TweetFollow Us on Twitter

Jul 98 Factory Floor

Volume Number: 14 (1998)
Issue Number: 7
Column Tag: From The Factory Floor

The New C++ Standard: Namespaces

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

By the time you read this, the Final Draft International Standard for C++ should be finalized, approved, and made available for purchase (see last month's interview with Ron Liechty for specifics on where to go to get your copy). In this month's column, our old friend Andreas Hommel, along with new friend Howard Hinnant will tackle an important part of the new standard: C++ namespaces.

Andreas Hommel is currently the C/C++ front-end and 68K back-end architect at Metrowerks. After finishing his Master's degree in Computer Science, Andreas did some contract programming in the desktop publishing area and also published several games on the Macintosh and Amiga. He has been with Metrowerks for five years.

Andreas lives in a small country village about 20 kilometers north of Hamburg, with his wife, two Australian Shepherd dogs, two Arabian horses and one Quarter horse. When he is not coding, riding horses or walking his dogs, Andreas likes running, traveling, playing a good video game, and driving really fast on the Autobahn. He also likes cooking and fine red wines (California cabernets, in particular).

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 are C++ namespaces?

Andreas: Namespaces are one of the newer ANSI C++ features. They allow a programmer to define new named or unnamed declarative regions. The original C++ (and ANSI C) only had one global namespace, but now it is possible to have many user defined namespaces.

For example:

namespace metro {
 int foo();
 int bar;
}

defines a namespace 'metro' and declares the namespace member function 'foo' and defines a namespace member variable 'bar'. You can define anything inside a namespace that can be defined in the global C++ namespace, even other nested namespaces. All these namespace members can then be used using qualified-ids.

For example:

int i = metro::foo() + metro::bar;

would call metro's member 'foo()'. So this is very similar to a C++ class definition. In fact we could have created something very similar using static class members:

class metro_class {
public:
 static int foo();
 static int bar;
};

int j = metro_class::foo() + metro_class::bar;

However, there are differences between a class and a namespace. You cannot create any instances of a namespace (ie a namespace variable or a pointer to a namespace) and all data and function namespace members behave like static class members (non-static namespace members wouldn't make any sense when you cannot have a namespace instance). So namespaces have more restrictions than classes. However, they have the advantage that you can split the definition of a namespace over several parts of one or more translation units (ie source and header files). So you can add new definitions later on.

For example

namespace metro {
 int baz();
}

adds the function 'baz' to our 'metro' namespace and this could be done in the same or any other source or header file.

Dave: So you have to use qualified-ids to access namespace members. Are there any mechanisms in C++ to simplify this?

Andreas: Yes, First, you don't have to use qualified-ids if you are accessing namespace members within the same or nested namespace. So you can do something like this:

namespace metro {
  int k = bar;  // no qualification necessary, uses metro::bar
}

or even:

int metro::baz()  // define metro::baz outside of namespace
{
  return foo();  // no qualification necessary, uses metro::foo
}

But, there are also language extensions that simplify the use of namespace members outside of the namespace. One is a 'using-declaration' that can be useful if you are using a particular namespace member over and over again. For example:

using metro::bar;  // using declaration
int m = bar;  // no qualification necessary, uses metro::bar

introduces metro's member bar to the current namespace which enables you to use metro::bar without any qualification.

The other major extension is called a 'using-directive' which will introduce all names defined in a namespace. For example:

using namespace metro;  // using-directive
int n = foo() + baz();  // no qualification necessary, uses 
                        // metro::foo and metro::baz

Dave: What is an unnamed namespace?

Andreas: An unnamed namespace like:

namespace { int o; }

behaves as if was replaced by:

namespace <unique> { }
using namespace <unique>;
namespace <unique> { int o; }

where <unique> is replaced with a translation unit specific identifier that is different from all other identifiers in a program. So it will be possible to access 'o' without any qualifications in the same translation unit:

void f() { o++; } // uses <unique>::o

but, not from any other translation unit. So this is very similar to:

static int o;
void f() { o++; }

The use of the 'static' keyword in namespace scope is actually deprecated in the current C++ draft.

Dave: Why would you want to use namespaces?

Andreas: Namespaces are very useful for libraries because they can be used to avoid name collisions. If you have a program that has a function 'foo' and you want to use a third party library that has a different function with the same name you would have to change your on program or the library to be able to use this library. If this library would have used its own namespace (e.g., 'metro' from the example above) you wouldn't have this problem. A good example for this is the std:: namespace that is used to implement the standard C++ library.

Dave: What did it take to get the libraries under namespace std?

Howard: Putting MSL (Metrowerks Standard Libraries) under namespace std took a lot more work than we had originally envisioned. There were many issues which needed sorting out, prioritizing, and dealing with within the time allowed. Without the aid of the entire MSL Team (headed by Vicki Scott) we would never have pulled it off so quickly. But it was more than just team work within the MSL Team that counted. Tight and rapid response between the MSL Team and the Compiler Team was crucial. There were many very subtle effects and interactions between the compiler's implementation of namespaces, and the library's use of namespaces. Andreas is great at getting to the heart of a problem, and providing a solution in an amazingly short amount of time.

Dave: What effect will namespaces have on legacy code?

Howard: Standard namespaces have quite a bit of backward compatibility built into them to ease the porting of namespace-ignorant code. We have implemented all of this compatibility, and where we thought was important, added even more backward compatibility.

Standard header names have become very particular, and very important. In previous releases, <iostream> and <iostream.h> were interchangeable. So were <cstdio> and <stdio.h>. This is no longer the case. As a general rule of thumb, namespace-ignorant code should use headers that end with the .h extension. If you use the extension-less headers, then your code should be "namespace std aware".

For example, the following HelloWorld will compile and run correctly:

#include <iostream.h>

int main()
{
 cout << "Hi\n";
}

But this will fail with a compiler error:

#include <iostream>

int main()
{
 cout << "Hi\n";
}

You can make the above HelloWorld work in one of two ways: You can provide a using declaration:

#include <iostream>

int main()
{
 using namespace std;
 cout << "Hi\n";
}

Or you can provide the full name of objects in the standard library:

#include <iostream>

int main()
{
 std::cout << "Hi\n";
}

The purpose of namespace std is to keep library names from conflicting with your own. With a name like cout, a conflict doesn't seem likely. But, consider the name vector or stack. Many users might want to use such names. Such users may even be unaware of their existence in the library. Now that these names are under a namespace, such conflicts are less likely.

Dave: Does namespace affect the C library?

Howard: The "C" library is also under namespace std when used from a C++ program. That is, printf's new name is std::printf. Remember, this is only when using the C++ compiler. C programs will continue to use just plain printf. Also note that if a C++ program includes <stdio.h> instead of <cstdio>, then printf is promoted to the global namespace. So again, just plain printf can be used.

 
AAPL
$100.58
Apple Inc.
+0.01
MSFT
$45.22
Microsoft Corpora
+0.27
GOOG
$583.37
Google Inc.
-1.12

MacTech Search:
Community Search:

Software Updates via MacUpdate

Pages 5.2.2 - Apple's word processo...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices. And... Read more
Quicken 2015 2.0.1 - Complete personal f...
The new Quicken 2015 helps you manage all your personal finances in one place, so you can see where you're spending and where you can save. Quicken automatically categorizes your financial... Read more
CleanMyMac 2.2.7 - Delete files that was...
CleanMyMac makes space for the things you love. Sporting a range of ingenious new features, CleanMyMac 2 lets you safely and intelligently scan and clean your entire system, delete large, unused... Read more
MacFamilyTree 7.2.4 - 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
Videobox 4.1.1 - Download Flash video th...
Videobox allows you to quickly and easily download Flash video from most all of the popular video sites on the internet. Videobox will convert the video into a native Quicktime format so it's ready... Read more
Web Snapper 3.3.5 - Capture entire Web p...
Web Snapper lets you capture Web pages exactly as they appear in your browser. You can send them to a file as images or vector-based, multi-page PDFs. It captures the whole Web page - eliminating... Read more
Picasa 3.9.138 - Organize, edit, and sha...
Picasa and Picasa Web Albums allows you to organize, edit, and upload your photos to the Web from your computer in quick, simple steps. Arrange your photos into folders and albums and erase their... Read more
Tidy Up 3.0.15.0 - Find duplicate files...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Parallels Desktop 10.0 - Run Windows app...
Parallels Desktop is simply the world's bestselling, top-rated, and most trusted solution for running Windows applications on your Mac. With Parallels Desktop for Mac, you can seamlessly run both... Read more
Apple Final Cut Pro X 10.1.3 - Professio...
Apple Final Cut Pro X is a professional video editing solution.Completely redesigned from the ground up, Final Cut Pro adds extraordinary speed, quality, and flexibility to every part of the post-... Read more

