TweetFollow Us on Twitter

Sept 98 Factory Floor

Volume Number: 14 (1998)
Issue Number: 9
Column Tag: From The Factory Floor

The New C++ Standard:
Partial Template Specialization

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

In last month's column, we continued our exploration of the new C++ standard, specifically taking a look at the subject of locales. In this month's column, Howard Hinnant is back once again, and will take us through partial template specialization.

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 exactly is partial template specialization?

Howard: In order to explain partial template specialization, it might first be easier to describe full template specialization. Consider:

template <class T>
class vector
{
...
};

This works well for most items that we might want to keep a list of. However, if we wanted to keep a vector<bool>, then an obvious optimization would be store each value in one bit, instead of actually storing a list of bools. This could be neatly accomplished by defining a specialization of our vector class:

template <>
class vector<bool>
{
...
};

This is known as template specialization.

Now consider a class template with two type parameters. I'll keep picking on vector:

template <class T, class Allocator>
class vector {
...
};

And let's say we still want to optimize for when T == bool. We can do this by:

template <class Allocator>
class vector<bool, Allocator> {
...
};

Since not all template parameters have been nailed down, this is known as partial template specialization. So now when I say:

vector<bool, myAllocator> a;

I get the optimized implementation for bool.

Dave: Cool. Any other useful optimizations you could do?

Howard: You can do some really cool things with this concept. For example, let's say you wanted to make a special vector for holding pointers. Here is a possibility:

template <class T, class Allocator>
class vector<T*, Allocator>
{
...
};

This might come in handy if you wanted some special behavior for when the element type was a pointer to something. For instance, one might want to treat vectors of pointers as pointing to heap based objects, and manage those pointers with new and delete. The standard library, of course, does not do this.

Dave: So how does partial template specialization affect the standard library?

Howard: MSL does use partial template specialization to implement vector<bool> as alluded to earlier. Additionally, there is a struct in <iterator> that looks like:

template <class Iterator>
struct iterator_traits
{
  typedef typename Iterator::difference_type  difference_type;
  typedef typename Iterator::value_type  value_type;
  typedef typename Iterator::pointer    pointer;
  typedef typename Iterator::reference    reference;
  typedef typename Iterator::iterator_category  iterator_category;
};

The purpose of this class is to help code that takes iterators find out valuable things about the iterator that it's working on. For example, consider the standard algorithm iter_swap that takes two iterators, and swaps the values that the iterators point to:

template <class ForwardIterator1, class ForwardIterator2>
void
iter_swap(ForwardIterator1 a, ForwardIterator2 b)
{
  typedef typename                               iterator_traits<ForwardIterator1>::value_type Value;
  Value tmp(*a);
  *a = *b;
  *b = tmp;
}

The routine iter_swap must create a temporary variable in order to accomplish the swap. But what is the type of the temporary? It queries the iterator to find out the proper type.

Thus all valid iterators (at least those that want to be used in standard algorithms and containers) must create the typedefs referred to above so that they can be queried via iterator_traits. This can be easily accomplished by deriving your custom iterators from the standard iterator struct:

In <iterator>:

template <class Category, class T, class Distance = ptrdiff_t,
     class Pointer = T*, class Reference = T&>  
struct iterator
{
   typedef Distance    difference_type;
   typedef T        value_type;
   typedef Pointer    pointer;
   typedef Reference  reference;
   typedef Category    iterator_category;
};

In your code:

class MyIterator
  : public std::iterator<random_access_iterator_tag, MyClass>
{
...
};

Dave: I see. But if I remember right, an iterator is just a generalization of a built-in pointer. In fact, built-in pointers can be used in all the standard algorithms. So how does iter_swap query a built-in pointer for its value_type? You can't derive int* from std::iterator.

Howard: Ahh... Exactly! This is where partial template specialization rides in to save the day. The standard library defines a specialization of iterator_traits for a pointer to anything:

template <class T>
struct iterator_traits<T*>
{
  typedef ptrdiff_t    difference_type;
  typedef T          value_type;
  typedef T*        pointer;
  typedef T&        reference;
  typedef random_access_iterator_tag  iterator_category;
};

So now when iter_swap tries to define Value:

typedef typename iterator_traits<ForwardIterator1>::value_type Value;

And ForwardIterator1 has the type int*, iterator_traits<int*> picks up the partial specialization and answers back with int. This is really a pretty slick design.

