《C++ Primer Plus》第18章：探讨 C++ 新标准（9）

编程练习

1. 下面是一个简短程序的一部分：

   int main() {using namespace std;// list of double deduced from list contentsauto q = average_list ({15.4, 10.7, 9.0});cout << q << endl;// list of int deduced from list contentscout << average_list({20, 30, 19, 17, 45, 38} ) << endl;// forced list of doubleauto ad = average_list<double>({'A', 70, 65.33 });cout << ad << endl;return 0;
   }
   

   请提供函数 average_list()，让该程序变得完整。它应该是一个模板函数，其中的类型参数指定了用作函数参数的 initialize_list 模板的类型以及函数的返回类型。
   答：

   #include<iostream>
   #include<initializer_list>template<typename T>
   T average_list(std::initializer_list<T> List){T t;for(auto iter = List.begin(); iter != List.end(); iter++ ){t += *iter;}t = t / List.size();return t;
   }int main() {using namespace std;// list of double deduced from list contentsauto q = average_list ({15.4, 10.7, 9.0});cout << q << endl;// list of int deduced from list contentscout << average_list({20, 30, 19, 17, 45, 38} ) << endl;// forced list of doubleauto ad = average_list<double>({'A', 70, 65.33 });cout << ad << endl;return 0;
   }
   

2. 下面是类 Cpmv 的声明：

   class Cpmv {
   public:struct Info {std::string qcode;std::string zcode;};
   private:Info *pi;
   public:Cpmv();Cpmv(std::string q, std::string z);Cpmv(const Cpmv & cp);Cpmv(Cpmv && mv);~Cpmv();Cpmv & operator=(const Cpmv & cp);Cpmv & operator=(Cpmv && mv);Cpmv operator+(const Cpmv & obj) const;void Display() const;
   };
   

   函数 operator+() 应创建一个对象，其成员 qcode 和 zcode 有操作数的相应成员拼接而成。请提供为移动构造函数和移动复制运算符实现移动语义的代码。编写一个使用所有这些方法的程序。为方便测试，让各个方法都显示特定的内容，以便知道它们被调用。

   #include<string>
   #include<iostream>class Cpmv {
   public:struct Info {std::string qcode;std::string zcode;};
   private:Info *pi;
   public:Cpmv();Cpmv(std::string q, std::string z);Cpmv(const Cpmv & cp);Cpmv(Cpmv && mv);~Cpmv();Cpmv & operator=(const Cpmv & cp);Cpmv & operator=(Cpmv && mv);Cpmv operator+(const Cpmv & obj) const;void Display() const;
   };Cpmv::Cpmv() {std::cout << "default constructer" << std::endl;pi = new Info;pi->qcode = "null";pi->zcode = "null";
   }Cpmv::Cpmv(std::string q, std::string z){std::cout << "constructer" << std::endl;pi = new Info;pi->qcode = q;pi->zcode = z;
   }Cpmv::Cpmv(const Cpmv & cp){std::cout << "copy constructer" << std::endl;pi = new Info(*cp.pi);
   }Cpmv::Cpmv(Cpmv && mv){std::cout << "move constructor" << std::endl;pi = mv.pi;mv.pi = nullptr;
   }Cpmv::~Cpmv() {std::cout << "destructor" << std::endl;delete pi;
   }Cpmv & Cpmv::operator=(const Cpmv & cp) {std::cout << "assigment" << std::endl;if(this == &cp){return *this;}delete pi;pi = new Info(*cp.pi);return *this;
   }Cpmv & Cpmv::operator=(Cpmv && mv) {std::cout << "move assignment" << std::endl;delete pi;pi = mv.pi;mv.pi = nullptr;return *this;
   }Cpmv Cpmv::operator+(const Cpmv & obj) const {std::cout << "operator+" << std::endl;Cpmv c;c.pi->qcode = pi->qcode + obj.pi->qcode;c.pi->zcode = pi->zcode + obj.pi->zcode;return c;
   }void Cpmv::Display() const {std::cout << pi->qcode << " " << pi->zcode << std::endl;
   }int main(){{Cpmv c1;c1.Display();Cpmv c2("c2c2c2", "2c2c2c");c2.Display();Cpmv c3("c3c3c3", "c3c3c3");c3.Display();Cpmv c4(c3);c4.Display();Cpmv c5(c2+c3);c5.Display();c1 = c5;c1.Display();c1 = c5+c5;c1.Display();}return 0;
   }
   

3. 编写并测试可变参数模板函数 sum_value()，它接受任意长度的参数列表（其中包含数值，但可以是任何类型），并以 long double 的方式返回这些数值的和。

   #include <ios>
   #include<iostream>template<typename T>
   long double sum_value(const T & t){return t;
   }
   template<typename T, typename... Args>
   long double sum_value(const T & t, const Args&... args){return t + sum_value(args...);
   }int main() {std::cout << sum_value(1,2,3,4,5) << std::endl;std::cout << sum_value(1,2,3,4,5,6,7) << std::endl;;std::cout.precision(5);std::cout.setf(std::ios_base::fixed, std::ios_base::floatfield);std::cout << sum_value(1,2,3,4,5,6,7,8,9) << std::endl;return 0;
   }
   

4. 使用 lambda 重新编写程序清单 16.15。具体地说，使用一个有名称的 lambda 替换函数 outint()，并将函数符替换为两个匿名 lambda 表达式。

   // functor.cpp -- using a functor
   #include <iostream>
   #include <list>
   #include <iterator>
   #include <algorithm>// template<class T> // functor class defines operator()()
   // class TooBig{
   // private:
   //     T cutoff;
   // public:
   //     TooBig(const T & t) : cutoff(t) {}
   //     bool operator()(const T & v) { return v > cutoff; }
   // };//void outint(int n) { std::cout << n << " ";}
   auto outint = [](int n) { std::cout << n << " ";};int main(){using std::list;using std::cout;using std::endl;//TooBig<int> f100(100);  // limit = 100int vals[10] = {50, 100, 90, 180, 60, 210, 415, 88, 188, 201};list<int> yadayada(vals, vals+10); // range constructorlist<int> etcetera(vals, vals+10);// C++11 cna use the following instead// list<int> yadayada = {50, 100, 90, 180, 60, 210, 415, 88, 188, 201};// list<int> etcetera {50, 100, 90, 180, 60, 210, 415, 88, 188, 201};cout << "Original lists:\n";for_each(yadayada.begin(), yadayada.end(), outint);cout << endl;std::for_each(etcetera.begin(), etcetera.end(), outint);cout << endl;yadayada.remove_if([](int n){return n > 100;});etcetera.remove_if([](int n){return n > 200;});cout << "Trimmed lists:\n";for_each(yadayada.begin(), yadayada.end(), outint);cout << endl;for_each(etcetera.begin(), etcetera.end(), outint);cout << endl;return 0;
   }