TweetFollow Us on Twitter

Aug 98 Factory Floor

Volume Number: 14 (1998)
Issue Number: 8
Column Tag: From The Factory Floor

The New C++ Standard: Locales

by by Howard Hinnant and Dave Mark, ©1998 by Metrowerks, Inc., all rights reserved.

In last month's column, we took our first pass at the Final Draft International Standard for C++. Specifically, we covered the topic of namespaces. In this month's column, Howard Hinnant is back, and will take us through the concepts behind C++ locales.

Howard Hinnant is a software engineer on the MSL team at Metrowerks, and is responsible for the C++ and EC++ libraries. Howard is a refugee from the aerospace industry where FORTRAN still rules. He has extensive experience in scientific computing including C++ implementations of linear algebra, finite difference and finite element solvers.

Dave: What is a C++ locale and why would I want to use one?

Howard: A locale is a C++ class in the standard library that allows you to easily customize I/O so that users of your software feel at home, wherever that home may be. For example, let's say you're writing some high finance report software. This stuff is for executives, and you know how much they like their formatting. So when you print out ten thousand bucks, it has to look like:

$10,000.00

Ok, so you've put 9 months into this software, and your boss is really pleased with it. In fact, he/she likes it so much, they want to use it for the offices world wide. Except that the formatting needs to be customized for each country... (you did plan for this didn't you!). The French executives might want to see:

10.000,00 Franc

And the German executives might want to see:

10.000,00 Mark

Does this mean you need seperate formatting routines for every country? No. This is precisely what locale was meant to solve. In fact, with locale it is not difficult to have several formats current at the same time (think international client/server).

Dave: But wait a minute, US$10,000 is not equivalent to 10.000,00 German Marks!

Howard: Good point. An important thing to understand about locale is that it only helps you with formatting and I/O. Any calculations (like exchange rates) are totally up to you. This all might become more clear with a specific example. Let's say you wrote a class called Money in your killer app that looked something like:

class Money
{
public:
  Money(double amount) : amount_(100*amount) {}
  double to_double() const {return amount_;}
private:
  double amount_;
};

std::ostream& operator<< (std::ostream& os,
                        const Money& amount);

Your top level code might look like:

Money amount; // Your Money class
is >> amount; // input from stream (perhaps from US)
os << amount; // output to stream (perhaps to France)

Dave: How do these streams know which country to format for? Where is the formatting logic?

Howard: You (the programmer) do have to do some more work here. In the previous example, you have to "imbue" each stream with a locale that knows how to format your Money.

This might look like:

std::locale US_Locale(...);
is.imbue(US_Locale);
std::locale Fr_Locale(...);
os.imbue(Fr_Locale);

Dave: The "...": Is this an exercise left to the reader?

Howard: Always hated those! A locale consists of a collection of facets. Each facet has a specialized responsibility. One facet knows how to format money. We need to create a locale with a money-formatting-facet that knows how we want our Money formatted. Here is how you might write a French franc formatter:

class Fr_Moneypunct : public std::moneypunct<char, false>
{
protected:
 virtual char do_decimal_point() const {return ',';}
 virtual char do_thousands_sep() const {return '.';}
 virtual std::string do_curr_symbol() const
                          {return "Franc";}
 virtual pattern do_pos_format() const {
  pattern result = {{char(sign), char(value), char(space), 
char(symbol)}};
  return result;
 }
 virtual pattern do_neg_format() const {
  pattern result = {{char(sign), char(value), char(space), 
char(symbol)}};
  return result;
 }
};

Then you can install it in a locale with:

std::locale Fr_Locale(std::locale(), new Fr_Moneypunct);

That ought to do it. Ok, I know it may look a little unfamiliar, but you've got to admit, it's not much code.

Dave: Agreed. How about a little more detail?

Howard: The base class std::moneypunct is a standard facet. Its default behavior is to format US currency. However all of this behavior lives under virtual methods that are overridable. The behaviors which are most likely to be changed, are the most easily overridable. For example, changing the decimal point character to ',' is demonstrated above with the do_decimal_point method.

The do_pos_format and do_neg_format methods have the responsibility for dictating the relative position of the sign, value and currency symbols. The default order is "symbol, sign, none, value". A "none" means that there may be zero or more spaces. A "space" means that there are one or more spaces. In our example, it was not enough to change the currency symbol from "$" to "Franc", we also wanted it to be printed after the amount, instead of before. That is why we changed the pattern order to "sign, value, space, symbol".

There is a do_negative_sign() method that returns a string. The first character of this string gets put in the "sign" spot designated by the pattern returned from do_neg_format(). If the string has more than one character, the remaining characters get put at the end of the entire formatting sequence. Thus if do_negative_sign() returns "()", then negative amounts can be formatted as:

(10.000,00 Franc)

Dave: There's that 10.000,00 Franc again. Where does the exchange rate go in?

