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(, ) << endl;//
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(, ) << endl;//
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 << '\t';
}
}; void show(const int &a)
{
cout << a << '\t';
} void main()
{
vector<int> vec = {, , , , , , , , , };
//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 = ;
public: void operator ()(const T &a)
{
m_data += a;
} T result() const
{
return m_data;
} }; void main()
{
vector<int> vec = {, , , , , , , , , }; 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));
}
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>::difference_type
_Count_if(_InIt _First, _InIt _Last, _Pr _Pred)
{
// count elements satisfying _Pred
typename iterator_traits<_InIt>::difference_type _Count = ; 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));
}
count_if