Messages Formatting (Translation)


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

Boost.Locale uses the GNU Gettext localization model. We recommend you read the general documentation of GNU Gettext, as it is outside the scope of this document.

The model is following:

  • First, our application foo is prepared for localization by calling the translate function for each message used in user interface.
    For example:
        cout << "Hello World" << endl;
    Is changed to
        cout << translate("Hello World") << endl;
  • Then all messages are extracted from the source code and a special foo.po file is generated that contains all of the original English strings.
        msgid "Hello World"
        msgstr ""
  • The foo.po file is translated for the supported locales. For example, de.po, ar.po, en_CA.po , and he.po.
        msgid "Hello World"
        msgstr "שלום עולם"
    And then compiled to the binary mo format and stored in the following file structure:

    When the application starts, it loads the required dictionaries. Then when the 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.

Loading dictionaries

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

  1. The search path of the dictionaries
  2. The application domain (or name)

This is done by calling the following member functions of the generator class:

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

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

using namespace std;
using namespace boost::locale;

int main()
    generator gen;

    // Specify location of dictionaries

    // Generate locales and imbue them to iostream

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

Message Translation

There are two ways to translate messages:

  • using boost::locale::translate() family of functions:
    These functions create a special proxy object basic_message that can be converted to string according to given locale or written to std::ostream formatting the message in the std::ostream's locale.
    It is very convenient for working with std::ostream object and for postponing message translation
  • Using boost::locale::gettext() family of functions:
    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 std::basic_string in given locale.
    These functions have similar names to thous used in the GNU Gettext library.

Indirect Message Translation

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

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

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

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

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

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

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

Plural Forms

GNU Gettext catalogs have simple, robust and yet powerful plural forms support. We recommend to read the original GNU documentation here.

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

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

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: translate(single,plural,count)

For example:

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

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:

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

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:

У вас есть 1 файл в каталоге
У вас есть 3 файла в каталоге
У вас есть 5 файлов в каталоге

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: 11.2.6 Additional functions for plural forms

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.


The context information is provided as the first parameter to the 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 po file would look:

msgctxt "File"
msgid "open"
msgstr "öffnen"

msgctxt "Internet Connection"
msgid "open"
msgstr "aufbauen"
Context information requires more recent versions of the gettext tools (>=0.15) for extracting strings and formatting message catalogs.

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

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 translate("Hello World").str() 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 in this section.

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

  • d - translation in specific domain
  • n - plural form translation
  • p - translation in specific context
    MessageBoxW(0,pgettext(L"File Dialog",L"Open?").c_str(),gettext(L"Question").c_str(),MB_YESNO);

Extracting messages from the source code

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

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

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

Now we run:

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

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

#: 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] ""

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

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

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

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

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

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 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 boost::function based callback that allows user to provide custom functionality to load message catalog files.

For example:

// Configure all options for message catalog
namespace blg = boost::locale::gnu_gettext;
blg::messages_info info;
info.language = "he"; = "IL";
info.paths.push_back(""); // You need some even empty path"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));

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

// 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"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.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));


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.

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

using namespace std;
using namespace boost::locale;

int main()
    generator gen;

    // Specify location of dictionaries
    // Specify the encoding of the source string

    // Generate locales and imbue them to iostream
    // In Windows 1255 (C) symbol is encoded as 0xA9
    cout << translate("© 2001 All Rights Reserved") << endl;

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.

Questions and Answers

  • Do I need GNU Gettext to use Boost.Locale?
    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.
    You would probably need:
    1. Boost.Locale itself -- for runtime.
    2. A tool for extracting strings from source code, and managing them: GNU Gettext provides good tools, but other implementations are available as well.
    3. A good translation program like Lokalize, Pedit or GTranslator.
  • 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?
    1. 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.
    2. 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.
    3. I see no reason to reimplement well debugged, working tools like xgettext, msgfmt, 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.
      Windows users can get GNU Gettext utilities via MinGW project. See Using Gettext Tools on 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.
    There are two important differences between the GNU Gettext runtime library and the Boost.Locale implementation:
    1. 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.
    2. The GNU Gettext API supports only 8-bit encodings, making it irrelevant in environments that natively use wide strings.
    3. The GNU Gettext runtime library distributed under LGPL license which may be not convenient for some users.