C++ primer 习题练习：12.13

#include <iostream>
#include <string>
using namespace std;
class Screen {
public:
	typedef string::size_type index;
	char get() const{return contents[cursor];}
	inline char get(index ht,index wd) const;
	index get_cursor() const;
	Screen (index hght,index wdth,const string &cntnts);
//	Screen& display(std::ostream &os){do_display(os);return *this;	}
	const Screen& display(std::ostream &os) const{	do_display(os);	return *this;}
	//三个成员函数
	Screen& move(index r,index c);
	Screen& set(char);
	Screen& display(ostream &os);

private:
	std::string contents;
	index cursor;
	index height,width;
	void do_display(std::ostream &os) const	{os<<contents;}
};
///////////////////////////
int main(){
	Screen myScreen(5,5,"aaaaaaaaaaaaaaaa/naaaaaaaaaaaaaa/naaaaaaaaaaaaaaa/naaaaaaaaaa/n");
	myScreen.move(4,0).set('#').display(cout);
	return 0;
}
//////////////////////////////////////////////////////////////////////////
//下面是类的实现部分
//////////////////////////////////////////////////////////////////////////
char Screen::get( index r,index c ) const
{
	index row=r*width;
	return contents[row+c];
}
inline Screen::index Screen::get_cursor() const
{
	return cursor;
}
Screen& Screen::set( char c )
{
	contents[cursor]=c;
	return *this;
}
Screen& Screen::display( ostream &os )
{
	os<<contents;
	return *this;
}
Screen& Screen::move( index r,index c )
{
	index row=r*width;
	cursor=row+c;
	return *this;
}
Screen::Screen( index hght,index wdth,const string &cntnts ):
		contents(cntnts),cursor(0),height(hght),width(wdth)
{}