Latest Forum Discussions

See All

DotEmu Summer Sale Cuts Several Games’ P...
DotEmu Summer Sale Cuts Several Games’ Prices to $0.99 Posted by Ellis Spice on August 21st, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
King Announces its Newest “Saga” – Diamo...
King Announces its Newest “Saga” – Diamond Digger Saga Posted by Jessica Fisher on August 21st, 2014 [ permalink ] Diamond Digger Saga, by King Digital Entertainment, promises to be another addictive, adorable game in the “Saga” | Read more »
Spacetime Studios’ Greg Mueller Tells Al...
| Read more »
TourStar Review
TourStar Review By Rob Thomas on August 21st, 2014 Our Rating: :: BORING TOURINGUniversal App - Designed for iPhone and iPad When The Ramones sang “Touring, touring/It’s never boring,” they clearly weren’t referring to TourStar.... | Read more »
Track Your Pregnancy with Glow Nurture
Track Your Pregnancy with Glow Nurture Posted by Jessica Fisher on August 21st, 2014 [ permalink ] iPhone App - Designed for the iPhone, compatible with the iPad | Read more »
Outcast Odyssey – An Interview with the...
At this year’s San Diego Comic-Con, fans got the chance to meet and mingle with several of the artists behind Magic Pixel Games and Namco Bandai’s upcoming card battler, Outcast Odyssey. Considering many of these artists have worked on comics in the... | Read more »
BoxPop Review
BoxPop Review By Jennifer Allen on August 21st, 2014 Our Rating: :: UNTAXING PUZZLESUniversal App - Designed for iPhone and iPad Move from tile to tile to clear the board in this slightly too simple and easy puzzle game.   | Read more »
Breakdown Buddy Review
Breakdown Buddy Review By Jennifer Allen on August 21st, 2014 Our Rating: :: FLAWED CONCEPTiPhone App - Designed for the iPhone, compatible with the iPad There’s a good idea here, but Breakdown Buddy isn’t as useful as it tries to... | Read more »
jamstik, the Guitar You Can Take Everywh...
jamstik, the Guitar You Can Take Everywhere, is Now Available for Purchase Posted by Jessica Fisher on August 21st, 2014 [ permalink ] Zivix has announced that their guitar-in-your-pocket, | Read more »
Star Walk 2 - Guide to the Sky Day and N...
Star Walk 2 - Guide to the Sky Day and Night 1.0.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0.0 (iTunes) Description: The next generation of the best-selling Star Walk, winner of Apple Design Award, used by... | Read more »

Price Scanner via MacPrices.net

21-inch iMacs on sale for up to $150 off MSRP
B&H Photo has 21″ iMacs on sale for up to $150 off MSRP including free shipping plus NY sales tax only. B&H will also include a free copy of Parallels Desktop software: - 21″ 2.7GHz iMac: $... Read more
27-inch 3.2GHz iMac on sale for $1698, save $...
Abt has the 27″ 3.2GHz iMac on sale for $1698 including free shipping. Their price is $101 off MSRP. Read more
Mac Backup Guru 2.0 Drive Backup/Cloneing Uti...
Mac Backup Guru developer MacDaddy has released Mac Backup Guru 2.0, offering new and enhanced advanced features, such as bootable backups, synchronised volumes and folders, and a Snapshot mode that... Read more
Operate GE’s New Free-Standing KItchen Range...
Think you accidentally left the oven on? Switch it off while on the go. The new free-standing Profile™ Series gas and electric ranges are GE’s second cooking appliances, following their wall oven, to... Read more
Parallels Announces Parallels Desktop 10 for...
The no. 1-selling software for running Windows applications on a Mac becomes an even easier choice for millions of consumers and IT professionals worldwide with the launch of the most powerful... Read more
Apple now offering certified refurbished 2014...
 The Apple Store is now offering Apple Certified Refurbished 2014 MacBook Airs for up to $180 off the cost of new models. An Apple one-year warranty is included with each MacBook, and shipping is... Read more
Best Buy’s College Student Deals: $100 off Ma...
Take an additional $100 off all MacBooks and iMacs, $50 off iPad Airs and iPad minis, at Best Buy Online with their College Students Deals Savings, valid through September 6th. Anyone with a valid .... Read more
MacBook Airs on sale for $100 off MSRP, free...
B&H Photo has three 2014 MacBook Airs on sale for $100 off MSRP. Shipping is free, and B&H charges NY sales tax only. They also include free copies of Parallels Desktop and LoJack for Laptops... Read more
Razer Taipan Mouse For Gamers And Non-Gamers...
If you’re a serious gamer on either Mac or Windows PCs, a serious gaming mouse is a necessity for first-tier performance. However, even if like me you’re not much of a gamer, there’s still a strong... Read more
15-inch 2.2GHz MacBook Pro on sale for $1899,...
Adorama has the new 15″ 2.2GHz Retina MacBook Pro on sale for $1899 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP, and it’s the lowest price available for this... 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
Position Opening at *Apple* - Apple (United...
**Job Summary** As more and more people discover Apple , they visit our stores seeking ways to incorporate our products into their lives. It's your job, as a Store 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
Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and Read more
*Apple* Solutions Consultant (ASC)- Retail S...
**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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.