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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Sibelius 2020.6 - Music notation solutio...
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
Bookends 13.4.2 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
OmniGraffle Pro 7.16 - 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
Drive Genius 6.1.0 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
Tor Browser 9.5 - Anonymize Web browsing...
The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. It contains a modified copy of Firefox that... Read more
VueScan 9.7.28 - 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
OmniGraffle 7.16 - Create diagrams, flow...
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
WALTR 2 2.6.26 - $39.95
WALTR 2 helps you wirelessly drag-and-drop any music, ringtones, videos, PDF, and ePub files onto your iPhone, iPad, or iPod without iTunes. It is the second major version of Softorino's critically-... Read more
Airmail 4.1 - Powerful, minimal email cl...
Airmail is an mail client with fast performance and intuitive interaction. Support for iCloud, MS Exchange, Gmail, Google Apps, IMAP, POP3, Yahoo!, AOL, Outlook.com, Live.com. Airmail was designed... Read more
Iridient Developer 3.3.12 - Powerful ima...
Iridient Developer (was RAW Developer) is a powerful image-conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more

Latest Forum Discussions

See All

Willy Jetman: Astromonkey's Revenge...
Barcelona-based developer Last Chicken Games are set to bring their game Willy Jetman: Astromonkey's Revenge to both iOS and Android on 9th July. The Metroidvania is already available on the likes of PS4, Switch and PC but now mobile folk will be... | Read more »
The 5 Best Mobile Real Time Strategy Gam...
Real-time strategy games feel like they’d be a perfect fit for mobile, but they’re trickier to pull off that you might think. The traditional mold of base-building and micro management can work on touch screens, but needs to be carefully honed so... | Read more »
Using your phone in a protest
I can't write about games today. There is a struggle happening in the streets right now and it needs everyone's attention. Here's some good info on how you can use your iOS device safely amidst a protest. | Read more »
Dungonian is a card-based dungeon crawle...
Dungonian is a card-based dungeon crawler from developer SandFish Games that only recently launched as a free-to-play title. It offers an extensive roster of playable heroes to collect and enemies to take down, and it's available right now for iOS... | Read more »
Steam Link Spotlight - Signs of the Sojo...
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was XCOM: Chimera Squad. Read about how it plays using Steam Link's new mouse and keyboard support over here. | Read more »
Steampunk Tower 2, DreamGate's sequ...
Steampunk Tower 2 is a DreamGate's follow up to their previous tower defence game. It's available now for both iOS and Android as a free-to-play title and will see players defending their lone base by kitting it out with a variety of turrets. [... | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Pokemon Go Fest 2020 will be a virtual e...
Niantic has announced that Pokemon Go Fest will still take place this year although understandably it won't be a physical event. Instead, it will become a virtual celebration and is set to be held on 25th and 26th July. [Read more] | Read more »
Marvel Future Fight's major May upd...
Marvel Future Fight's latest update has now landed, and it sounds like a big one. The focus this time around is on Marvel's Guardians of the Galaxy, and it introduces all-new characters, quests, and uniforms for players to collect. [Read more] | Read more »
SINoALICE, Yoko Taro and Pokelabo's...
Yoko Taro and developer Pokelabo's SINoALICE has now opened for pre-registration over on the App Store. It's already amassed 1.5 million Android pre-registrations, and it's currently slated to launch on July 1st. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple restocks 27″ iMacs, Certified Refurbish...
Apple has restocked Certified Refurbished 2019 27″ iMacs starting at $1529 and up to $350 off the cost of new models. Apple’s one-year warranty is standard, shipping is free, and each iMac features a... Read more
Apple’s new 2020 13″ 4-Core MacBook Airs on s...
B&H Photo has Apple’s new 2020 13″ 4-Core MacBook Airs on sale today for $100 off Apple’s MSRP, only $1199. Expedited shipping is free to many addresses in the US. The MacBook Airs are the same... Read more
New Verizon promo: $150 off any Apple Watch w...
Verizon is offering $150 off any Apple Watch when purchased alongside an iPhone through June 10, 2020. They’re also offering up to $100 on any Apple Watch trade-in. Here are the details: “Get $150... Read more
Last year’s 13″ 2.4GHz MacBook Pros are avail...
Apple has Certified Refurbished 2019 13″ 2.4GHz/256GB 4-Core Touch Bar MacBook Pros available for $1359, $440 off original MSRP. Apple’s one-year warranty is included, shipping is free, and each... Read more
Apple’s new 2020 13″ MacBook Pros on sale for...
Apple reseller Abt Electronics has new 2020 13″ MacBook Pros on sale today for up to $140 off MSRP, starting at $1208. Shipping is free, and most configurations are in stock today. Note that Abt’s... Read more
Apple CEO Reacts To Nationwide Protests Over...
NEWS: 06.03.20 – With the recent death of a black man in the custody of a white police officer igniting outrage among Americans from all walks of life, which resulted in protests and civil unrest... Read more
At up to $420 off MSRP, these Certified Refur...
Apple has Certified Refurbished 2019 16″ MacBook Pros available for up to $420 off the cost of new models, starting at $2039. Each model features a new outer case, shipping is free, and an Apple 1-... Read more
Apple restocks refurbished 3rd generation 12....
Apple restocked select 3rd generation 12.9″ WiFi iPad Pros starting at only $699 and up to $330 off original MSRP. Each iPad comes with a standard Apple one-year warranty, outer cases are new, and... Read more
These wireless carriers are offering Apple’s...
Looking for a deal on Apple’s new iPhone SE? Apple itself offers the unlocked 64GB iPhone SE for $399 or $16.62/month. If you’re willing to try a new carrier, two of Apple’s wireless carriers are... Read more
Save $80 on the 64GB 10.5″ iPad Air with this...
Apple has 10.5″ 64GB WiFi iPad Airs models available for $80 off MSRP, Certified Refurbished. Each iPad comes with Apple’s standard one-year warranty and includes a new outer case. Shipping is free... Read more

Jobs Board

*Apple* Architect - SAIC (United States)
**Description** We are currently seeking a motivated, career and customer oriented Apple Architect to join our team in Washington, DC to begin an exciting and Read more
*Apple* Support Engineer - SAIC (United Stat...
**Description** We are currently seeking a motivated, career and customer oriented Apple Support Engineer to join our team in Washington, DC to begin an exciting and Read more
Perioperative RN - ( *Apple* Hill Surgical C...
Perioperative RN - ( Apple Hill Surgical Center) Tracking Code 60593 Job Description Monday - Friday - Full Time Days Possible Saturdays General Summary: Under the Read more
Senior Practice Manager - *Apple* Hill Eye...
Senior Practice Manager - Apple Hill Eye Center Tracking Code 61713 Job Description Apple Hill Medical Center General Summary: Under general supervision, manages Read more
*Apple* Mac Desktop Support - Global Dimensi...
…Operate and support an Active Directory (AD) server-client environment for all Apple devices operating on the BUMED network + Leverage necessary industry enterprise Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.