NAME

       functor - a function wrapper suitable for field expressions

DESCRIPTION

       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);

IMPLEMENTATION

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

SEE ALSO

       field(2), geo(2), quadrature_option_type(2), integrate(4)