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    <p>As <a href="https://stackoverflow.com/questions/1358400/what-is-external-linkage-and-internal-linkage-in-c/1358622#1358622">dudewat said</a> <em>external</em> linkage means the symbol (function or global variable) is accessible throughout your program and <em>internal</em> linkage means that it's only accessible in one <a href="http://en.wikipedia.org/wiki/Translation_unit_%28programming%29" rel="noreferrer">translation unit</a>. </p> <p>You can explicitly control the linkage of a symbol by using the <code>extern</code> and <code>static</code> keywords. If the linkage isn't specified then the default linkage is <code>extern</code> for non-<code>const</code> symbols and <code>static</code> (internal) for <code>const</code> symbols.</p> <pre><code>// in namespace or global scope int i; // extern by default const int ci; // static by default extern const int eci; // explicitly extern static int si; // explicitly static // the same goes for functions (but there are no const functions) int foo(); // extern by default static int bar(); // explicitly static </code></pre> <p>Note that instead of using <code>static</code> for internal linkage it is better to use <a href="https://stackoverflow.com/questions/154469/unnamed-anonymous-namespaces-vs-static-functions">anonymous namespaces</a> into which you can also put <code>class</code>es. The linkage for anonymous namespaces has changed between C++98 and C++11 but the main thing is that they are unreachable from other translation units.</p> <pre><code>namespace { int i; // external linkage but unreachable from other translation units. class invisible_to_others { }; } </code></pre>
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    1. POWhat is external linkage and internal linkage?
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    1. COThe implementation of the "export" keyword highlighted a difference between a function declared 'static' and a function declared in the unnamed namespace. To summarise as best I can, an function template declared with the export keyword in one translation unit can refer to a function defined in an unnamed namespace of a different translation unit as a result of 2-phase lookup. (http://www.ddj.com/showArticle.jhtml?articleID=184401584)
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      1. USRichard Corden
    2. COWhat if I do following: 1.cpp <code>const int ci;</code> 2.cpp <code>extern const int ci;</code>
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      1. USRajendra Uppal
    3. CO@Rajenda you will get an unresolved symbol error (sorry for the nine month delay in answering I missed this comment).
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