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                        <h1 id="use-linkages-correctly"><a class="header" href="linkage.html#use-linkages-correctly">use linkages correctly</a></h1>
<blockquote>
<p>what is translation unit ?</p>
</blockquote>
<p>cpp file fed to compiler after it has been pro-processed.</p>
<blockquote>
<p>what does linkage mean ?</p>
</blockquote>
<p>There may be multiple translation units ( compilation units) and they re related with each other - using each other... at linking step linker will search these parts and link memory addresses.</p>
<blockquote>
<p>types of linkages in c++ ?</p>
</blockquote>
<p>internal and external.</p>
<p>Firstly we need to understand =&gt; declaration and definition differences. In declaration we just need the signature of things like function or variables... In definition we define the memory size of these variables . Definition of things can be once because of ODR ( one definition rule) but we can repeat declaration many times around the code. At linkage phase compiler will try to link them to their definition and at this step we should follow ODR.</p>
<p>.hh is external</p>
<p>.cc is internal</p>
<p>external: globally defined items. linker can access them from other translation units.</p>
<pre><code class="language-c++">
// .h : declarations
extern int i; //
extern void func(int i); // functions implicitly external - you dont need to use

// .cpp : definitions
int i = 0;
void func(int i) {}

</code></pre>
<p>internal: belongs to only one translation unit.</p>
<p>External items use one memory resource =&gt; unique - memory saving
Internal items re copied =&gt; multiple copy means more memory usage</p>
<blockquote>
<p>how to define internal linkage items in a file ?</p>
</blockquote>
<p>Declare and define internal linkage items in source code (*.cpp) if possible.</p>
<ul>
<li>static</li>
<li>const</li>
<li>constexpr</li>
<li>typedef</li>
</ul>
<blockquote>
<p>how to define external linkage items in a file ?</p>
</blockquote>
<ul>
<li>extern</li>
<li>macros</li>
<li>non-const global items</li>
</ul>
<blockquote>
<p>how forward declaration used with linkage ?</p>
</blockquote>
<p>FD helps loosely coupling and late linking - if we change definition of functions of the FD items; sources will not compiled again.</p>
<blockquote>
<p>What does inline do ?</p>
</blockquote>
<p>It defines the variable.</p>
<pre><code class="language-c++">
struct MyS
{
static int i; // declaration
inline static int j; // decl + definition
static constexpr int k; // implicitly inline =&gt; decl + defines variable
};
</code></pre>
<blockquote>
<p>No linkage items ?</p>
</blockquote>
<p>Local variable in functions have no linkage info</p>
<blockquote>
<p>Global variables with internal linkage ?</p>
</blockquote>
<p>Define them in unnamed namespaces for ODR. but best is decl + define them in cpp not header.</p>
<blockquote>
<p>Inline linkage ?</p>
</blockquote>
<p>Inline functions and variables have external linkage - one definition multiple copy</p>
<blockquote>
<p>static functions and variables linkage ?</p>
</blockquote>
<p>Internal</p>
<blockquote>
<p>unnamed namespace linkage ?</p>
</blockquote>
<p>Internal</p>
<blockquote>
<p>named namespace linkage ?</p>
</blockquote>
<p>External</p>
<blockquote>
<p>static inline linkage ?</p>
</blockquote>
<p>Internal... static is more dominant than inline...or internal feature &gt; external feature</p>

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