C++ – Unnamed/anonymous namespaces vs. static functions

cnamespaces

A feature of C++ is the ability to create unnamed (anonymous) namespaces, like so:

namespace {
    int cannotAccessOutsideThisFile() { ... }
} // namespace

You would think that such a feature would be useless — since you can't specify the name of the namespace, it's impossible to access anything within it from outside. But these unnamed namespaces are accessible within the file they're created in, as if you had an implicit using-clause to them.

My question is, why or when would this be preferable to using static functions? Or are they essentially two ways of doing the exact same thing?

Best Answer

~~The C++ Standard reads in section 7.3.1.1 Unnamed namespaces, paragraph 2:~~

~~The use of the static keyword is deprecated when declaring objects in a namespace scope, the unnamed-namespace provides a superior alternative.~~

Static only applies to names of objects, functions, and anonymous unions, not to type declarations.

Edit:

The decision to deprecate this use of the static keyword (affecting visibility of a variable declaration in a translation unit) has been reversed (ref). In this case using a static or an unnamed namespace are back to being essentially two ways of doing the exact same thing. For more discussion please see this SO question.

Unnamed namespace's still have the advantage of allowing you to define translation-unit-local types. Please see this SO question for more details.

Credit goes to Mike Percy for bringing this to my attention.

Related Solutions

C++ – Why are unnamed namespaces used and what are their benefits

Unnamed namespaces are a utility to make an identifier translation unit local. They behave as if you would choose a unique name per translation unit for a namespace:

namespace unique { /* empty */ }
using namespace unique;
namespace unique { /* namespace body. stuff in here */ }

The extra step using the empty body is important, so you can already refer within the namespace body to identifiers like ::name that are defined in that namespace, since the using directive already took place.

This means you can have free functions called (for example) help that can exist in multiple translation units, and they won't clash at link time. The effect is almost identical to using the static keyword used in C which you can put in in the declaration of identifiers. Unnamed namespaces are a superior alternative, being able to even make a type translation unit local.

namespace { int a1; }
static int a2;

Both a's are translation unit local and won't clash at link time. But the difference is that the a1 in the anonymous namespace gets a unique name.

Read the excellent article at comeau-computing Why is an unnamed namespace used instead of static? (Archive.org mirror).

C++ – Documenting namespaces with Doxygen

I have performed an experiment using Doxygen and the two examples and here are the results. The class names in the examples have been renamed to avoid confusion with Doxygen.

Example 1, Outside Namespace

/*!
 *  \addtogroup Records
 *  @{
 */

//! Generic record interfaces and implementations
namespace Records
{

  //! Describes the record interface  
  class Interface;

} // End namespace Records

/*! @} End of Doxygen Groups*/

Doxygen Results:

Click on Modules button (in the main bar).
Click on "Records" module in the window.

Records & Namespaces screen snapshot

Example 2: Within Namespace (class renamed to Fields)

//! Generic record interfaces and implementations
namespace Fields
{
/*!
 *  \addtogroup Fields
 *  @{
 */


  //! Describes the record interface  
  class Interface;

/*! @} End of Doxygen Groups*/

} // End namespace Fields

Doxygen Results:

Click on Modules button (in the main bar).
Click on "Records" module in the window.

Records & Namespaces screen snapshot within namespace

Summary

The location of Doxygen \addtogroup command has different results depending on whether it is located within a namespace definition or outside. When declared outside of a namespace, the Doxygen Modules tab will show the namespace, as shown in Example 1 above. When the \addtogroup command is placed inside a namespace, the Doxygen Modules tab will not display the namespaces as shown in Example 2 above. If you want your namespace to be listed in the Doxygen Modules tab, locate the \addtogroup command outside of the namespace.

Best Answer

Edit:

Related Solutions

C++ – Why are unnamed namespaces used and what are their benefits

C++ – Documenting namespaces with Doxygen

Example 1, Outside Namespace

Doxygen Results:

Example 2: Within Namespace (class renamed to Fields)

Doxygen Results:

Summary

Related Topic