# libsigc++: Functors

Functors are copyable types that define operator()(). More...

## Modules | |

Slots | |

Slots are type-safe representations of callback methods and functions. | |

mem_fun() | |

mem_fun() Creates a functor from a pointer to a method. | |

ptr_fun() | |

ptr_fun() is used to convert a pointer to a function to a functor. | |

## Classes | |

class | sigc::can_deduce_result_type_with_decltype< T_functor > |

Helper class, to determine if decltype() can deduce the result type of a functor. More... | |

struct | sigc::functor_base |

A hint to the compiler. More... | |

struct | sigc::functor_trait< T_functor, I_derives_functor_base, I_can_use_decltype > |

Trait that specifies the return type of any type. More... | |

struct | sigc::visitor< T_functor > |

sigc::visitor<T_functor>::do_visit_each() performs a functor on each of the targets of a functor. More... | |

## Macros | |

#define | SIGC_FUNCTOR_TRAIT(T_functor, T_return) |

Helper macro, if you want to mix user-defined and third party functors with libsigc++. More... | |

#define | SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE |

Helper macro, if you want to mix user-defined and third party functors with libsigc++. More... | |

#define | SIGC_FUNCTORS_HAVE_RESULT_TYPE |

Helper macro, if you want to mix user-defined and third party functors with libsigc++. More... | |

## Functions | |

template<class T_action , class T_functor > | |

void | sigc::visit_each (const T_action& _A_action, const T_functor& _A_functor) |

This function performs a functor on each of the targets of a functor. More... | |

template<class T_type , class T_action , class T_functor > | |

void | sigc::visit_each_type (const T_action& _A_action, const T_functor& _A_functor) |

This function performs a functor on each of the targets of a functor limited to a restricted type. More... | |

## Detailed Description

Functors are copyable types that define operator()().

Types that define operator()() overloads with different return types are referred to as multi-type functors. Multi-type functors are only partially supported in libsigc++.

Closures are functors that store all information needed to invoke a callback from operator()().

Adaptors are functors that alter the signature of a functor's operator()().

libsigc++ defines numerous functors, closures and adaptors. Since libsigc++ is a callback library, most functors are also closures. The documentation doesn't distinguish between functors and closures.

The basic functor types libsigc++ provides are created with ptr_fun() and mem_fun() and can be converted into slots implicitly. The set of adaptors that ships with libsigc++ is documented in the Adaptors module.

If you want to mix user-defined and third party functors with libsigc++, and you want them to be implicitly convertible into slots, libsigc++ must know the result type of your functors. There are different ways to achieve that.

- Derive your functors from sigc::functor_base and place
`typedef T_return result_type;`

in the class definition. - Use the macro SIGC_FUNCTOR_TRAIT(T_functor,T_return) in namespace sigc. Multi-type functors are only partly supported.
- For functors not derived from sigc::functor_base, and not specified with SIGC_FUNCTOR_TRAIT(), libsigc++ tries to deduce the result type with the C++11 decltype() specifier. That attempt usually succeeds if the functor has a single operator()(), but it fails if operator()() is overloaded.
- Use the macro SIGC_FUNCTORS_HAVE_RESULT_TYPE, if you want libsigc++ to assume that result_type is defined in all user-defined or third party functors, whose result type can't be deduced in any other way.

If all these ways to deduce the result type fail, void is assumed.

With libsigc++ versions before 2.6, the macro SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE activated the test with decltype(). That macro is now unneccesary and deprecated.

## Macro Definition Documentation

#define SIGC_FUNCTOR_TRAIT | ( | T_functor, | |

T_return | |||

) |

Helper macro, if you want to mix user-defined and third party functors with libsigc++.

If you want to mix functors not derived from sigc::functor_base with libsigc++, and these functors don't define `result_type`

, use this macro inside namespace sigc to expose the return type of the functors like so:

#define SIGC_FUNCTORS_DEDUCE_RESULT_TYPE_WITH_DECLTYPE |

Helper macro, if you want to mix user-defined and third party functors with libsigc++.

If you want to mix functors not derived from sigc::functor_base with libsigc++, and your compiler can deduce the result type of the functor with the C++11 keyword `decltype`

, use this macro inside namespace sigc like so:

Functors with overloaded operator()() are not supported.

**Deprecated:**- This macro does nothing. The test it activated in libsigc++ versions before 2.6, is now unconditionally activated.

#define SIGC_FUNCTORS_HAVE_RESULT_TYPE |

Helper macro, if you want to mix user-defined and third party functors with libsigc++.

If you want to mix functors not derived from sigc::functor_base with libsigc++, and these functors define `result_type`

, use this macro inside namespace sigc like so:

## Function Documentation

void sigc::visit_each | ( | const T_action & | _A_action, |

const T_functor & | _A_functor |
||

) |

This function performs a functor on each of the targets of a functor.

void sigc::visit_each_type | ( | const T_action & | _A_action, |

const T_functor & | _A_functor |
||

) |

This function performs a functor on each of the targets of a functor limited to a restricted type.