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函数对象

(转)

函数指针的一种替代策略是Function object(函数对象)。

函数对象与函数指针相比较有两个方面的优点:首先如果被重载的调用操作符是inline函数则编译器能够执行内联编译,提供可能的性能好处;其次函数对象可以拥有任意数目的额外数据,用这些数据可以缓冲结果,也可以缓冲有助于当前操作的数据。

函数对象是一个类,它重载了函数调用操作符operator() ,该操作符封装了一个函数的功能。典型情况下函数对象被作为实参传递给泛型算法,当然我们也可以定义独立的函数对象实例。

来看下面的二个例子: 比较理解会更好些:

#include<vector>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
class Sum {
int val;
public:
Sum(int i) :val(i) { }

//当在需要int的地方,Sum将自动转换为int类型
//这里是为了方便cout<<Sum的实例;
operator int() const { return val; }

//写在类中的函数代码一般默认为内联代码
int operator()(int i) { return val+=i; }
};

void f(vector<int> v)
{
Sum s = 0; //Sum s = 0等价于Sum s(0),不等价于Sum s;s = 0;

//对vector<int>中的元素求和
//函数对象被作为实参传递给泛型算法
s = for_each(v.begin(), v.end(), s);

cout << "the sum is " << s << "\n";

//更简单的写法,定义独立的函数对象实例
cout << "the sum is " << for_each(v.begin(), v.end(), Sum(0)) << "\n";
}


int main()
{
vector<int> v;
v.push_back(3); v.push_back(2); v.push_back(1);
f(v);
system("pause");
return 0;
}
-----------------------------------------------
#include <iostream>
#include <list>
#include <algorithm>
#include "print.hpp"
using namespace std;

// function object that adds the value with which it is initialized
class AddValue {
  private:
    int theValue;    // the value to add
  public:
    // constructor initializes the value to add
    AddValue(int v) : theValue(v) {
    }

    // the ``function call'' for the element adds the value
    void operator() (int& elem) const {
        elem += theValue;
    }
};

int main()
{
    list<int> coll;

    // insert elements from 1 to 9
    for (int i=1; i<=9; ++i) {
        coll.push_back(i);
    }

    PRINT_ELEMENTS(coll,"initialized:                ");

    // add value 10 to each element
    for_each (coll.begin(), coll.end(),    // range
              AddValue(10));               // operation

    PRINT_ELEMENTS(coll,"after adding 10:            ");

    // add value of first element to each element
    for_each (coll.begin(), coll.end(),    // range
              AddValue(*coll.begin()));    // operation

    PRINT_ELEMENTS(coll,"after adding first element: ");
}
-------------------------------------------------------------------------
operator()中的参数为container中的元素
---------------------------

另外的实例:
Function Objects as Sorting Criteria

Programmers often need a sorted collection of elements that have a special class (for example, a collection of persons). However, you either don't want to use or you can't use the usual operator < to sort the objects. Instead, you sort the objects according to a special sorting criterion based on some member function. In this regard, a function object can help. Consider the following example:

   // fo/sortl.cpp

   #include <iostream>
   #include <string>
   #include <set>
   #include <algorithm>
   using namespace std;


   class Person {
     public:
       string firstname() const;
       string lastname() const;
       ...
   };


   /* class for function predicate
    * - operator() returns whether a person is less than another person
    */
   class PersonSortCriterion {
     public:
       bool operator() (const Person& p1, const Person& p2) const {
           /* a person is less than another person
            * - if the last name is less
            * - if the last name is equal and the first name is less
            */
           return p1.lastname()<p2.1astname() ||
                  (! (p2.1astname()<p1.lastname()) &&
                   p1.firstname()<p2.firstname());
       }
   };


   int main()
   {

       //declare set type with special sorting criterion
       typedef set<Person,PersonSortCriterion> PersonSet;

       //create such a collection
       PersonSet coll;
       ...


       //do something with the elements
       PersonSet::iterator pos;
       for (pos = coll.begin(); pos != coll.end();++pos) {
           ...
       }
       ...
   }


//fo/foreach3.cpp #include <iostream> #include <vector> #include <algorithm> using namespace std; //function object to process the mean value class MeanValue { private: long num; //number of elements long sum; //sum of all element values public: //constructor MeanValue() : num(0), sum(0) { } //"function call" //-process one more element of the sequence void operator() (int elem) { num++; //increment count sum += elem; //add value } //return mean value double value() { return static_cast<double>(sum) / static_cast<double>(num); } }; int main() { vector<int> coll; //insert elments from 1 to 8 for (int i=1; i<=8; ++i) { coll.push_back(i); } //process and print mean value MeanValue mv = for_each (coll.begin(), coll.end(), //range MeanValue()); //operation cout << "mean value: " << mv.value() << endl; }

posted on 2005-12-14 16:53 梦在天涯 阅读(4655) 评论(1)  编辑 收藏 引用 所属分类: CPlusPlusSTL/Boost

评论

# re: 函数对象 2006-01-10 11:55 shanzy

Function object(函数对象)

个人觉得应该叫:“功能对象”更好一点,JJHOU叫“仿函数”只是说它用起来像函数  回复  更多评论   


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