Why are unnamed namespaces used and what are their benefits?


Question

I just joined a new C++ software project and I'm trying to understand the design. The project makes frequent use of unnamed namespaces. For example, something like this may occur in a class definition file:

// newusertype.cc
namespace {
  const int SIZE_OF_ARRAY_X;
  const int SIZE_OF_ARRAY_Y;
  bool getState(userType*,otherUserType*);
}

newusertype::newusertype(...) {...

What are the design considerations that might cause one to use an unnamed namespace? What are the advantages and disadvantages?

1
215
8/9/2010 2:41:22 AM

Accepted Answer

(In the following, the things are stuff that does not apply anymore to C++11, but did apply to C++03. C++11 makes almost no differences anymore (if there are, they are just language lawyer differences which I can't recall).).

Unnamed namespaces are a utility to make an identifier translation unit local. They behave as if you would choose an 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).

171
3/20/2019 12:50:00 PM

Having something in an anonymous namespace means it's local to this translation unit (.cpp file and all its includes) this means that if another symbol with the same name is defined elsewhere there will not be a violation of the One Definition Rule (ODR).

This is the same as the C way of having a static global variable or static function but it can be used for class definitions as well (and should be used rather than static in C++).

All anonymous namespaces in the same file are treated as the same namespace and all anonymous namespaces in different files are distinct. An anonymous namespace is the equivalent of:

namespace __unique_compiler_generated_identifer0x42 {
    ...
}
using namespace __unique_compiler_generated_identifer0x42;

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