functor(2) a function wrapper suitable for field expressions


A functor is a class-function, i.e. a class that defines the operator(): it can be used in place of an usual function. Moreover, functors can be used in Rheolef field expressions, mixed with fields (see field(2)). For instance, assuming that uh is a field and u_ex is a functor:

    Float err_l1 = integrate (omega, abs(uh - uh_ex), qopt);

where omega denotes a mesh (see geo(2)) and qopt a quadrature formula (see quadrature_option_type(2)). See also the integrate(4) function. An usual function u_ex_f cannot always be mixed so nicely in expressions, due to c++ language rules. For instance, the following exprtession is valid:

    Float err_l1 = integrate (omega, abs(uh - u_ex_f), qopt);

In some case, the compiler cannot build a field expression using usual functionsn e.g.

    Float I = integrate (omega, 0.5*u_ex_f), qopt);

because 0.5*u_ex_f is a direct algebraic operation between usual functions and flmoating points, that is not defined in the c++ language. A way to circumvent this difficulty is to convert the usual function into a functor, as in:

    Float I = integrate (omega, 0.5*functor(u_ex_f)), qopt);


template<class R, class... Args>
functor (R(*f)(Args...)) {
  return std::function<R(Args...)>(f);