函数对象(仿函数 functor)

简单地说，函数对象就是一个重载了()运算符的类实例，它可以像一个函数一样使用。

#include <iostream>
using namespace std;

class Add
{
public:
    int operator ()(const int &a, const int &b)
    {
        return (a + b);
    }
    double operator ()(const double &a, const double &b)
    {
        return (a + b);
    }
};


void main()
{
    Add plus;
    cout << plus(45, 54) << endl;//99
    cout << plus(44.3, 58.4) << endl;//102.7
}

用模板：

#include <iostream>
using namespace std;

template <typename T>
class Add
{
public:
    T operator ()(const T &a, const T &b)
    {
        return (a + b);
    }
};


void main()
{
    Add<int> plus1;
    cout << plus1(45, 54) << endl;//99
    Add<double> plus2;
    cout << plus2(44.3, 58.4) << endl;//102.7
}

标准库中的find_if、count_if、for_each等与之相关：

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

template <typename T>
class ShowT
{
public:
    void operator ()(const T &a)
    {
        cout << a << '	';
    }
};

void show(const int &a)
{
    cout << a << '	';
}

void main()
{
    vector<int> vec = {1, 3, 5, 7, 9, 64, 128, 156, 200, 256};
    //for_each(vec.begin(), vec.end(), show);    //ok
    for_each(vec.begin(), vec.end(), ShowT<int>()); //ok
    cout << endl;
}

与普通函数不同，因为类可以有数据成员，所以仿函数可以有状态的：

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

template <typename T>
class AddT
{
    T m_data = 0;
public:

    void operator ()(const T &a)
    {
        m_data += a;
    }
    
    T result() const
    {
        return m_data;
    }

};

void main()
{
    vector<int> vec = {1, 3, 5, 7, 9, 64, 128, 156, 200, 256};

    AddT<int> sum = for_each(vec.begin(), vec.end(), AddT<int>()); //for_each返回第三个参数的【拷贝】(A copy of the function object )
    cout << sum.result()
         << endl;
}

---

//函数功能:对一区间[beg,end)执行_Func.并返回_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));
}

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)));
         }

template<class _InIt,
         class _Pr> inline
         typename iterator_traits<_InIt>::difference_type
                   _Count_if(_InIt _First, _InIt _Last, _Pr _Pred)
        {       
            // count elements satisfying _Pred
            typename iterator_traits<_InIt>::difference_type _Count = 0;
     
            for (;_First != _Last; ++_First)
                if(_Pred(*_First))
                    ++_Count;
            return(_Count);
        }
 
template<class _InIt,
         class _Pr> inline
         typename iterator_traits<_InIt>::difference_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));
        }