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 
     
      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 
     
      using namespace std;template 
      
       class Add{public:    T operator ()(const T &a, const T &b)    {        return (a + b);    }};void main(){    Add
       
         plus1;    cout << plus1(45, 54) << endl;//99    Add
        
          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 
     
      #include 
      
       #include 
       
        using namespace std;template 
        
         class ShowT{public:    void operator ()(const T &a)    {        cout << a << '\t';    }};void show(const int &a){    cout << a << '\t';}void main(){    vector
         
           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
          
           ()); //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 
     
      #include 
      
       #include 
       
        using namespace std;template 
        
         class AddT{    T m_data = 0;public:    void operator ()(const T &a)    {        m_data += a;    }        T result() const    {        return m_data;    }};void main(){    vector
         
           vec = {
   1, 3, 5, 7, 9, 64, 128, 156, 200, 256};    AddT
          
            sum = for_each(vec.begin(), vec.end(), AddT
           
            ()); //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
      
       inline _Fn1 _For_each(_InIt _First, _InIt _Last, _Fn1 _Func){    for (;_First != _Last; ++_First)        _Func(*_First);    return(_Func);} template
       
        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
      
        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
       
         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
      
        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
       
         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));        }