Dave: That is neat. But partial template specialization is a relatively new feature of CodeWarrior. How did MSL handle this problem before this feature was available?

Howard: Oh, yes... The Dark Time. We took advantage of the fact that we did have full template specialization available to us. So we created full specializations for iterator_traits for every type we could think of: char*, int*, bool*, short*, long* ... plus all combinations of unsigned and const modifiers. This worked pretty well except for one minor little detail. We could not create a specialization for myType*. Here myType represents all of the classes which the customer created.

In order to combat this last problem, we (actually Dennis C. De Mars) created a macro which defined a full specialization of iterator_traits for its argument myType:

#define __MSL_FIX_ITERATORS__(myType) \
 template<> \
 struct std::iterator_traits  <myType*> { \
   typedef ptrdiff_t      difference_type; \
   typedef myType        value_type; \
   typedef myType*        pointer; \
   typedef myType&        reference; \
   typedef random_access_iterator_tag  iterator_category; \
 };

Yes, everyone knows about __MSL_FIX_ITERATORS__. And you now know the full story behind this macro. It is a macro that no one liked, but we could not live without it.

Even if you didn't use the standard algorithms, you got bit by this when you used the standard containers, because they used it. We had plans to reduce MSL's dependence on iterator_traits, but Andreas (our C++ compiler guru) came out with partial specialization before we could implement those plans. This was really best as we could have only reduced the dependence, not eliminated it.

 
AAPL
$524.94
Apple Inc.
+5.93
MSFT
$40.01
Microsoft Corpora
-0.39
GOOG
$536.10
Google Inc.
-20.44

MacTech Search:
Community Search:

Software Updates via MacUpdate

Tweetbot 1.5.1 - Popular iOS twitter cli...
Tweetbot is a full-featured OS X Twitter client with a lot of personality. Whether it's the meticulously-crafted interface, sounds and animation, or features like multiple timelines and column views... Read more
Mac DVDRipper Pro 4.1.7 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
PDFpenPro 6.2 - Advanced PDF toolkit for...
PDFpenPro allows users to edit PDF's easily. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Create... Read more
PDFpen 6.2 - Edit and annotate PDFs with...
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
Monolingual 1.5.9 - Remove unwanted OS X...
Monolingual is a program for removing unnecesary language resources from OS X, in order to reclaim several hundred megabytes of disk space. It requires a 64-bit capable Intel-based Mac and at least... Read more
Maya 2015 - 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
Starcraft II: Wings of Liberty 1.1.1.180...
Download the patch by launching the Starcraft II game and downloading it through the Battle.net connection within the app. Starcraft II: Wings of Liberty is a strategy game played in real-time. You... Read more
Sibelius 7.5.0 - Music notation solution...
Sibelius is the world's best-selling music notation software for Mac. It is as intuitive to use as a pen, yet so powerful that it does most things in less than the blink of an eye. The demo includes... Read more
Typinator 5.9 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
MYStuff Pro 2.0.16 - Create inventories...
MYStuff Pro is the most flexible way to create detail-rich inventories for your home or small business. Add items to MYStuff by dragging and dropping existing information, uploading new images, or... Read more

