Wherefore inline unnamed namespaces?

Question

A quick one for the gurus: C++11 allows unnamed namespaces to be declared inline. This seems redundant to me; things declared in an unnamed namespace are already us

Answer 1

Here is one use that I have found:

namespace widgets { inline namespace {

void foo();

} } // namespaces

void widgets::foo()
{
}

In this example, foo has internal linkage and we can define the function later on by using the namespace::function syntax to ensure that the function's signature is correct. If you were to not use the widgets namespace then the void foo() definition would define a totally different function. You also don't need to re-open the namespace saving you a level of indentation.

If there is another function called foo in the widgets namespace already then this will give you an ambiguity instead rather than a nasty ODR violation.