Consider the following snippet:
void Foo() // 1
{
}
namespace
{
void Foo() // 2
{
}
}
int main()
{
Foo(); // Ambiguous.
::Foo(); // Calls the Foo in the global namespace (Foo #1).
// I'm trying to call the `Foo` that's defined in the anonymous namespace (Foo #2).
}
How can I refer to something inside an anonymous namespace in this case?
An anonymous namespace makes the enclosed variables, functions, classes, etc. available only inside that file. In your example it's a way to avoid global variables. There is no runtime or compile time performance difference.
Unnamed NamespacesThey are directly usable in the same program and are used for declaring unique identifiers. In unnamed namespaces, name of the namespace in not mentioned in the declaration of namespace. The name of the namespace is uniquely generated by the compiler.
You can have the same name defined in two different namespaces, but if that is true, then you can only use one of those namespaces at a time. However, this does not mean you cannot use the two namespace in the same program. You can use them each at different times in the same program.
An object namespace protects named objects from unauthorized access. Creating a private namespace enables applications and services to build a more secure environment. A process can create a private namespace using the CreatePrivateNamespace function.
You can't. The standard contains the following section (§7.3.1.1, C++03):
An unnamed-namespace-definition behaves as if it were replaced by
namespace unique { /* empty body */ }
using namespace unique;
namespace unique { namespace-body }
where all occurrences of unique in a translation unit are replaced by the same identifier and this identifier differs from all other identifiers in the entire program.
Thus you have no way to refer to that unique name.
You could however technically use something like the following instead:
int i;
namespace helper {
namespace {
int i;
int j;
}
}
using namespace helper;
void f() {
j++; // works
i++; // still ambigous
::i++; // access to global namespace
helper::i++; // access to unnamed namespace
}
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