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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

OS X Server 4.1.3 - For OS X 10.10 Yosem...
Designed for OS X and iOS devices, OS X Server makes it easy to share files, schedule meetings, synchronize contacts, develop software, host your own website, publish wikis, configure Mac, iPhone,... Read more
pwSafe 4.1 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
Kodi 15.0.rc1 - Powerful media center to...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
Coda 2.5.11 - One-window Web development...
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 surprises... Read more
Bookends 12.5.7 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Access the power of Bookends directly from Mellel, Nisus Writer Pro, or MS Word (... Read more
Maya 2016 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
RapidWeaver 6.2.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
MacFamilyTree 7.5.2 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Paragraphs 1.0.1 - Writing tool just for...
Paragraphs is an app just for writers. It was built for one thing and one thing only: writing. It gives you everything you need to create brilliant prose and does away with the rest. Everything in... Read more
BlueStacks App Player 0.9.21 - Run Andro...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Version 0.9.21: Note: Now requires OS X 10.8 or later running on a 64-bit Intel processor. Initial stable... Read more

Rage of Bahamut is Giving Almost All of...
The App Store isn't what it used to be back in 2012, so it's not unexpected to see some games changing their structures with the times. Now we can add Rage of Bahamut to that list with the recent announcement that the game is severely cutting back... | Read more »
Adventures of Pip (Games)
Adventures of Pip 1.0 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** ONE WEEK ONLY — 66% OFF! *** “Adventures of Pip is a delightful little platformer full of charm, challenge and impeccable... | Read more »
Divide By Sheep - Tips, Tricks, and Stre...
Who would have thought splitting up sheep could be so involved? Anyone who’s played Divide by Sheep, that’s who! While we’re not about to give you complete solutions to everything (because that’s just cheating), we will happily give you some... | Read more »
NaturalMotion and Zynga Have Started Tea...
An official sequel to 2012's CSR Racing is officially on the way, with Zynga and NaturalMotion releasing a short teaser trailer to get everyone excited. Well, as excited as one can get from a trailer with no gameplay footage, anyway. [Read more] | Read more »
Grab a Friend and Pick up Overkill 3, Be...
Overkill 3 is a pretty enjoyable third-person shooter that was sort of begging for some online multiplayer. Fortunately the begging can stop, because its newest update has added an online co-op mode. [Read more] | Read more »
Scanner Pro's Newest Update Adds Au...
Scanner Pro is one of the most popular document scanning apps on iOS, thanks in no small part to its near-constant updates, I'm sure. Now we're up to update number six, and it adds some pretty handy new features. [Read more] | Read more »
Heroki (Games)
Heroki 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: CLEAR THE SKIES FOR A NEW HERO!The peaceful sky village of Levantia is in danger! The dastardly Dr. N. Forchin and his accomplice,... | Read more »
Wars of the Roses (Games)
Wars of the Roses 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: | Read more »
TapMon Battle (Games)
TapMon Battle 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: It's time to battle!Tap! Tap! Tap! Try tap a egg to hatch a Tapmon!Do a battle with another tapmons using your hatched tapmons! *... | Read more »
Alchemic Dungeons (Games)
Alchemic Dungeons 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ### Release Event! ### 2.99$->0.99$ for limited time! ### Roguelike Role Playing Game! ### Alchemic Dungeons is roguelike... | Read more »

Price Scanner via MacPrices.net

Seagate Backup Plus Drives Feature 200GB of C...
Seagate Technology plc has announced that its Backup Plus family of external storage offerings will now include 200GB of OneDrive cloud storage, a major added value, and the addition of Lyve’s photo... Read more
Canon PIXMA MG3620 Wireless Inkjet All-in-One...
Canon U.S.A., Inc. has announced the PIXMA MG3620 Wireless (1) Inkjet All-in-One (AIO) printer for high-quality photo and document printing. Built with convenience in mind for the everyday home user... Read more
July 4th Holiday Weekend 13-inch MacBook Pro...
Save up to $150 on the purchase of a new 2015 13″ Retina MacBook Pro at the following resellers this weekend. Shipping is free with each model: 2.7GHz/128GB MSRP $1299 2.7GHz/... Read more
27-inch 3.5GHz 5K iMac on sale for $2149, sav...
Best Buy has the 27″ 3.5GHz 5K iMac on sale for $2149.99. Choose free shipping or free local store pickup (if available). Sale price for online orders only, in-store prices may vary. Their price is $... Read more
Apple now offering refurbished 2015 11-inch...
The Apple Store is now offering Apple Certified Refurbished 2015 11″ MacBook Airs as well as 13″ MacBook Airs (the latest models), available for up to $180 off the cost of new models. An Apple one-... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.5GHz Retina MacBook Pro on sale for $2274 including free shipping. Their price is $225 off MSRP, and it’s the lowest price available for this model. Read more
Finally Safe To Upgrade To Yosemite’?
The reason I’ve held back from upgrading my MacBook Air from OS X 10.9 Mavericks to 10.10 Yosemite for nearly a year isn’t just procrastination. Among other bugs reported, there have been persistent... Read more
Logo Pop Free Vector Logo Design App For OS X...
128bit Technologies has released of Logo Pop Free 1.2 for Mac OS X, a vector based, full-fledged, logo design app available exclusively on the Mac App Store for the agreeable price of absolutely free... Read more
21-inch 1.4GHz iMac on sale for $999, save $1...
B&H Photo has new 21″ 1.4GHz iMac on sale for $999 including free shipping plus NY sales tax only. Their price is $100 off MSRP. Best Buy has the 21″ 1.4GHz iMac on sale for $999.99 on their... Read more
16GB iPad mini 3 on sale for $339, save $60
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $339 including free shipping plus NY tax only. Their price is $60 off MSRP. Read more

Jobs Board

Frameworks Engineer, *Apple* Watch - Apple...
**Job Summary** Join the team that is shaping the future of software development for Apple Watch! As a software engineer on the Apple Watch Frameworks team you will Read more
Mobile Payments Counsel, *Apple* Pay (digit...
**Job Summary** Apple is looking for an atto ey to join Apple 's Legal Department to support Apple Pay. **Key Qualifications** 4+ years of relevant experience Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** The ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store which delivers Read more
Partner Marketing Manager, Merchant- *Apple*...
**Job Summary** The Apple Pay partner marketing team is looking for a marketing manager to develop and drive US marketing programs with our merchant partners. The right Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.