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copied!<p>(In the following, the <strike>striked-through</strike> 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).). </p> <p>Unnamed namespaces are a utility to make an identifier <strike>effectively</strike> translation unit local. They behave as if you would choose an unique name per translation unit for a namespace:</p> <pre><code>namespace unique { /* empty */ } using namespace unique; namespace unique { /* namespace body. stuff in here */ } </code></pre> <p>The extra step using the empty body is important, so you can already refer within the namespace body to identifiers like <code>::name</code> that are defined in that namespace, since the using directive already took place. </p> <p>This means you can have free functions called (for example) <code>help</code> that can exist in multiple translation units, and they won't clash at link time, <strike>since they all got an unique name due to their unique namespace they are in</strike>. The effect is almost identical to using the <code>static</code> keyword used in C which you can put in in the declaration of identifiers. <strike><code>static</code> used in that manner is deprecated in C++, since</strike> unnamed namespaces are a superior alternative, being able to even make a type translation unit local. </p> <pre><code>namespace { int a1; } static int a2; </code></pre> <p>Both <code>a</code>'s are translation unit local and won't clash at link time. But the difference is that the <code>a1</code> in the anonymous namespace <strike>just</strike> gets an unique name. <strike>It has still external linkage and may be exported into the symbol table of the object file being created. This becomes important if you want to use its address as a template argument:</p> <pre><code>template<int * ptr> struct sample { }; // OK - a1 has external linkage sample<&a1> s1; // NOT OK - translation unit locality is done by giving a2 internal linkage. sample<&a2> s2; </code></pre> <p>Template parameters has to have external linkage so in this case the identifier has to be put into an anonymous namespace.</strike></p> <p>Read the excellent article at comeau-computing <a href="http://www.comeaucomputing.com/techtalk/#nostatic" rel="noreferrer">`Why is an unnamed namespace used instead of static?</a>.</p>

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