练习7.31

定义一对类X和Y，其中X包含一个指向Y的指针，而Y包含一个类型为X的对象

#ifndef ex7_31_H
#define ex7_31_H

class Y;

class X{
	Y *p = nullptr; //第一次使用Y，上面要声明
};
class Y{
	X a; //定义一个类X的对象a
};
#endif

练习7.32

定义Screen和Window_mgr，其中clear是Window_mgr的成员，是Screen的友元

#ifndef ex7_32_H
#define ex7_32_H

#include <string>
#include <vector>

class Screen; //类声明

class Window_mng {
public:
	//类型别名
    using ScreenIndex = std::vector<Screen>::size_type;
    //clear函数声明
    void clear(ScreenIndex);

private:
	//函数定义
    std::vector<Screen> screens{Screen(24, 80, ' ')};
};
//clear函数定义
void Window_mng::clear(ScreenIndex i)
{
    Screen &s = screens[i];
    s.contents = std::string(s.height * s.width, ' ');
}

class Screen {
	//友元函数声明，类Screen可以访问类Window_mng中的成员函数clear
    friend void Window_mng::clear(ScreenIndex);

public:
	//类型别名
    typedef std::string::size_type pos;
    //默认构造函数
    Screen() = default;
    //两参构造函数并初始化数据成员
    Screen(pos ht, pos wd) : height(ht), width(wd), contents(ht*wd, ' ') {}
    //三参构造函数并初始化数据成员
    Screen(pos ht, pos wd, char c) : height(ht), width(wd), contents(ht*wd, c) {}
	//get无参函数定义
    char get() const { return contents[cursor]; }
    //get内联两参函数声明
    inline char get(pos r, pos c) const;
    //单参set成员函数声明
    Screen &set(char);
    //三参set成员函数声明
    Screen &set(pos, pos, char);
	//两参move函数声明
    Screen &move(pos r, pos c);
	//返回对象引用，之后可以操作改变对象的属性
    Screen &display(std::ostream &os)
        { do_display(os); return *this; }
   	//返回对象引用，之后对对象的操作就是对其本身的操作
   	//第一个const:之后的操作无法改变对象的属性
   	//第二个const：修饰隐式形参this指针，不能够改变this所指的对象
    const Screen&display(std::ostream &os) const
        { do_display(os); return *this; }

private:
	//const修饰隐式形参this指针，不能够改变this所指的对象
    void do_display(std::ostream &os) const { os << contents; }

    pos cursor = 0;
    pos height = 0, width = 0;
    std::string contents;
};
//const修饰隐式形参this指针，不能够改变this所指的对象
char Screen::get(pos r, pos c) const
{
    pos row = r * width;
    return contents[row + c]; //返回拷贝值给临时对象
}

inline Screen &Screen::set(char c)
{
    contents[cursor] = c;
    return *this; //返回引用类型
}

inline Screen &Screen::set(pos r, pos col, char c)
{
    contents[r*width + col] = c;
    return *this; //返回引用类型
}

inline Screen &Screen::move(pos r, pos c)
{
    pos row = r * width;
    cursor = row + c;
    return *this;
}

#endif

练习7.33

如果我们给Screen添加一个如下所示的size成员将发生什么情况？请尝试修改

unknown type name ‘pos’

//类Screen内的类型别名 typedef std::string::size_type pos;
//const修饰隐式形参this指针，不能够改变this所指的对象
pos Screen::size() const{
	return height * width;
}

fixed

//pos在Screnn的作用域内
//const修饰隐式形参this指针，不能够改变this所指的对象
Screen::pos Screen::size() const{
	return height * width;
}

练习7.34

P256 Screen类

There is an error in

dummy_fcn(pos height)
		   ^
Unknown type name 'pos'

练习7.35

typedef string Type;//Type为string的类型别名
Type initVal(); //use 'string'
class Exercise{
public:
	typedef double Type;//Type为double的类型别名
	Type setVal(Type); //use 'double'
	Type initVal(); //use 'double'
private:
	int val;
};
Type Exercise::setVal(Type parm){ //first is 'string', second is 'double'
	val = parm + initVal(); //Exercise::initVal()
	return val;
}

Fixed：
changed

Type Exercise::setVal(Type parm){ //first is 'string', second is 'double'
	val = parm + initVal(); //Exercise::initVal()
	return val;
}

to

Exercise::Type Exercise::setVal(Type parm){ //first is 'double', second is 'double'
	val = parm + initVal(); //Exercise::initVal()
	return val;
}

练习7.36

In this case, the constructor initializer makes it appear as if base is initialized with i and then base is used to initialize rem. However, base is initialized first. The effect of this initializer is to initialize rem with the undefined value of base

// :base(i),rem(base % j){ }
base = i; 
rem = base % j; //用未定义base值初始化rem，在定义int rem, base; 中base后被定义，但初始化时先使用的base

Fixed：

struct X{
	X(int i, int j): base(i),rem(base & j){ }
	int base, rem; //初始值列表的顺序应该与数据成员声明顺序一致
};

练习7.37

本节提供的Sales_data类

//use Sales_data(std::istream &is);
Sales_data first_item(cin); //its value are up to your input
int main(){
//use Sales_data(std::string s = " "); 
//bookNo = " ", cnt = 0, revenue = 0.0
	Sales_data next; 
//use Sales_data(std:string s = " ");
//bookNo = "9-999-99999-9", cnt = 0, revenue = 0.0
	Sales_data last("9-999-99999-9");
}

练习7.38

Sales_data(std::istream &is = std::cin){ read(is, *this); }

练习7.39

illegal.
cause the call of overloaded ‘Sales_data()’ is ambiguous(含糊的,含混的)
如果使用同一参数，并不知道调用哪个重载版本

练习7.40

Such as Book

class Book{
public:
	Book() = default;
	Book(unsigned no, std::string name, std::string author, std::string pubdate):no_(no),name_(name),author_(author),pubdate_(pubdate){ }
	Book(std::istream &in) { in >> no_ >> name_ >> author_ >> pubdate; }
private:
	unsigned no_;
	std::string name_;
	std::string author_;
	std::string pubdate_;
};

最后

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