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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

OmniGraffle Pro 6.1.4 - 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
OmniGraffle 6.1.4 - Create diagrams, flo...
OmniGraffle 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 Graffle to... Read more
MegaSeg 5.9.5 - Professional MP3 DJ appl...
MegaSeg is a complete solution for pro audio/video DJ mixing, radio automation, and music scheduling with rock-solid performance and an easy-to-use design. Mix with visual waveforms and Magic... Read more
MarsEdit 3.6.8 - Quick and convenient bl...
MarsEdit is a blog editor for OS X that makes editing your blog like writing email, with spell-checking, drafts, multiple windows, and even AppleScript support. It works with with most blog services... Read more
BBEdit 11.0.3 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Microsoft Office Preview 15.8 - Popular...
Welcome to the new and modern Microsoft Office for Mac. You will receive regular updates automatically until the official release in the second half of 2015. With the redesigned Ribbon and your... Read more
Yosemite Cache Cleaner 9.0.5 - Clear cac...
Yosemite Cache Cleaner is an award-winning general purpose tool for OS X. YCC makes system maintenance simple with an easy point-and-click interface to many OS X functions. Novice and expert users... Read more
ExpanDrive 4.3.2 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
RapidWeaver 6.0.8 - 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
Artlantis Studio 5.1.2.7 - 3D rendering...
Artlantis Studio is a unique and ideal tool for performing very high resolution rendering easily and in real time. The new FastRadiosity engine now lets you compute images in radiosity-even in... Read more

Bored? MyLeisure FreeTime Maximizer Will...
Bored? MyLeisure FreeTime Maximizer Will Take Care of That! Posted by Jessica Fisher on March 5th, 2015 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
New Publisher Allstar Games Heads West w...
Allstar Games has announced its first mobile title designed for western audiences, Allstar Heroes. The game will be a massive online battle arena (MOBA) that offers dozens of heroes for you to collect and pit against your opponents. As each hero has... | Read more »
RAD Boarding Review
RAD Boarding Review By Jennifer Allen on March 5th, 2015 Our Rating: :: NEARLY RADUniversal App - Designed for iPhone and iPad RAD Boarding isn’t quite one of the greats, but it has potential.   | Read more »
Presenting the International Mobile Gami...
11th Annual International Mobile Gaming Awards ceremony, hosted by actress Allison Haislip, gathered mobile game developers and publishers from around the world. They chose 13 winners out of the 93 nominations. British studio USTWO won the the Grand... | Read more »
AG Drive Review
AG Drive Review By Tre Lawrence on March 5th, 2015 Our Rating: :: FUTURISTIC STREET RACING.Universal App - Designed for iPhone and iPad Futuristic racing… interstellar style.   | Read more »
GDC 2015 – Nightmare Guardians is an Int...
GDC 2015 – Nightmare Guardians is an Interesting Hybrid of MOBA and Lane Defense Posted by Rob Rich on March 5th, 2015 [ permalink ] I have to say that lane defense (i.e. | Read more »
Overkill 3 Review
Overkill 3 Review By Tre Lawrence on March 5th, 2015 Our Rating: :: WHO'S NEXT?Universal App - Designed for iPhone and iPad Cover system gameplay in the third-person.   Developer: Craneballs Price: Free Version Reviewed: 1.1.6... | Read more »
Warner Bros. Interactive Entertainment A...
Warner Bros. has some exciting games coming down the pipe! | Read more »
GDC 2015 – Star Trek Timelines will Prob...
GDC 2015 – Star Trek Timelines will Probably Make Your Inner Trekkie Squeal With Glee Posted by Rob Rich on March 4th, 2015 [ permalink ] Any popular fictional universe has its fair share of fan fiction – where belo | Read more »
Protect Yourself from an Onslaught of Ca...
Surprise Attack Games has announced a Cat-astrophic new physics puzzler called Fort Meow! In the game, a young girl named Nia finds her grandfather’s journal which triggers an all mighty feline attack! Why do the cats want the journal? Who knows,... | Read more »

Price Scanner via MacPrices.net

Apple restocks refurbished 15-inch Retina Mac...
The Apple Store has restocked Apple Certified Refurbished 2014 15″ Retina MacBook Pros, available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model,... Read more
Roundup of MacBook Air sale prices, models up...
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more
New Firstrade Mobile App Enables On-The-Go Tr...
Firstrade Securities Inc. has announced its new mobile app, which gives investors immediate access to the company’s trading platform on all mobile devices. The app was developed in-house and was... Read more
Sonnet Introduces USB 3.0 + eSATA Thunderbolt...
Sonnet has announced the launch of its new USB 3.0 + eSATA Thunderbolt Adapter for easy connectivity to USB 3.0 devices and eSATA storage, and USB 3.0 + Gigabit Ethernet Thunderbolt Adapter for easy... Read more
Apple restocks refurbished 27-inch 5K iMacs f...
The Apple Store has restocked Apple Certified Refurbished 27″ 3.5GHz 5K iMacs for $2119 including free shipping. Their price is $380 off the cost of new models, and it’s the lowest price available... Read more
Free Clean Reader Mobile App Hides Swear Word...
The new Clean Reader app, now available in the Apple App Store and Google Play, delivers the opportunity of reading any book without being exposed to profanity. By selecting how clean they want their... Read more
Kinsa Launches “Groups” App to Monitor Illnes...
Kinsa, makers of the first FDA approved app-enabled smartphone thermometer thst won the 2013 Cleveland Clinic Medical Innovation Grand Prize and recently appeared in Apple’s “Parenthood” TV... Read more
iPad: A More Positive Outlook – The ‘Book Mys...
It’s good to hear someone saying positive things about the iPad. I’ve been trying to bend my mind around how Apple’s tablet could have gone from zero to bestselling personal computing device on the... Read more
Mac Pros on sale for up to $279 off MSRP
Amazon has Mac Pros in stock and on sale for up to $279 off MSRP. Shipping is free: - 4-Core Mac Pro: $2725.87, $273 off MSRP (9%) - 6-Core Mac Pro: $3719.99, $279 off MSRP (7%) Read more
Sale! 13-inch Retina MacBook Pros for up to $...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $205 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1219.99 save $80 - 13″ 2.... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the 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
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You 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
*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.