Consider implementing a "automated" downcasting tecniche from FunctionExpressions with single functions into their specific function

[rafaelrojasmiliani]

Use

gsplines::GSpline curve(...);
gsplines::functions::FunctionExpression curve_dot = curve.derivate()
gsplines::GSpline curve_2 = curve_dot;

See here

[rafaelrojasmiliani]

template<T>
T FunctionExpression::operator(T){return T(*static_cast<T*>(this));}