Howard: Let's take a closer look at our Money class. We could put the exchange logic as a method in there with a method called us_to_france:

class Money
{
public:
  Money(double amount) : amount_(100*amount) {}
  Money& us_to_france() {amount_ /= .1685; return *this;}
  double to_double() const {return amount_;}
private:
  double amount_;
};

So now, assuming you had read in US $10,000.00, you might print a French version like:

os << amount.us_to_france();

and "59.347,18 Franc" will be printed.

I'm going to throw my standard disclaimer in here: The Money class presented here is just to demo locale. I really haven't put any serious design thought into Money itself.

Dave: Ok, so we've created a customized locale, with a customized facet. How does our Money class make use of this machinery?

Howard: The work we've done so far will affect both input and output. To keep this discussion short, let's just look at the output routine for our example Money class:

std::ostream&
operator<< (std::ostream& os, const Money& amount)
{
  using namespace std;
  ostream::sentry ok(os);
  if (ok) 
  {
    const money_put<char>& fmt = use_facet<money_put<char> > (os.getloc());
    ios_base::fmtflags saveflags = os.flags();
    showbase(os);
    if (fmt.put(os, false, os, os.fill(), amount.to_double()).failed())
      os.setstate(ios_base::badbit);
    os.flags(saveflags);
  }
  return os;
}

The bad news is that this code may look strange to you. The good news is that it is pretty much boiler plate code. Most locale-savey output routines you write will look similar to this.

The first line with the sentry animal prepares the stream for output, and ensures that the stream is in good shape. Next the ostream is queried for its locale (which you earlier imbued). Then the locale is searched for its money_put facet via the global method use_facet. The money_put facet is a standard facet that will use your Fr_Moneypunct class through the magic of virtual functions. Finally, you simply ask the money_put facet to "put" your Money on the ostream. money_put knows how to handle amounts expressed as doubles or strings, so you must be able to convert to one of those. That is why the to_double method has been part of the Money class from the beginning.

Note that I've specialized Money to only output to narrow (char) streams. We could have easily templated everything on the character type in order to handle both narrow and wide streams. This is in fact what the standard library does.

Dave: Can you give me some other possibilities for locale?

Howard: I think space will prohibit us from discussing all the facets in this much detail, but I'll try to hit some highlights.

A collate facet is available for sorting characters. The default behavior is the case sensitive stuff you've come to know and love.

A collection of ctype facets wrap the functionality found in the C header <ctype.h> (and <wctype.h>).

Numeric facets control the formatting of both integers and floating point numbers (input and output). There are several similarities here with the monetary facets described above, including the ability to specify decimal point characters and thousands seperators.

A collection of time facets are available to help you format times and dates. If you need to read or write "janvier" instead of "January", you can do it here. CodeWarrior offers a timepunct facet similar to monepunct and numpunct. This facet is non-standard, but so common as to have become a defacto standard. This makes it much easier to customize the names of the days of the week, or the names of the months.

Dave: locale sounds both powerful and flexible. But I can still safely ignore it if I want to right?

Howard: Yes, the default locale will give you the traditional formatting that we are all accustomed to.

 
AAPL
$99.76
Apple Inc.
+2.09
MSFT
$44.08
Microsoft Corpora
+0.45
GOOG
$520.84
Google Inc.
+9.67

MacTech Search:
Community Search:

Software Updates via MacUpdate

TechTool Pro 7.0.5 - Hard drive and syst...
TechTool Pro is now 7, and this is the most advanced version of the acclaimed Macintosh troubleshooting utility created in its 20-year history. Micromat has redeveloped TechTool Pro 7 to be fully 64... Read more
PDFKey Pro 4.0.2 - Edit and print passwo...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Yasu 2.9.1 - System maintenance app; per...
Yasu was originally created with System Administrators who service large groups of workstations in mind, Yasu (Yet Another System Utility) was made to do a specific group of maintenance tasks... Read more
Hazel 3.3 - Create rules for organizing...
Hazel is your personal housekeeper, organizing and cleaning folders based on rules you define. Hazel can also manage your trash and uninstall your applications. Organize your files using a... Read more
Autopano Giga 3.7 - Stitch multiple imag...
Autopano Giga allows you to stitch 2, 20, or 2,000 images. Version 3.0 integrates impressive new features that will definitely make you adopt Autopano Pro or Autopano Giga: Choose between 9... Read more
MenuMeters 1.8 - CPU, memory, disk, and...
MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking... Read more
Coda 2.5 - One-window Web development su...
Coda is a powerful Web editor that puts everything in one place. An editor. Terminal. CSS. Files. With Coda 2, we went beyond expectations. With loads of new, much-requested features, a few... Read more
Arq 4.6.1 - Online backup to Google Driv...
Arq is super-easy online backup for the Mac. Back up to your own Google Drive storage (15GB free storage), your own Amazon Glacier ($.01/GB per month storage) or S3, or any SFTP server. Arq backs up... Read more
Airfoil 4.8.10 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
Apple iMovie 10.0.6 - Edit personal vide...
With an all-new design, Apple iMovie lets you enjoy your videos like never before. Browse your clips more easily, instantly share your favorite moments, and create beautiful HD movies and Hollywood-... Read more

