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    <p>This is a common problem when befriending non-member functions with templates. The <code>friend</code> declaration inside the <code>TemplateClass</code> does not befriend your <code>swap</code> template, but rather a non-templated free function <code>swap</code> that takes <code>TemplateClass&lt;T&gt;</code> for which ever <code>T</code> the template is instantiated (i.e. the specialization <code>TemplateClass&lt;int&gt;</code> will befriend a free function <code>void swap( TemplateClass&lt;int&gt;&amp;,TemplateClass&lt;int&gt;&amp; );</code> that is not templated).</p> <p>The best solution is to provide the <code>swap</code> definition inlined inside the class template definition, as that will make the compiler generate a non-templated <code>swap</code> function for the exact type whenever needed. As another positive side effect, that <code>swap</code> function will only be found during Argument Dependent Lookup, so it will not take part of overload resolution for anything that does not involve your template.</p> <p>Other alternatives are befriending the whole <code>swap</code> template function, or befriending the particular specialization of the <code>swap</code> function when applied to the same <code>T</code> that the template has been instantiated with. The first of the options is simple in code, but it grants access to <em>all</em> of the specializations of the <code>swap</code> template, and that might have bad side effects. Befriending the particular <code>swap</code> specialization solves that issue, but is a bit more complex to implement (you need to forward declare the class template, then the <code>swap</code> template, then define the class template, and finally define the <code>swap</code> template).</p> <p>More on this in this other <a href="https://stackoverflow.com/questions/4660123/overloading-friend-operator-for-template-class/4661372#4661372">answer</a>, where the different options and syntaxes are explained with more detail.</p> <p>As to the particular error message of <code>unresolved external</code>, that is due to how identifier lookup works. When you used <code>swap(*this,other);</code> inside a member function, lookup starts inside the class, and tries to find an appropriate <code>swap</code>. It first looks in the class context and finds the declaration of the <code>friend</code> free function, so lookup does not continue going outwards and adds a dependency to that particular free function. It adds the dependency and waits for the linker to locate the appropriate symbol. Because the compiler never considered the templated <code>swap</code> at namespace level, it never actually instantiated it, but even if it had instantiated that template, the dependency inside the <code>operator=</code> member function is on a free function, not that specialization.</p>
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    1. POUnresolved external using template class with copy-and-swap
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    1. USDavid Rodríguez - dribeas
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    1. COInlining itself did not do the job, but in the link you posted I realized they did not have the < T > stuff in the friend void declaration. Check the edit in my post to see the code I have chosen, if you are interested.
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      1. USTim Meyer
    2. CO@Tim Meyer: If you did not misspelled anything, just inlining the function definition inside the class template definition should have worked with or without the `<T>` (if `T` is the actual template argument). Inside the class template scope, the name of the class template is a short hand for the specialization of the template that is being instantiated, which means that `TemplateClass` and `TemplateClass<T>` are exact synonyms if the template argument is `T`.
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      1. USDavid Rodríguez - dribeas
    3. COI have precompiled headers active (though the header file only excludes windows/third party libs), maybe there was an issue there. I did do a full rebuild at some point after removing the < T >
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