函数对象function object，又称仿函数functors,是定义了operator()的对象。

class FunctionObjectType{    public:        void operator() ()            {                //statement            }}

1 /* The following code example is taken from the book 2 * "The C++ Standard Library - A Tutorial and Reference, 2nd Edition" 3 * by Nicolai M. Josuttis, Addison-Wesley, 2012 4 * 5 * (C) Copyright Nicolai M. Josuttis 2012. 6 * Permission to copy, use, modify, sell and distribute this software 7 * is granted provided this copyright notice appears in all copies. 8 * This software is provided "as is" without express or implied 9 * warranty, and with no claim as to its suitability for any purpose.10 */11 #include 
      
       12 #include 
       
        13 #include 
        
         14 #include 
         
          15 #include 
          
           16 using namespace std;17 18 19 /* class Person20 */21 class Person {22 private:23     string fn;    // first name24     string ln;    // last name25 public:26     Person() {27     }28     Person(const string& f, const string& n)29         : fn(f), ln(n) {30     }31     string firstname() const;32     string lastname() const;33     // ...34 };35 36 inline string Person::firstname() const {37     return fn;38 }39 40 inline string Person::lastname() const {41     return ln;42 }43 44 ostream& operator<< (ostream& s, const Person& p)45 {46     s << "[" << p.firstname() << " " << p.lastname() << "]";47     return s;48 }49 50 51 /* class for function predicate52 * - operator () returns whether a person is less than another person53 */54 class PersonSortCriterion {55 public:56     bool operator() (const Person& p1, const Person& p2) const {57         /* a person is less than another person58         * - if the last name is less59         * - if the last name is equal and the first name is less60         */61         return p1.lastname()
           
             PersonSet;80 81 // create such a collection82 PersonSet coll;83 coll.insert(p1);84 coll.insert(p2);85 coll.insert(p3);86 coll.insert(p4);87 coll.insert(p5);88 coll.insert(p6);89 coll.insert(p7);90 91 // do something with the elements92 // - in this case: output them93 cout << "set:" << endl;94 PersonSet::iterator pos;95 for (pos = coll.begin(); pos != coll.end(); ++pos) {96 cout << *pos << endl;97 }98 }
           
          
         
        
       
      

for_each获取function object的状态

#include 
      
       #include 
       
        #include 
        
         using namespace std;class MeanValue{private:    long num; //num of elements    long sum; //sum of valuespublic:    MeanValue():num(0),sum(0)    {    }    void operator() (int elem)    {        ++num;        sum += elem;    }    double value()    {        return static_cast
         
          (sum) / static_cast
          
           (num);    }};int main(){    vector
           
             coll = { 1,2,3,4,5,6,7,8 }; MeanValue mv = for_each(coll.begin(), coll.end(), MeanValue()); cout << "mean value:" << mv.value() << endl; return 0;}
           
          
         
        
       
      

 预定义的function object

C++标准库提供了许多预定义的function object和binder,后者允许你合成更多精巧的function object。使用时需包含头文件<functional>

bind()函数对象适配器

1 #include 
      
        2 #include 
       
         3 using namespace std; 4 using namespace std::placeholders; //定义占位符 5  6 int main() 7 { 8     auto plus10 = bind(plus
        
         (), _1, 10); //以占位符_1表示第一个参数，以10为第二个参数 9     cout << "+10: " << plus10(7) << endl;   //1710 11     auto plus10times2 = bind(multiplies
         
          (),12                             bind(plus
          
           (),13                                 _1, 10),14                                 2);15     cout << "+10*2: " << plus10times2(7) << endl; //3416 17     auto pow3 = bind(multiplies
           
            (),bind(multiplies
            
             (),_1, _1),_1);18 cout << "x*x*x: " << pow3(7) << endl;19 20 auto inversDivide = bind(divides
             
              (), _2, _1);21 cout << "invdiv: " << inversDivide(49, 7) << endl; // 1/722 return 0;23 }
             
            
           
          
         
        
       
      

 运用Lambda

Lambda提供了一种直观、易读的方式，可将独特的行为传递给算法和容器的成员函数。

Lambda表达式完整声明格式如下：

[capture list] (params list) mutable exception-> return type { function body }

其中：

1. capture list：捕获外部变量列表
2. params list：形参列表
3. mutable指示符：用来说用是否可以修改捕获的变量
4. exception：异常设定
5. return type：返回类型
6. function body：函数体

Lambda调用全局函数

1 /* The following code example is taken from the book 2 * "The C++ Standard Library - A Tutorial and Reference, 2nd Edition" 3 * by Nicolai M. Josuttis, Addison-Wesley, 2012 4 * 5 * (C) Copyright Nicolai M. Josuttis 2012. 6 * Permission to copy, use, modify, sell and distribute this software 7 * is granted provided this copyright notice appears in all copies. 8 * This software is provided "as is" without express or implied 9 * warranty, and with no claim as to its suitability for any purpose.10 */11 #include 
      
       12 #include 
       
        13 #include 
        
         14 #include 
         
          15 using namespace std;16 17 char myToupper(char c)18 {19     std::locale loc;20     return std::use_facet
          
           
             >(loc).toupper(c);21 }22 23 int main()24 {25 string s("Internationalization");26 string sub("Nation");27 28 // search substring case insensitive29 string::iterator pos;30 pos = search(s.begin(), s.end(), // string to search in31 sub.begin(), sub.end(), // substring to search32 [](char c1, char c2) { // compar. criterion33 return myToupper(c1) == myToupper(c2);34 });35 if (pos != s.end()) {36 cout << "\"" << sub << "\" is part of \"" << s << "\""37 << endl;38 }39 }
           
          
         
        
       
      

Lambda调用成员函数

1 /* The following code example is taken from the book 2 * "The C++ Standard Library - A Tutorial and Reference, 2nd Edition" 3 * by Nicolai M. Josuttis, Addison-Wesley, 2012 4 * 5 * (C) Copyright Nicolai M. Josuttis 2012. 6 * Permission to copy, use, modify, sell and distribute this software 7 * is granted provided this copyright notice appears in all copies. 8 * This software is provided "as is" without express or implied 9 * warranty, and with no claim as to its suitability for any purpose.10 */11 #include 
      
       12 #include 
       
        13 #include 
        
         14 #include 
         
          15 #include 
          
           16 using namespace std;17 using namespace std::placeholders;18 19 class Person {20 private:21     string name;22 public:23     Person(const string& n) : name(n) {24     }25     void print() const {26         cout << name << endl;27     }28     void print2(const string& prefix) const {29         cout << prefix << name << endl;30     }31     //...32 };33 34 int main()35 {36     vector
           
             coll37 = { Person("Tick"), Person("Trick"), Person("Track") };38 39 // call member function print() for each person40 for_each(coll.begin(), coll.end(),41 [](const Person& p) {42 p.print();43 });44 cout << endl;45 46 // call member function print2() with additional argument for each person47 for_each(coll.begin(), coll.end(),48 [](const Person& p) {49 p.print2("Person: ");50 });51 }