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copied!<p>"How can I declare and define in this class"</p> <p>In what class? You've defined a class template, not a class. You can't call a static function of a class template itself, you have to call a particular version of the static function that's part of a real class.</p> <p>So, do you want the template (and hence all instantiations of it) to have a function returning a StaticVector<3>, or do you want one particular instantiation of that template to have a function returning a StaticVector<3>?</p> <p>If the former:</p> <pre><code> template <unsigned int N> struct SV { int contents[N]; static SV<3> get3dVec(int x, int y, int z) { SV<3> v; v.contents[0] = x; v.contents[1] = y; v.contents[2] = z; return v; } }; int main() { SV<3> v = SV<1>::get3dVec(1,2,3); } </code></pre> <p>works for me.</p> <p>If the latter (you only want get3dVec to be a member of SV<3>, not of all SV<whatever>), then you want template specialisation:</p> <pre><code> template <unsigned int N> struct SV { int contents[N]; }; template<> struct SV<3> { int contents[3]; // must be re-declared in the specialization static SV<3> get3dVec(int x, int y, int z) { SV<3> v; v.contents[0] = x; v.contents[1] = y; v.contents[2] = z; return v; } }; int main() { SV<3> v = SV<1>::get3dVec(1,2,3); // compile error SV<3> v = SV<3>::get3dVec(1,2,3); // OK } </code></pre> <p>If for no other reason than to make the calling code look nicer by omitting the basically irrelevant template parameter, I agree with Iraimbilanja that normally a free function (in a namespace if you're writing for re-use) would make more sense for this example. </p> <p>C++ templates mean that you don't need static functions as much in C++ as you do in Java: if you want a "foo" function that does one thing for class Bar and another thing for class Baz, you can declare it as a function template with a template parameter that can be Bar or Baz (and which may or may not be inferred from function parameters), rather than making it a static function on each class. But if you do want it to be a static function, then you have to call it using a specific class, not just a template name.</p>

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