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POC++ about function templates
 

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  1. POC++ about function templates
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    <p>since 2 hours I struggle with templates, lets consider these files :</p> <h2>ChildrenOfBodom.h :</h2> <pre><code>#include&lt;iostream&gt; using namespace std; void AreYouDeadYet(string); template&lt;typename T1&gt;void InYourFace(T1); </code></pre> <h2>ChildreOfBodom.cc :</h2> <pre><code>#include"ChildrenOfBodom.h" void AreYouDeadYet(string s){ InYourFace(s); } template&lt;typename T1&gt;void InYourFace(T1 t1){ cout &lt;&lt; t1 &lt;&lt; endl; } </code></pre> <h2>main.cc :</h2> <pre><code>// g++ ChildrenOfBodom.cc main.cc #include"ChildrenOfBodom.h" int main(){ AreYouDeadYet("Awesome"); InYourFace("Awesome"); int i=8; InYourFace(i); return 0; } </code></pre> <p>at the compilation I got this message :</p> <pre><code>/tmp/ccN1fClI.o: In function `main': main.cc:(.text+0x54): undefined reference to `void InYourFace&lt;char const*&gt;(char const*)' main.cc:(.text+0x65): undefined reference to `void InYourFace&lt;int&gt;(int)' collect2: error: ld returned 1 exit status </code></pre> <p>my immediate solution was to write the function in <code>main.cc</code> (and still in <code>ChildrenOfBodom.cc</code>),</p> <h2>main.cc :</h2> <pre><code>#include"ChildrenOfBodom.h" template&lt;typename T1&gt;void InYourFace(T1 t1){ cout &lt;&lt; t1 &lt;&lt; endl; } int main(){ AreYouDeadYet("Awesome"); InYourFace("Awesome"); int i=8; InYourFace(i); return 0; } </code></pre> <p>this works. But I guess I store two times <code>InYourFace()</code>, so I did some searches and found what Tyler McHenry wrote (<a href="https://stackoverflow.com/questions/1353973/c-template-linking-error">here</a>), </p> <blockquote> <p>BlockquoteTemplates functions, including member functions, must be written entirely in headers.</p> </blockquote> <p>so I did it. But I would like to understand, because irc all of my teachers tell to don't write functions in a header. So is it really correct to write templates in headers ? or there is another way to write them in cc files, I mean a different way that one I used before, because doesn't sound really efficient if I need to change the template... and what does exactly the compiler when I use a template ? here I used a <code>string</code> and <code>int</code> so I guess the compiler create <code>InYourFace(int)</code> and <code>InYourFace(string)</code> and I guess if I call in <code>main.cc</code> thirty times <code>InYourFace(int)</code> it will not create thirty times <code>InYourFace(int)</code> but create only one and uses it thirty times. But if I use <code>InYourFace(int)</code> in <code>ChildrenOfBodom.cc</code>, does it will create <code>InYourFace(int)</code> too, or it will create <code>InYourFace(int)</code> one time only for <code>main.cc</code> and <code>ChildrenOfBodom.cc</code> ?</p> <p>Hope my question and my english are clear...</p>
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    1. COA general hint is to usually put templated definitions only in header files. (i.e. move definition of `template<typename T1>void InYourFace(T1 t1)` -with its body- to file `ChildrenOfBodom.h`
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    2. CO`template` functions aren't functions. They are instructions on how to write a function, after the `template` parameters are supplied. They participate in overload resolution (the compiler says "what kind of function could I generate from this `template`?), and there is deduction magic where their arguments are inferred from what you pass to the `template` function: but until the types are fixed, they aren't functions. In order to produce the function "on demand", the compiler needs to be able to see the full body of the `template` function where it is used, basically.
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