---
name: "functor"
section: "2rheolef"
platform: "linux"
package: "librheolef-dev"
version: "7.0-2+b1"
release: "Debian 10 (Buster)"
platforms: ["linux"]
url: "https://new.carta.tech/functor"
---

# functor(2rheolef)

a function wrapper suitable for field expressions

## 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(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(2), integrate(4)

## 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.