Latest Forum Discussions

See All

Have a Special Dead Trigger 2 Easter Bas...
Have a Special Dead Trigger 2 Easter Basket Full of Goodies, Courtesy of Madfinger Games Posted by Rob Rich on April 18th, 2014 [ permalink ] Dead Trigger 2 | Read more »
Zynga Launches Brand New Farmville Exper...
Zynga Launches Brand New Farmville Experience with Farmville 2: Country Escape Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
David. Review
David. Review By Cata Modorcea on April 18th, 2014 Our Rating: :: MINIMALISTIC IN A DIFFERENT WAYUniversal App - Designed for iPhone and iPad David is a minimalistic game wrapped inside of a soothing atmosphere in which the hero... | Read more »
Eyefi Unveils New Eyefi Cloud Service Th...
Eyefi Unveils New Eyefi Cloud Service That Allows Users to Share Media Across Personal Devices Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Tales from the Dragon Mountain: The Lair...
Tales from the Dragon Mountain: The Lair Review By Jennifer Allen on April 18th, 2014 Our Rating: :: STEADY ADVENTURINGiPad Only App - Designed for the iPad Treading a safe path, Tales from the Dragon Mountain: The Lair is a... | Read more »
Yahoo Updates Flickr App with Advanced E...
Yahoo Updates Flickr App with Advanced Editing Features and More Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
My Incredible Body - A Kid's App to...
My Incredible Body - A Kid's App to Learn about the Human Body 1.1.00 Device: iOS Universal Category: Education Price: $2.99, Version: 1.1.00 (iTunes) Description: Wouldn’t it be cool to look inside yourself and see what was going on... | Read more »
Trials Frontier Review
Trials Frontier Review By Carter Dotson on April 18th, 2014 Our Rating: :: A ROUGH LANDINGUniversal App - Designed for iPhone and iPad Trials Frontier finally brings the famed stunt racing franchise to mobile, but how much does its... | Read more »
Evernote Business Notebook by Moleskin I...
Evernote Business Notebook by Moleskin Introduced – Support Available in Evernote for iOS Posted by Tre Lawrence on April 18th, 2014 [ permalink ] | Read more »
Sparkle Unleashed Review
Sparkle Unleashed Review By Jennifer Allen on April 18th, 2014 Our Rating: :: CLASSY MARBLE FLINGINGUniversal App - Designed for iPhone and iPad It’s a concept we’ve seen before, but Sparkle Unleashed is a solidly enjoyable orb... | Read more »

Price Scanner via MacPrices.net

iMacs on sale for up to $160 off MSRP this we...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
iPad Airs on sale this weekend for up to $100...
Best Buy has WiFi iPad Airs on sale for $50 off MSRP and WiFi + Cellular iPad Airs on sale for $100 off MSRP on their online store for a limited time, with prices now starting at $449. Choose free... Read more
Apple restocks refurbished Mac minis starting...
The Apple Store has restocked Apple Certified Refurbished Mac minis for up to $150 off the cost of new models. Apple’s one-year warranty is included with each mini, and shipping is free: - 2.5GHz Mac... Read more
Hyundai Brings Apple CarPlay To The 2015 Sona...
Hyundai Motor America has announced it will bring Apple CarPlay functionality to the 2015 Sonata. CarPlay is pitched as a smarter, safer and easier way to use iPhone in the car and gives iPhone users... Read more
Updated iPads Coming Sooner Than We Had Thoug...
MacRumors, cites KGI securities analyst Ming Chi Kuo, well-respected as an Apple product prognisticator, saying that Apple will introduce an upgraded iPad Air and iPad mini in 2014/Q3, meaning the... Read more
Toshiba Unveils New High And Low End Laptop M...
Toshiba has announced new laptop models covering both the high-end and low-end of the notebook computer spectrum. Toshiba 4K Ultra HD Laptop Toshiba’s new Satellite P55t features one of the world’s... Read more
Save up to $270 with Apple refurbished 13-inc...
The Apple Store has Apple Certified Refurbished October 2013 13″ Retina MacBook Pros available starting at $1099, with models up to $270 off MSRP. Apple’s one-year warranty is standard, and shipping... Read more
Apple now offering refurbished iPad mini with...
The Apple Store has Certified Refurbished 2nd generation iPad minis with Retina Displays now available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free.... Read more
Microsoft Blinks – Drops Microsoft Office 365...
Microsoft has dropped the annual subscription fee for Microsoft Office 365 Personal – which is needed in order to create and edit documents in Microsoft Office for iPad. However, Apple’s iOS and OS X... Read more
New AVG Vault Apps for iOS and Android Help K...
AVG Technologies N.V. an online security company for 177 million active users, has announced the launch of its latest mobile application, AVG Vault. The free app introduces an innovative user... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Retail - Manager - Holyoke - Apple I...
Job Summary Keeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you’re a master of them all. In the store’s fast-paced, Read more
*Apple* Retail - Manager - Apple (United Sta...
Job SummaryKeeping an Apple Store thriving requires a diverse set of leadership skills, and as a Manager, you're a master of them all. In the store's fast-paced, dynamic Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Retail - Market Leader - Cincinnati...
…challenges of developing individuals, building teams, and affecting growth across Apple Stores. You demonstrate successful leadership ability - focusing on excellence Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.