《C++ Primer Plus》（第6版）第18章编程练习

1. 函数average_list()

下面是一个简短程序的一部分：

int main()
{
    using namespace std;
    // list of double deduced from list contents
    auto q = average_list({15.4, 10.7, 9.0});
    cout << q << endl;
    // list of int deduced from list contents
    cout << average_list({20, 30, 19, 17, 45, 38} ) << endl;
    // forced list of double
    auto 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initializer_list模板的类型以及函数的返回类型。

程序：

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

template <typename T>
T average_list(initializer_list<T> l);

int main()
{
    // list of double deduced from list contents
    auto q = average_list({15.4, 10.7, 9.0});
    cout << q << endl;
    // list of int deduced from list contents
    cout << average_list({20, 30, 19, 17, 45, 38}) << endl;
    // forced list of double
    auto ad = average_list<double>({'A', 70, 65.33});
    cout << ad << endl;

    system("pause");
    return 0;
}

template <typename T>
T average_list(initializer_list<T> l)
{
    T sum = 0;
    if (l.size() == 0)
        return 0;
    for_each(l.begin(), l.end(), [&sum](T t) { sum += t; });
    return sum / l.size();
}

运行结果：

在这里插入图片描述

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赋值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。

程序：

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

class Cpmv
{
public:
    struct Info
    {
        string qcode;
        string zcode;
    };

private:
    Info *pi;

public:
    Cpmv();
    Cpmv(string q, 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;
};

int main()
{
    Cpmv c1;
    Cpmv c2("abc", "123");
    Cpmv c3(c2);
    c1 = c2;
    c1.Display();
    Cpmv c4(move(c1));
    c4.Display();
    Cpmv c5;
    c5 = move(c2);
    c5.Display();
    Cpmv c6("abc", "123");
    Cpmv c7("def", "456");
    Cpmv c8 = c6 + c7;
    c8.Display();

    system("pause");
    return 0;
}

Cpmv::Cpmv()
{
    pi = new Info;
    pi->qcode = "";
    pi->zcode = "";
    cout << "Cpmv() called.\n";
}

Cpmv::Cpmv(string q, string z)
{
    pi = new Info;
    pi->qcode = q;
    pi->zcode = z;
    cout << "Cpmv(string q, string z) called.\n";
}

Cpmv::Cpmv(const Cpmv &cp)
{
    pi = new Info;
    pi->qcode = cp.pi->qcode;
    pi->zcode = cp.pi->zcode;
    cout << "Cpmv(const Cpmv &cp) called.\n";
}

Cpmv::Cpmv(Cpmv &&mv)
{
    pi = mv.pi;
    mv.pi = nullptr;
    cout << "Cpmv(Cpmv &&mv) called.\n";
}

Cpmv::~Cpmv()
{
    delete pi;
    cout << "~Cpmv()  called.\n";
}

Cpmv &Cpmv::operator=(const Cpmv &cp)
{
    cout << "Cpmv &operator=(const Cpmv &cp) called.\n";
    if (this == &cp)
        return *this;
    delete pi;
    pi = new Info;
    pi->qcode = cp.pi->qcode;
    pi->zcode = cp.pi->zcode;
    return *this;
}

Cpmv &Cpmv::operator=(Cpmv &&mv)
{
    cout << "Cpmv &operator=(Cpmv &&mv) called.\n";
    if (this == &mv)
        return *this;
    delete pi;
    pi = mv.pi;
    mv.pi = nullptr;
    return *this;
}

Cpmv Cpmv::operator+(const Cpmv &obj) const
{
    cout << "Cpmv operator+(const Cpmv &obj) const called.\n";
    Cpmv cv;
    cv.pi->qcode = pi->qcode + obj.pi->qcode;
    cv.pi->zcode = pi->zcode + obj.pi->zcode;
    return cv;
}

void Cpmv::Display() const
{
    cout << "The qcode is " << pi->qcode << endl;
    cout << "The zcode is " << pi->zcode << endl;
}