c++ - unnamed namespace

Question

What does it exactly mean when the Standard states

$7.3.1.1/2 - "The use of the static keyword is deprecated when declaring variables in a namespace scope (see annex D); the unnamed-namespace provides a superior alternative."

I have referred this but it does not cover what I am looking for.

Is there an example where the superiority is clearly demonstrated.

NB: I know about how unnamed namespaces can make extern variables visible in the translation unit and yet hide them from other translation units. But the point of this post is about 'static namespace scope' names (e.g global static variables)

Answer 1

What does it exactly mean?

Technically deprecated means that a future standard may remove the feature.

In practice that isn't going to happen, because of the need to support old code.

So in practice it means, "strongly discouraged".

Example of superiority of unnamed namespace

An unnamed namespace is generally superior because what you have in that namespace can have external linkage.

In C++98 external linkage is necessary for things that can be template parameters, e.g., if you want to templatize on a char const*, it must be pointer to char that has external linkage.

#include <iostream>

// Compile with "-D LINKAGE=static" to see problem with "static"
#ifndef LINKAGE
#   define LINKAGE extern
#endif

template< char const* s >
void foo()
{
    std::cout << s << std::endl;
}

namespace {
    LINKAGE char const message[] = "Hello, world!";
}  // namespace anon

int main()
{
    foo<message>();
}

That said, it's a bit inconsistent that static isn't also deprecated for functions.