Just declare the template function and don't define it, then define two specializations:
template <int nSize> Eigen::Matrix<double, nSize, 1> myFunc(); template <> Eigen::Matrix<double, 2, 1> myFunc<2>() { return ExternalLibrary::generate_Vector_2d(); } template <> Eigen::Matrix<double, 3, 1> myFunc<3>() { return ExternalLibrary::generate_Vector_3d(); }
The compiler, while it will eliminate dead code as an optimization, must verify the validity of the dead code anyway, which means it must ensure that the return type of ExternalLibrary::generate_Vector_3d() can be converted to Eigen::Matrix<double, 2, 1>, even though it can be proven that that return statement can't be executed. Because this conversion is not allowed, you get a compile-time error.
By separating out the two return statements into separate specializations, you eliminate this problem as it removes the return ExternalLibrary::generate_Vector_3d(); statement from a function that could return something other than an Eigen::Matrix<double, 3, 1>.
Note that using myFunc() with an nSize other than 2 or 3 will cause a link-time error. We can move this error to compile-time by providing a body that will fail to instantiate, but that (ostensibly) depends on nSize so that the compiler won't try to instantiate it before it is used:
template <int nSize> Eigen::Matrix<double, nSize, 1> myFunc() { // With C++11, the following line provides a more useful diagnostic. If // you don't have C++11 support, remove it and the bad return statement // will do the job, albeit with a more cryptic error message. static_assert((static_cast<void>(nSize), false), "Invalid template parameter nSize"); return static_cast<class INVALID_NSIZE_PARAMETER**********>(nSize); }
This will provide a diagnostic pointing to the incorrect use of myFunc(). For example, without C++11 support you would see something like:
main.cpp:12:12: error: cannot cast from type 'int' to pointer type 'class INVALID_NSIZE_PARAMETER **********' return static_cast<class INVALID_NSIZE_PARAMETER**********>(nSize); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ main.cpp:29:5: note: in instantiation of function template specialization 'myFunc<4>' requested here myFunc<4>(); ^
