Screen类：primer12.14

#include<iostream>
#include<string>

class Screen
{
public:
    typedef std::string::size_type index;
    Screen(const std::string &con, index hx, index wx);
    char get() const
        { return contents[cursor];}
    inline char get(index hx, index wx) const;
    inline index get_cursor() const;

    Screen& move(index r, index c);
    Screen& set(char ch);
    const Screen& display(std::ostream &os) const;
    Screen& display(std::ostream &os);

private:
    std::string contents;
    index cursor;
    index height, width;
    void Screen::do_display(std::ostream &os) const;
};

Screen::Screen(const std::string &con, index hx, index wx):
    contents(con),height(hx),width(wx),cursor(0)
{
    if(contents.size() > height * width)
        height = contents.size()/width + 1;
    else contents.replace(con.size(), height*width - con.size(), 1, ' ');
}

char Screen::get(index hx, index wx) const
{
    return contents[wx + hx*width];
}

Screen::index Screen::get_cursor() const
{
    return cursor;
}

Screen& Screen::move(index r, index c)
{
    if(r >= height || c >= width)
    {
        std::cerr << "invalid row or column" << std::endl;
        throw EXIT_FAILURE;
    }
    cursor = r * width + c;
    return *this;
}

Screen& Screen::set(char ch)
{
    contents[cursor] = ch;
    return *this;
}

const Screen& Screen::display(std::ostream &os) const
{
    do_display(os);
    return *this;
}

Screen& Screen::display(std::ostream &os)
{
    do_display(os);
    return *this;
}

void Screen::do_display(std::ostream &os) const
{
    for(index i = 0; i < contents.size(); ++i)
    {
        os << contents[i];
        if( (i+1) % width == 0 )
            os << '\n';
    }
}


int main()
{
    std::string str = "Youth,beautiful youth，I love you!love";
    Screen scr(str,1,5);
    str = "kkkkkkkkkkkkkkkkkkkk";  // 这个时候str的改变不会改变Screen。
    scr.move(1,2).set('Z').display(std::cout).set('Z');
//    scr.move(1,2);
    scr.display(std::cout);
    return 0;
}