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copied!<p>It is <em>not</em> necessary to put the implementation in the header file, see the alternative solution at the end of this answer.</p> <p>Anyway, the reason your code is failing is that, when instantiating a template, the compiler creates a new class with the given template argument. For example:</p> <pre><code>template<typename T> struct Foo { T bar; void doSomething(T param) {/* do stuff using T */} }; // somewhere in a .cpp Foo<int> f; </code></pre> <p>When reading this line, the compiler will create a new class (let's call it <code>FooInt</code>), which is equivalent to the following:</p> <pre><code>struct FooInt { int bar; void doSomething(int param) {/* do stuff using int */} } </code></pre> <p>Consequently, the compiler needs to have access to the implementation of the methods, to instantiate them with the template argument (in this case <code>int</code>). If these implementations were not in the header, they wouldn't be accessible, and therefore the compiler wouldn't be able to instantiate the template.</p> <p>A common solution to this is to write the template declaration in a header file, then implement the class in an implementation file (for example .tpp), and include this implementation file at the end of the header.</p> <pre><code>// Foo.h template <typename T> struct Foo { void doSomething(T param); }; #include "Foo.tpp" // Foo.tpp template <typename T> void Foo<T>::doSomething(T param) { //implementation } </code></pre> <p>This way, implementation is still separated from declaration, but is accessible to the compiler.</p> <p>Another solution is to keep the implementation separated, and explicitly instantiate all the template instances you'll need:</p> <pre><code>// Foo.h // no implementation template <typename T> struct Foo { ... }; //---------------------------------------- // Foo.cpp // implementation of Foo's methods // explicit instantiations template class Foo<int>; template class Foo<float>; // You will only be able to use Foo with int or float </code></pre> <p>If my explanation isn't clear enough, you can have a look at the <a href="https://isocpp.org/wiki/faq/templates#templates-defn-vs-decl" rel="noreferrer">C++ Super-FAQ on this subject</a>.</p>

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