## DESCRIPTION

This class is now obsolete, from Rheolef version 6.7
and is maintained for backward compatibility purpose only.
Until Rheolef version 6.6, this class was used
to mark functors with profil compatible with fields, i.e. that accepts
**point** as parameter and returns a field value (scalar, vector, tensor).
This mark was used to filter field expression arguments in **interpolate**
and **integrate**.
From version 6.7, this mark is no more required, and any function or functor
that is callable with a **point** as argument is valid in a field expression.

A functor is a class-function, i.e. a class that defines
the **operator()**. A variable **f** of a class-function can be
used as **f(arg)** and when its argument is of type **point**
see point(2), the function **f** interprets as a continuous field field.
Thus, it can be interpolated see interpolate(4) and
it can be combined within field expressions see field(2)
that appears in arguments of see integrate(4).

## EXAMPLE

struct f : field_functor<f,Float> { Float operator() (const point& x) const { return 1-norm(x); } }; // ... geo omega ("square"); space Xh (omega, "P1"); field fh = interpolate (Xh, f); test (Xh); field lh = integrate (f*v);

## IMPLEMENTATION NOTE

The current implementation of a **field_functor** class
bases on the curiously recurring template pattern (CRTP) C++ idiom:
the definition of the class **f** derives from
**field_functor**<**f**,Float> that depend itself upon **f**.
So, be carrefull when using copy-paste, as there is no checks if
you write e.g. **field_functor**<**g**,Float> with another function **g**
instead of **f**.

## IMPLEMENTATION

template <class Function, class Result> struct field_functor : std::unary_function<point_basic<float_traits<Result> >,Result> { const Function& get_ref() const { return static_cast<const Function&>(*this); } operator Function() const { return get_ref(); } Result operator() (const point& x) const { return get_ref().operator()(x); } };