[笔记] 《c++ primer》显示器程序 Chapter7

补充Sales_data没有体现出的其他类特性

Screen.h

#include <string>
#include <iostream>

class Screen {
public:
    typedef std::string::size_type pos;
#if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
    Screen() = default;  // needed because Screen has another constructor
#else
    Screen(): cursor(0), height(0), width(0) { }
#endif
    // cursor initialized to 0 by its in-class initializer
    Screen(pos ht, pos wd, char c): height(ht), width(wd), 
                                    contents(ht * wd, c) { }
    friend class Window_mgr;
    Screen(pos ht = 0, pos wd = 0): 
       cursor(0), height(ht), width(wd), contents(ht * wd, ' ') { }
    char get() const              // get the character at the cursor
        { return contents[cursor]; }       // implicitly inline
    inline char get(pos ht, pos wd) const; // explicitly inline
    Screen &clear(char = bkground);
private:
    static const char bkground = ' ';
public:
    Screen &move(pos r, pos c);      // can be made inline later
    Screen &set(char);
    Screen &set(pos, pos, char);
    // other members as before
    // display overloaded on whether the object is const or not
    Screen &display(std::ostream &os) 
                  { do_display(os); return *this; }
    const Screen &display(std::ostream &os) const
                  { do_display(os); return *this; }
private:
     // function to do the work of displaying a Screen
     void do_display(std::ostream &os) const {os << contents;}
    // other members as before
private:
#ifdef IN_CLASS_INITS
    pos cursor = 0;
    pos height = 0, width = 0;
#else
    pos cursor;
    pos height, width;
#endif
    std::string contents;
};

Screen &Screen::clear(char c) 
{
    contents = std::string(height*width, c);
    return *this;
}

inline                   // we can specify inline on the definition
Screen &Screen::move(pos r, pos c)
{
    pos row = r * width; // compute the row location
    cursor = row + c;    // move cursor to the column within that row
    return *this;        // return this object as an lvalue
}

char Screen::get(pos r, pos c) const // declared as inline in the class
{
    pos row = r * width;      // compute row location
    return contents[row + c]; // return character at the given column
}

inline Screen &Screen::set(char c) 
{ 
    contents[cursor] = c; // set the new value at the current cursor location
    return *this;         // return this object as an lvalue
}
inline Screen &Screen::set(pos r, pos col, char ch)
{
    contents[r*width + col] = ch;  // set specified location to given value
    return *this;                  // return this object as an lvalue
}

useScreen.cpp

#include <iostream>
using std::cout; using std::endl;

#include <string>
using std::string;

#include "Screen.h"

int main()
{
    Screen myScreen(5,3);
    // move the cursor to a given position, and set that character 
    myScreen.move(4,0).set('#');

    Screen nextScreen(5, 5, 'X');
    nextScreen.move(4,0).set('#').display(cout);
    cout << "
";
    nextScreen.display(cout);
    cout << endl;

    const Screen blank(5, 3);
    myScreen.set('#').display(cout);  // calls nonconst version
    cout << endl;
    blank.display(cout);              // calls const version
    cout << endl;

    myScreen.clear('Z').display(cout); cout << endl;
    myScreen.move(4,0);
    myScreen.set('#');
    myScreen.display(cout); cout << endl;
    myScreen.clear('Z').display(cout); cout << endl;

    // if move returns Screen not Screen&
    Screen temp = myScreen.move(4,0); // the return value would be copied
    temp.set('#'); // the contents inside myScreen would be unchanged
    myScreen.display(cout);
    cout << endl;
}