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

Thunderbird 52.3.0 - Email client from M...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
coconutBattery 3.6.3 - Displays info abo...
With coconutBattery you're always aware of your current battery health. It shows you live information about your battery such as how often it was charged and how is the current maximum capacity in... Read more
Little Snitch 4.0.2 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
VueScan 9.5.82 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Postbox 5.0.17 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
CleanMyMac 3.8.6 - $39.95
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac lets you safely and intelligently scan and clean your entire system, delete large, unused files... Read more
Default Folder X 5.1.6b3 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click on... Read more
Amazon Chime 4.6.5852 - Amazon-based com...
Amazon Chime is a communications service that transforms online meetings with a secure, easy-to-use application that you can trust. Amazon Chime works seamlessly across your devices so that you can... Read more
VOX 2.8.30 - Music player that supports...
VOX just sounds better! 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 audio formats you should ever need.... Read more
iFFmpeg 6.4.3 - Convert multimedia files...
iFFmpeg is a comprehensive media tool to convert movie, audio and media files between formats. The FFmpeg command line instructions can be very hard to master/understand, so iFFmpeg does all the hard... Read more

War Wings beginner's guide - how to...
War Wings is the newest project from well-established game maker Miniclip. It's a World War II aerial dogfighting game with loads of different airplane models to unlock and battle. The game offers plenty of single player and multiplayer action. We... | Read more »
How to win every 2v2 battle in Clash Roy...
2v2 is coming back to Clash Royale in a big way. Although it's only been available for temporary periods of time, 2v2 has seen a hugely positive fan response, with players clamoring for more team-based gameplay. Soon we'll get yet another taste of... | Read more »
Roll to Win with Game of Dice’s new upda...
Joycity’s hit Game of Dice gets a big new update this week, introducing new maps, mechanics, and even costumes. The update sets players loose on an exciting new map, The Cursed Tower, that allows folks to use special Runes mid-match. If you feel... | Read more »
Bottom of the 9th (Games)
Bottom of the 9th 1.0.1 Device: iOS iPhone Category: Games Price: $4.99, Version: 1.0.1 (iTunes) Description: Play the most exciting moment of baseball in this fast-paced dice and card game! | Read more »
The best apps for viewing the solar ecli...
If you somehow missed the news, many parts of the United States will be witness to a total solar eclipse on August 21 for the first time in over 90 years. It'll be possible to see the eclipse in at least some capacity throughout the continental U... | Read more »
The 5 best mobile survival games
Games like ARK: Survival Evolved and Conan Exiles have taken the world of gaming by storm. The market is now flooded with hardcore survival games that send players off into the game's world with nothing but maybe the clothes on their back. Never... | Read more »
Portal Walk (Games)
Portal Walk 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Portal Walk is adventure and relaxing platform game about Eugene. Eugene stuck between worlds and trying to find way back home.... | Read more »
Technobabylon (Games)
Technobabylon 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: City of Newton, 2087. Genetic engineering is the norm, the addictive Trance has replaced almost any need for human interaction,... | Read more »
5 reasons why 2v2 is the best mode in Cl...
Supercell has been teasing fans with 2v2 windows that allow players to team up for limited periods of time. The Summer of 2v2 was just this past July, but players are already clamoring for more of that sweet, sweet team-based action. The fans have... | Read more »
The best deals on the App Store this wee...
It seems like the week's only just started, and yet here we are with a huge pile of discounted games to sort through. There are some real doozies on sale this week. We're talking some truly stellar titles. Let's take a look at four of the best... | Read more »

Price Scanner via MacPrices.net

Back To School With The Edge Desk All-in-one...
Back to school is just around the corner, and the ergonomically correct Edge Desk all-in-one portable kneeling desk is ideal for students living in dorms and small apartments, Edge Desk features:... Read more
Norton Core Secure Wi-Fi Router Now Available...
First introduced at the 2017 Consumer Electronics Show (CES), Norton Core, a secure, high-performance Wi-Fi router, fundamentally changed the concept of Wi-Fi routers by making security the primary... Read more
ViewSonic Adds New 27-inch 4K UHD Monitor to...
ViewSonic Corp. has introduced the VP2785-4K, a 27-inch 4K UHD (3840×2160) monitor that delivers precise and consistent color representation and performance to ensure incredible image quality. Built... Read more
Apple now offering Certified Refurbished 2017...
Apple is now offering Certified Refurbished 2017 27″ iMacs for up to $350 off original MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: – 27″ 3.... Read more
13-inch 2.3GHz MacBook Pros on sale for $100...
Amazon has the new 2017 13″ 2.3GHz MacBook Pros on sale today for $100 off MSRP, each including free shipping: – 13″ 2.3GHz/128GB Space Gray MacBook Pro (MPXQ2LL/A): $1199.99 $100 off MSRP – 13″ 2.... Read more
Clearance 2016 13-inch MacBook Airs available...
B&H Photo has clearance 2016 13″ MacBook Airs available for up to $200 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: – 13″ 1.6GHz/128GB MacBook Air (MMGF2LL... Read more
Clearance 21-inch and 27-inch iMacs available...
B&H Photo has clearance 21″ and 27″ Apple iMacs available for up to $500 off original MSRP, each including free shipping plus NY & NJ sales tax only: – 27″ 3.3GHz iMac 5K: $1799 $500 off... Read more
New iOS 11 Productivity Features Welcome But...
The iOS community is in late summer holding mode awaiting the September arrival of the iPhone 8 and iOS 11. iOS 11 public betas have been available for months — number six was released this week —... Read more
Samsung Electronics Launches New Portable SSD...
Samsung Electronics America, Inc. has announced the launch of Samsung Portable SSD T5 – its newest portable solid state drive (PSSD) that raises the bar for the performance of external memory... Read more
TrendForce Reports YoY Gain of 3.6% for 2Q17...
Market research firm TrendForce reports that the global notebook shipments for this second quarter registered a sequential quarterly increase of 5.7% and a year-on-year increase of 3.6%, totaling 39.... Read more

Jobs Board

Business Development Manager - *Apple* Medi...
Job Summary Apple Music is a single, intuitive app that...- all in one place. You can stream any Apple Music song, playlist or album, and download it Read more
Development Operations and Site Reliability E...
Development Operations and Site Reliability Engineer, Apple Payment Gateway Job Number: 57572631 Santa Clara Valley, California, United States Posted: Jul. 27, 2017 Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
Program Manager, *Apple* Pay Business Opera...
…Manager to deliver and sustain a seamless user and support experience for Apple Pay Cash, the upcoming person-to-person payments feature in iMessage. On the Business Read more
Sr. Software Engineer, Core Services, *Apple...
…You will be part of the server team that powers various features within the Apple client applications - iTunes, App Store, iBooks, Podcast, Apple Music etc. You Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.