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copied!<p>I have two quick tips that may save you some problems in the future.</p> <p><strong>Templates must be defined and implemented in headers, because they are translated/created at compile-time</strong></p> <p>If you have the following code:</p> <pre><code>// Foo.h template <typename T> class Foo { ... }; // Bar.cpp #include "Foo.h" void func() { Foo<int> fooInt; Foo<double> fooDouble; } </code></pre> <p>When processing the Bar.cpp file, the compiler will "create" a new definition of the Foo class with every T replaced with the int type for the fooInt instance, and a new definition of the Foo class with every T replaced with the double type for the fooDouble instance.</p> <p>In a nutshell, C++ does not support template classes, the compiler emulates them replacing the types at compile time and creating normal classes. So, the compiled type of fooInt and fooDouble are different.</p> <p>Because they are translated at compile-time and not compiled individually, they <em>must</em> be defined and implemented in headers.</p> <p>If you implement the template class as a normal class (header and source file) the error thrown by the compiler (the linker actually) is that there is no method implemented (it's because the source file is ignored).</p> <p><strong>You can still create forward declarations to avoid cyclic dependencies</strong></p> <p>It's just that you have to forward declare them like this:</p> <pre><code>template <typename T> class Foo; </code></pre> <p>And, as with normal forward declarations, you have to be sure that you are including all the headers necessary.</p>

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