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copied!<p>The Standard is very clear on this:</p> <blockquote> <p><strong>14.7.1 Implicit instantiation [temp.inst]</strong></p> <p>9 The implicit instantiation of a class template does not cause any static data members of that class to be implicitly instantiated.</p> </blockquote> <p>All the calls in <code>main()</code> to your <code>Fibo<n>::x</code> for <code>n > 1</code>, are explicit instantiations, that through the Fibonnaci recursion will implicitly instantiate <code>Fibo<n-1></code> and <code>Fibo<n-2></code> but not their members <code>x</code>. This means that at those points, the <code>static</code> members <code>x</code> will be evaluated to their default initialization of <code>0</code>. For <code>n=1</code> and <code>n=0</code>, the compiler will see the explicit initialization values of 1. So effectively, you get the following computation</p> <pre><code>Fibo<5>::x --> Fibo<4>::x + Fibo<3>::x --> 0 + 0 = 0 Fibo<4>::x --> Fibo<3>::x + Fibo<2>::x --> 0 + 0 = 0 Fibo<3>::x --> Fibo<2>::x + Fibo<1>::x --> 0 + 1 = 1 Fibo<2>::x --> Fibo<1>::x + Fibo<0>::x --> 1 + 1 = 2 Fibo<1>::x --> 1 Fibo<0>::x --> 1 </code></pre> <p>You need to instantiate the static member <code>x</code> before evaluating the Fibonacci recursion. You can do this through a <code>static const int</code> or <code>enum</code> member <code>x</code>, or through a function (possibly <code>constexpr</code> in C++11) as shown by @Jarod42.</p>

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