【C++标准库】STL函数对象及Lambda

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
            }
}

/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference, 2nd Edition"
* by Nicolai M. Josuttis, Addison-Wesley, 2012
*
* (C) Copyright Nicolai M. Josuttis 2012.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <string>
#include <deque>
#include <set>
#include <algorithm>
using namespace std;


/* class Person
*/
class Person {
private:
    string fn;    // first name
    string ln;    // last name
public:
    Person() {
    }
    Person(const string& f, const string& n)
        : fn(f), ln(n) {
    }
    string firstname() const;
    string lastname() const;
    // ...
};

inline string Person::firstname() const {
    return fn;
}

inline string Person::lastname() const {
    return ln;
}

ostream& operator<< (ostream& s, const Person& p)
{
    s << "[" << p.firstname() << " " << p.lastname() << "]";
    return s;
}


/* 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.lastname() ||
            (p1.lastname() == p2.lastname() &&
                p1.firstname()<p2.firstname());
    }
};


int main()
{
    Person p1("nicolai", "josuttis");
    Person p2("ulli", "josuttis");
    Person p3("anica", "josuttis");
    Person p4("lucas", "josuttis");
    Person p5("lucas", "otto");
    Person p6("lucas", "arm");
    Person p7("anica", "holle");

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

    // create such a collection
    PersonSet coll;
    coll.insert(p1);
    coll.insert(p2);
    coll.insert(p3);
    coll.insert(p4);
    coll.insert(p5);
    coll.insert(p6);
    coll.insert(p7);

    // do something with the elements
    // - in this case: output them
    cout << "set:" << endl;
    PersonSet::iterator pos;
    for (pos = coll.begin(); pos != coll.end(); ++pos) {
        cout << *pos << endl;
    }
}

for_each获取function object的状态

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

class MeanValue
{
private:
    long num; //num of elements
    long sum; //sum of values
public:
    MeanValue():num(0),sum(0)
    {
    }
    void operator() (int elem)
    {
        ++num;
        sum += elem;
    }
    double value()
    {
        return static_cast<double>(sum) / static_cast<double>(num);
    }
};

int main()
{
    vector<int> 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

#include <iostream>
#include <functional>
using namespace std;
using namespace std::placeholders; //定义占位符

int main()
{
    auto plus10 = bind(plus<int>(), _1, 10); //以占位符_1表示第一个参数，以10为第二个参数
    cout << "+10: " << plus10(7) << endl;   //17

    auto plus10times2 = bind(multiplies<int>(),
                            bind(plus<int>(),
                                _1, 10),
                                2);
    cout << "+10*2: " << plus10times2(7) << endl; //34

    auto pow3 = bind(multiplies<int>(),bind(multiplies<int>(),_1, _1),_1);
    cout << "x*x*x: " << pow3(7) << endl;

    auto inversDivide = bind(divides<double>(), _2, _1);
    cout << "invdiv: " << inversDivide(49, 7) << endl;  // 1/7
    return 0;
}

 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

/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference, 2nd Edition"
* by Nicolai M. Josuttis, Addison-Wesley, 2012
*
* (C) Copyright Nicolai M. Josuttis 2012.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <iostream>
#include <algorithm>
#include <locale>
#include <string>
using namespace std;

char myToupper(char c)
{
    std::locale loc;
    return std::use_facet<std::ctype<char> >(loc).toupper(c);
}

int main()
{
    string s("Internationalization");
    string sub("Nation");

    // search substring case insensitive
    string::iterator pos;
    pos = search(s.begin(), s.end(),           // string to search in
        sub.begin(), sub.end(),       // substring to search
        [](char c1, char c2) {      // compar. criterion
        return myToupper(c1) == myToupper(c2);
    });
    if (pos != s.end()) {
        cout << """ << sub << "" is part of "" << s << """
            << endl;
    }
}

Lambda调用成员函数

/* The following code example is taken from the book
* "The C++ Standard Library - A Tutorial and Reference, 2nd Edition"
* by Nicolai M. Josuttis, Addison-Wesley, 2012
*
* (C) Copyright Nicolai M. Josuttis 2012.
* Permission to copy, use, modify, sell and distribute this software
* is granted provided this copyright notice appears in all copies.
* This software is provided "as is" without express or implied
* warranty, and with no claim as to its suitability for any purpose.
*/
#include <functional>
#include <algorithm>
#include <vector>
#include <iostream>
#include <string>
using namespace std;
using namespace std::placeholders;

class Person {
private:
    string name;
public:
    Person(const string& n) : name(n) {
    }
    void print() const {
        cout << name << endl;
    }
    void print2(const string& prefix) const {
        cout << prefix << name << endl;
    }
    //...
};

int main()
{
    vector<Person> coll
        = { Person("Tick"), Person("Trick"), Person("Track") };

    // call member function print() for each person
    for_each(coll.begin(), coll.end(),
        [](const Person& p) {
        p.print();
    });
    cout << endl;

    // call member function print2() with additional argument for each person
    for_each(coll.begin(), coll.end(),
        [](const Person& p) {
        p.print2("Person: ");
    });
}