编程练习
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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; }
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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; }
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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; }
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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; }