Latest Forum Discussions

See All

This Week at 148Apps: October 13-17, 201...
Expert App Reviewers   So little time and so very many apps. What’s a poor iPhone/iPad lover to do? Fortunately, 148Apps is here to give you the rundown on the latest and greatest releases. And we even have a tremendous back catalog of reviews; just... | Read more »
Angry Birds Transformers Review
Angry Birds Transformers Review By Jennifer Allen on October 20th, 2014 Our Rating: :: TRANSFORMED BIRDSUniversal App - Designed for iPhone and iPad Transformed in a way you wouldn’t expect, Angry Birds Transformers is a quite... | Read more »
GAMEVIL Announces the Upcoming Launch of...
GAMEVIL Announces the Upcoming Launch of Mark of the Dragon Posted by Jessica Fisher on October 20th, 2014 [ permalink ] Mark of the Dragon, by GAMEVIL, put | Read more »
Interview With the Angry Birds Transform...
Angry Birds Transformers recently transformed and rolled out worldwide. This run-and-gun title is a hit with young Transformers fans, but the ample references to classic Transformers fandom has also earned it a place in the hearts of long-time... | Read more »
Find Free Food on Campus with Ypay
Find Free Food on Campus with Ypay Posted by Jessica Fisher on October 20th, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Strung Along Review
Strung Along Review By Jordan Minor on October 20th, 2014 Our Rating: :: GOT NO STRINGSUniversal App - Designed for iPhone and iPad A cool gimmick and a great art style keep Strung Along from completely falling apart.   | Read more »
P2P file transferring app Send Anywhere...
File sharing services like Dropbox have security issues. Email attachments can be problematic when it comes to sharing large files. USB dongles don’t fit into your phone. Send Anywhere, a peer-to-peer file transferring application, solves all of... | Read more »
Zero Age Review
Zero Age Review By Jordan Minor on October 20th, 2014 Our Rating: :: MORE THAN ZEROiPad Only App - Designed for the iPad With its mind-bending puzzles and spellbinding visuals, Zero Age has it all.   | Read more »
Hay Ewe Review
Hay Ewe Review By Campbell Bird on October 20th, 2014 Our Rating: :: SAVE YOUR SHEEPLEUniversal App - Designed for iPhone and iPad Pave the way for your flock in this line drawing puzzle game from the creators of Worms.   | Read more »
My Very Hungry Caterpillar (Education)
My Very Hungry Caterpillar 1.0.0 Device: iOS Universal Category: Education Price: $3.99, Version: 1.0.0 (iTunes) Description: Care for your very own Very Hungry Caterpillar! My Very Hungry Caterpillar will captivate you as he crawls... | Read more »

Price Scanner via MacPrices.net

2013 15-inch 2.0GHz Retina MacBook Pro availa...
B&H Photo has leftover previous-generation 15″ 2.0GHz Retina MacBook Pros now available for $1599 including free shipping plus NY sales tax only. Their price is $400 off original MSRP. B&H... Read more
Updated iPad Prices
We’ve updated our iPad Air Price Tracker and our iPad mini Price Tracker with the latest information on prices and availability from Apple and other resellers, including the new iPad Air 2 and the... Read more
Apple Pay Available to Millions of Visa Cardh...
Visa Inc. brings secure, convenient payments to iPad Air 2 and iPad mini 3as well as iPhone 6 and 6 Plus. Starting October 20th, eligible Visa cardholders in the U.S. will be able to use Apple Pay,... Read more
Textkraft Pocket – the missing TextEdit for i...
infovole GmbH has announced the release and immediate availability of Textkraft Pocket 1.0, a professional text editor and note taking app for Apple’s iPhone. In March 2014 rumors were all about... Read more
C Spire to offer iPad Air 2 and iPad mini 3,...
C Spire on Friday announced that it will offer iPad Air 2 and iPad mini 3, both with Wi-Fi + Cellular, on its 4G+ LTE network in the coming weeks. C Spire will offer the new iPads with a range of... Read more
Belkin Announces Full Line of Keyboards and C...
Belkin International has unveiled a new lineup of keyboard cases and accessories for Apple’s newest iPads, featuring three QODE keyboards and a collection of thin, lightweight folios for both the... Read more
Verizon offers new iPad Air 2 preorders for $...
Verizon Wireless is accepting preorders for the new iPad Air 2, cellular models, for $100 off MSRP with a 2-year service agreement: - 16GB iPad Air 2 WiFi + Cellular: $529.99 - 64GB iPad Air 2 WiFi... Read more
Price drops on refurbished Mac minis, now ava...
The Apple Store has dropped prices on Apple Certified Refurbished previous-generation Mac minis, with models now available starting at $419. Apple’s one-year warranty is included with each mini, and... Read more
Apple refurbished 2014 MacBook Airs available...
The Apple Store has Apple Certified Refurbished 2014 MacBook Airs available for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is free.... Read more
Refurbished 2013 MacBook Pros available for u...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.