    not provide tools for generation, translation, compilation and management of these catalogs.
    Boost.Locale only reimplements the GNU Gettext libintl.
    \n
    You would probably need:
    \n
    -#  Boost.Locale itself -- for runtime.
    -#  A tool for extracting strings from source code, and managing them: GNU Gettext provides good tools, but other
        implementations are available as well.
    -#  A good translation program like <a href="http://userbase.kde.org/Lokalize">Lokalize</a>, <a href="http://www.poedit.net/">Pedit</a> or <a href="http://projects.gnome.org/gtranslator/">GTranslator</a>.

-   Why doesn't Boost.Locale provide tools for extracting and management of message catalogs. Why should
    I use GPL-ed software? Are my programs or message catalogs affected by its license?
    \n
    -#  Boost.Locale does not link to or use any of the GNU Gettext code, so you need not worry about your code as
        the runtime library is fully reimplemented.
    -#  You may freely use GPL-ed software for extracting and managing catalogs, the same way as you are free to use
        a GPL-ed editor. It does not affect your message catalogs or your code.
    -#  I see no reason to reimplement well debugged, working tools like \c xgettext, \c msgfmt, \c msgmerge that
        do a very fine job, especially as they are freely available for download and support almost any platform.
    All Linux distributions, BSD Flavors, Mac OS X and other Unix like operating systems provide GNU Gettext tools
    as a standard package.\n
    Windows users can get GNU Gettext utilities via MinGW project. See \ref gettext_for_windows.


-   Is there any reason to prefer the Boost.Locale implementation to the original GNU Gettext runtime library?
    In either case I would probably need some of the GNU tools.
    \n
    There are two important differences between the GNU Gettext runtime library and the Boost.Locale implementation:
    \n
    -#  The GNU Gettext runtime supports only one locale per process. It is not thread-safe to use multiple locales
        and encodings in the same process. This is perfectly fine for applications that interact directly with
        a single user like most GUI applications, but is problematic for services and servers.
    -#  The GNU Gettext API supports only 8-bit encodings, making it irrelevant in environments that natively use
        wide strings.
    -#  The GNU Gettext runtime library distributed under LGPL license which may be not convenient for some users.

*/