Unnamed namespaces (C++ only)

A namespace with no identifier before an opening brace produces an unnamed namespace. Each translation unit may contain its own unique unnamed namespace. The following example demonstrates how unnamed namespaces are useful.

#include <iostream>

using namespace std;

namespace {
   const int i = 4;
   int variable;
   }

int main()
{
   cout << i << endl;
   variable = 100;
   return 0;
}

In the previous example, the unnamed namespace permits access to i and variable without using a scope resolution operator.

The following example illustrates an improper use of unnamed namespaces.

#include <iostream>

using namespace std;

namespace {
   const int i = 4;
   }

int i = 2;

int main()
{
   cout << i << endl; // error
   return 0;
}

Inside main, i causes an error because the compiler cannot distinguish between the global name and the unnamed namespace member with the same name. In order for the previous example to work, the namespace must be uniquely identified with an identifier and i must specify the namespace it is using.

You can extend an unnamed namespace within the same translation unit. For example:

#include <iostream>

using namespace std;

namespace {
   int variable;
   void funct (int);
   }

namespace {
   void funct (int i) { cout << i << endl; }
   }

int main()
{
   funct(variable);
   return 0;
}

both the prototype and definition for funct are members of the same unnamed namespace.

Related information

• Program linkage
• Internal linkage