Function object (functor functor)

Simply put, a function object is an instance of a class that overloads the () operator, which can be used like a function.

#include <iostream>using namespacestd;classadd{ Public:    int operator()(Const int&a,Const int&b) {return(A +b); }    Double operator()(Const Double&a,Const Double&b) {return(A +b); }};voidMain () {ADD plus; cout<< Plus ( $, Wu) << Endl;// Aboutcout << Plus (44.3,58.4) << Endl;//102.7}

Using templates:

#include <iostream>using namespacestd;template<typename t>classadd{ Public: Toperator()(ConstT &a,ConstT &b) {return(A +b); }};voidMain () {ADD<int>Plus1; cout<< Plus1 ( $, Wu) << Endl;// Aboutadd<Double>Plus2; cout<< Plus2 (44.3,58.4) << Endl;//102.7}

The find_if, count_if, For_each, etc. in the standard library are related to:

#include <iostream>#include<algorithm>#include<vector>using namespacestd;template<typename t>classshowt{ Public:    void operator()(ConstT &a) {cout<< a <<'\ t'; }};voidShowConst int&a) {cout<< a <<'\ t';}voidmain () {vector<int> VEC = {1,3,5,7,9, -, -,156, $, the}; //For_each (Vec.begin (), Vec.end (), show); //OKFor_each (Vec.begin (), Vec.end (), showt<int> ());//OKcout <<Endl;}

Unlike a normal function, a class can have a data member, so the functor can have a state:

#include <iostream>#include<algorithm>#include<vector>using namespacestd;template<typename t>classaddt{T m_data=0; Public:    void operator()(ConstT &a) {m_data+=A; } T result ()Const    {        returnm_data; }};voidmain () {vector<int> VEC = {1,3,5,7,9, -, -,156, $, the}; AddT<int> sum = For_each (Vec.begin (), Vec.end (), addt<int> ());//For_each Returns the "copy" of the third parameter (A copy of the function object)cout <<Sum.result ()<<Endl;}

//function function: Performs _func on an interval [beg,end]. and returns _FUNC.template<class_init,class_fn1>inline _fn1 _for_each (_init _first, _init _last, _fn1 _func) { for(; _first! = _last; + +_first) _func (*_first); return(_func);} Template<class_init,class_fn1>inline _fn1 for_each (_init _first,//Start Interval_init _last,//Endinterval, [_first,_last]_FN1_FUNC)//function object or function (unary_function){    //perform function for each element_debug_range (_first, _last);    _debug_pointer (_func); return(_for_each (_unchecked (_first), _unchecked (_last), _func));}

For_each

template<class_init,class_pr>inline _init _find_if (_init _first, _init _last, _pr _pred) {//Find first satisfying _pred             for(; _first! = _last; + +_first)if(_pred (*_first))//bool (*pfun) (T)                     Break; return(_first); } template<class_init,class_pr>inline _init find_if (_init _first, _init _last, _pr _pred) {//Find first satisfying _pred_debug_range (_first, _last);            _debug_pointer (_pred); return(_rechecked (_first, _find_if (_unchecked (_first), _unchecked (_last), _pred))); }

find_if

template<class_init,class_pr>Inline TypeName Iterator_traits<_InIt>::d ifference_type _count_if (_init _first, _init _last, _pr _pred) {//count elements satisfying _predTypeName Iterator_traits<_init>::d ifference_type _count =0;  for(; _first! = _last; + +_first)if(_pred (*_first)) ++_count; return(_count); } template<class_init,class_pr>Inline TypeName Iterator_traits<_InIt>::d ifference_type count_if (_init _first, _init _last, _pr _pred) {//count elements satisfying _pred_debug_range (_first, _last);            _debug_pointer (_pred); return(_count_if (_unchecked (_first), _unchecked (_last), _pred)); }

count_if

Function object (functor functor)