.\"
.de Id
..
.de Sp
.if n .sp
.if t .sp 0.4
..
.TH functor 2rheolef "rheolef-7.0" "rheolef-7.0" "rheolef-7.0"
.\" label: /*Class:functor
.SH NAME
\fBfunctor\fP - a function wrapper suitable for field expressions
.SH DESCRIPTION

A \fIfunctor\fP is a class-function, i.e. a class that defines
the \fBoperator()\fP:
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 \fBuh\fP is a field and \fBu_ex\fP is a functor:
.\" begin_example
.Sp
.nf
    Float err_l1 = integrate (omega, abs(uh - uh_ex), qopt);
.Sp
.fi
.\" end_example
where \fBomega\fP denotes a mesh
(see geo(2))
and \fBqopt\fP a quadrature formula
(see quadrature_option(2)).
See also the integrate(4) function.
An usual function \fBu_ex_f\fP cannot always be mixed so nicely
in expressions, due to c++ language rules.
For instance, the following exprtession is valid:
.\" begin_example
.Sp
.nf
    Float err_l1 = integrate (omega, abs(uh - u_ex_f), qopt);
.Sp
.fi
.\" end_example
In some case, the compiler cannot build a field expression using
usual functionsn e.g.
.\" begin_example
.Sp
.nf
    Float I = integrate (omega, 0.5*u_ex_f), qopt);
.Sp
.fi
.\" end_example
because \fB0.5*u_ex_f\fP 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:
.\" begin_example
.Sp
.nf
    Float I = integrate (omega, 0.5*functor(u_ex_f)), qopt);
.Sp
.fi
.\" end_example
.\" skip start:AUTHOR:
.\" skip start:DATE:
.\" END
.SH IMPLEMENTATION
.\" begin_example
.Sp
.nf
template<class R, class... Args>
std::function<R(Args...)>
functor (R(*f)(Args...)) {
  return std::function<R(Args...)>(f);
}
.Sp
.fi
.\" end_example

.\" LENGTH = 4
.SH SEE ALSO
field(2), geo(2), quadrature_option(2), integrate(4)
.SH COPYRIGHT
Copyright (C) 2000-2018 Pierre Saramito <Pierre.Saramito@imag.fr> GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to the extent permitted by law.