On 15 February 2012 22:17, Mattia Jona-Lasinio wrote: > Hi Jonathan, > > I don´t think this link applies. It is clear that linking will fail in > the link examples because the static variables are declared but never > defined, Right, just like your code. > i.e. they are never instantiated. No, it's not to do with instantiation. It's the same problem, your program is invalid and can be fixed by adding definitions of the variables. > On the other hand the difference between the non working versions > > std::cout << (b = aa< X<4> >(3))[1].v << std::endl; The operator<< overload used here takes its argument by reference. Binding a reference to an object requires the object to have an address, so you need to provide a definition. Read the FAQ more carefully. Your program is invalid. The compiler is not required to issue a diagnostic, so it *might* accept it, as in your final example, but that doesn't mean the code is correct. The code is still wrong. You can fix it by adding a definition: template<int T> const int X<T>::v;
