C++ Primer(第五版) 7.3.1--7.3.4节练习

7.23    

#ifndef	SCREEN_H
#define	SCREEN_H

class Screen {
public:
	using pos = std::string::size_type;
	Screen() = default;
	Screen(pos ht, pos wd): height(ht), width(wd), contents(ht * wd, ' ') { }
	Screen(pos ht, pos wd, char c): height(ht), width(wd), contents(ht * wd, c) { }
	char get() const { return contents[cursor]; }
	inline char get(pos r, pos c) const;
	Screen &move(pos r, pos c);
private:
	pos cursor = 0;
	pos height = 0, width = 0;
	std::string contents;
};

char Screen::get(pos r, pos c) const
{
	return contents[r * width + c];
}

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

#endif

7.24    见7.23代码

7.25    可以使用默认版本，其数据成员cursor，height和width都是内置类型，contents是string类型，定义了拷贝和赋值运算符，默认版本能够正确处理。

7.26    

class Sales_data {
public:
	/* ... */
	double avg_price