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

CrossOver 14.1.6 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Printopia 2.1.14 - Share Mac printers wi...
Run Printopia on your Mac to share its printers to any capable iPhone, iPad or iPod Touch. Printopia will also add virtual printers, allowing you to save print-outs to your Mac and send to apps.... Read more
Google Drive 1.24 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more
Chromium 45.0.2454.85 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 45.0.2454.85: Note: Does not contain the "... Read more
OmniFocus 2.2.5 - GTD task manager with...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
iFFmpeg 5.7.1 - Convert multimedia files...
iFFmpeg is a graphical front-end for FFmpeg, a command-line tool used to convert multimedia files between formats. The command line instructions can be very hard to master/understand, so iFFmpeg does... Read more
VOX 2.6 - Music player that supports man...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
Box Sync 4.0.6567 - Online synchronizati...
Box Sync gives you a hard-drive in the Cloud for online storage. Note: You must first sign up to use Box. What if the files you need are on your laptop -- but you're on the road with your iPhone? No... Read more
Carbon Copy Cloner 4.1.4 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
OmniGraffle Pro 6.3.1 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more

You Can Play Madfinger Games' Unkil...
Madfinger Games - probably best known for the Dead Trigger series - has officially launched their newest zombie shooter (that isn't called Dead Trigger), named Unkilled. [Read more] | Read more »
Rock(s) Rider - HD Edition (Games)
Rock(s) Rider - HD Edition 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: *** PLEASE NOTE: Compatible with iPhone 4s, iPad 2, iPad mini, iPod touch (5th generation) or newer *** Do you... | Read more »
Rebuild 3: Gangs of Deadsville (Games)
Rebuild 3: Gangs of Deadsville 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: It's been a few years since the zombpocalypse turned the world's cities into graveyards and sent the few... | Read more »
Power Ping Pong (Games)
Power Ping Pong 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Do you wield your bat with zen-like focus or do your balls of fury give you a killer spin? Table tennis goes mobile with a... | Read more »
Z.O.N.A Project X (Games)
Z.O.N.A Project X 1.00 Device: iOS Universal Category: Games Price: $1.99, Version: 1.00 (iTunes) Description: Z.O.N.A Project X - shooter in the post-apocalyptic world. | Read more »
Trick Shot (Games)
Trick Shot 1.0.6 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.6 (iTunes) Description: A game where all you have to do is throw a ball into a box, simple? Trick Shot is a minimalist physics puzzler with 90 levels... | Read more »
Goat Simulator MMO Simulator (Games)
Goat Simulator MMO Simulator 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** Coffee Stain Studios brings next-gen... | Read more »
Worms™ 4 (Games)
Worms™ 4 1.02 Device: iOS Universal Category: Games Price: $4.99, Version: 1.02 (iTunes) Description: The latest instalment in the worldwide mega hit franchise! Coming soon to iPhone, iPad and iPod touch. When the guys and girls at... | Read more »
The Deer God (Games)
The Deer God 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: 30% off launch sale!!! “It can be a struggle, but it's all worth it when you're shooting fire out your antlers.” Kotaku “The... | Read more »
AppSpy's Patreon campaign kicks off
Occasionally you'll see us use AppSpy's videos here on 148Apps to support an article we've written. That's because we're part of Steel Media, and AppSpy is Steel's video arm, so we're all part of one happy family. [Read more] | Read more »

Price Scanner via MacPrices.net

Near-Office Input Functionality Virtually Any...
Today Logitech introduced the Logitech K380 Multi-Device Bluetooth Keyboard and the Logitech M535 Bluetooth Mouse, giving users the freedom to work on any device, most anywhere. According to... Read more
College Student Deals: Additional $100 off Ma...
Take an additional $100 off all MacBooks and iMacs at Best Buy Online with their College Students Deals Savings, valid through September 4, 2015. Anyone with a valid .EDU email address can take... Read more
Will You Buy An iPad Pro? – The ‘Book Mystiqu...
It looks like we may not have to wait much longer to see what finally materializes as a new, larger-panel iPad (Pro/Plus?) Usually reliable Apple product prognosticator KGI Securities analyst Ming-... Read more
eFileCabinet Announces SMB Document Managemen...
Electronic document management (EDM) eFileCabinet, Inc., a hosted solutions provider for small to medium businesses, has announced that its SecureDrawer and eFileCabinet Online services will be... Read more
WaterField Designs Unveils American-Made, All...
San Francisco’s WaterField Designs today unveiled their all-leather Cozmo 2.0 — an elegant attach laptop bag with carefully-designed features to suit any business environment. The Cozmo 2.0 is... Read more
Apple’s 2015 Back to School promotion: Free B...
Purchase a new Mac or iPad at The Apple Store for Education and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free,... Read more
128GB MacBook Airs on sale for $100 off MSRP,...
B&H Photo has 11″ & 13″ MacBook Airs with 128GB SSDs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air: $799.99, $100 off MSRP... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
27-inch 3.2GHz iMac on sale for $1679, save $...
B&H Photo has the 27″ 3.2GHz iMac on sale for $1679.99 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Apple and Cisco Partner to Deliver Fast-Lane...
Apple and Cisco have announced a partnership to create a “fast lane” for iOS business users by optimizing Cisco networks for iOS devices and apps. The alliance integrates iPhone with Cisco enterprise... Read more

Jobs Board

*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
Simply Mac *Apple* Specialist- Repair Techn...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the greatest premier retailer of Apple products expertise in North America. We're looking for dedicated individuals to provide personalized service and Read more
*Apple* Desktop Analyst - KDS Staffing (Unit...
…field and consistent professional recruiting achievement. Job Description: Title: Apple Desktop AnalystPosition Type: Full-time PermanentLocation: White Plains, NYHot Read more
Simply Mac- *Apple* Specialist- Store Manag...
Simply Mac is the largest premier retailer for Apple products and solutions. We're looking for dedicated individuals with a passion to simplify and enhance the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.