Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




Thinking in C++
Prev Contents / Index Next

Namespaces in headers

You’ll notice that using directives are present in nearly all the cpp files in this book, usually in the form:

using namespace std;

Since std is the namespace that surrounds the entire Standard C++ library, this particular using directive allows the names in the Standard C++ library to be used without qualification. However, you’ll virtually never see a using directive in a header file (at least, not outside of a scope). The reason is that the using directive eliminates the protection of that particular namespace, and the effect lasts until the end of the current compilation unit. If you put a using directive (outside of a scope) in a header file, it means that this loss of “namespace protection” will occur with any file that includes this header, which often means other header files. Thus, if you start putting using directives in header files, it’s very easy to end up “turning off” namespaces practically everywhere, and thereby neutralizing the beneficial effects of namespaces.

In short: don’t put using directives in header files.

Thinking in C++
Prev Contents / Index Next

   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire