关于声明为const的成员函数的问题

[skineffect]

刚看了C++Primer 3第四篇，按书上说的拼凑了个Screen类，有个关于const成员函数的问题，源代码如下

//Screen.h
#include<string>
#include<iostream>
using namespace std;
class Screen{
  friend istream& operator>>(istream&,Screen&);
  friend ostream& operator<<(ostream&,const Screen&);
private:
  string _screen;
  mutable string::size_type _cursor;     //_cursor 被声明为mutable,因为move函数中要修改它，而move函数被声明为const
  short _height,_width;
  inline int remainingSpace();
public:
  Screen(int hi=8,int wid=40,char bkground='#');
  Screen& clear(char bkground='#');
  Screen& set(const string &s);
  Screen& set(char ch);
  int height(){return _height;}
  int width(){return _width;}
  void home(){_cursor=0;}
  inline Screen& move(int,int) const;    //这里将move函数声明为const
  char get() const{return _screen[_cursor];}
  inline char get(int,int);
  bool checkRange(int,int);
  void copy(const Screen& sobj);
  Screen& display();
  Screen& reSize(int h,int w,char bkground='#');
};
Screen& Screen::move(int r,int c) const{         //move函数定义
  if(checkRange(r,c)){
    int row=(r-1)*_width;
    _cursor=row+c-1;
  }
  return *this;
}
char Screen::get(int r,int c){
  move(r,c);
  return get();
}
inline int Screen::remainingSpace(){
  int sz=_width*_height;
  return(sz-_cursor);
}
//ScreenDef.cpp
#include "Screen.h"
ostream& operator<< (ostream& os,const Screen& s){
  os<<"<"<<s._height<<","<<s._width<<">";
  os<<s._screen;
  return os;
}
istream& operator>> (istream& is, Screen& s){
  is>>s._height>>s._width;
  return is;
}
bool isEqual(Screen& s1, Screen * s2){
  if(s1.height()!=s2->height()||s1.width()!=s2->width())
    return false;
  for(int ix=0;ix<s1.height();++ix)
    for(int jy=0;jy<s2->width();++jy)
      if(s1.get(ix,jy)!=s2->get(ix,jy))
        return false;
  return true;
}
Screen::Screen(int hi,int wid,char bk):_height(hi),_width(wid),_cursor(0),_screen(hi*wid,bk){}
bool Screen::checkRange(int row,int col){
  if(row<1||row>_height||col<1||col>_width){
    cerr<<"Screen coordinates("<<row<<","<<col<<")out of bounds.\n";
    return false;
  }
  return true;
}
void Screen::copy(const Screen& sobj){
  if(this!=&sobj)
  {
    _height=sobj._height;
    _width=sobj._width;
    _cursor=0;
    _screen=sobj._screen;
  }
}
Screen& Screen::set(const string &s){
  int space=remainingSpace();
  int len=s.size();
  if(space<len){
    cerr<<"Screen:warning:truncation:"<<"Space:"<<space<<"string length:"<<len<<endl;
    len=space;
  }
  _screen.replace(_cursor,len,s);
  _cursor +=len-1;
  return *this;
}
Screen& Screen::set(char ch){
  if(ch=='\0')
    cerr<<"Screen:warning:"<<"null character(ignored).\n";
  else
    _screen[_cursor]=ch;
  return *this;
}
Screen& Screen::clear(char bkground='#'){
  _cursor=0;
  _screen.assign(_screen.size(),bkground);
  return *this;
}
Screen& Screen::display(){
  typedef string::size_type idx_type;
  for(idx_type ix=0;ix<_height;++ix){
    idx_type offset=_width*ix;
    for(idx_type iy=0;iy<_width;++iy)
      cout<<_screen[offset+iy];
    cout<<endl;
  }
  return *this;
}
Screen& Screen::reSize(int h,int w,char bkground='#'){
  string local(_screen);
  _screen.assign(h*w,bkground);
  typedef string::size_type idx_type;
  idx_type local_pos=0;
  for(idx_type ix=0;ix<_height;++ix){
    idx_type offset=w*ix;
    for(idx_type iy=0;iy<_width;++iy)
      _screen[offset+iy]=local[local_pos++];
  }
  _height=h;
  _width=w;
  return *this;
}
//Screen.cpp
#include "Screen.h"
int main(){
  Screen sobj(3,3);
  string init("abcdefghi");
  cout<<"Screen Object("<<sobj.height()<<","<<sobj.width()<<")\n\n";
  string::size_type initpos=0;
  for(int ix=1;ix<=sobj.width();++ix)
    for(int iy=1;iy<=sobj.height();++iy){
      sobj.move(ix,iy);
      sobj.set(init[initpos++]);
    }
  for(int ix=1;ix<=sobj.width();++ix){
    for(int iy=1;iy<=sobj.width();++iy)
      cout<<sobj.get(ix,iy);
    cout<<"\n";
  }
  Screen myScreen(3,3),bufScreen;
  myScreen.clear().move(2,2).set('*').display();
  bufScreen.copy(myScreen);
  bufScreen.reSize(5,5).display();
  system("pause");
}

就是上面标注的move函数，他被声明为const后不能通过编译，去掉const就能，是不是返回Screen&的问题呢？
还有就是本人初学类，不知道这种工程文件的写法是否合适，如不合适应当怎么写，谢谢了。

[珍惜生命远离CPP]

move决定了checkRange也得是const的函数

[skineffect]

有点错误啊（又出来一个问题），就是在ScreenDef.cpp文件中的clear和reSize函数定义中必须去掉参数的默认值才行：
Screen& Screen::reSize(int h,int w,char bkground='#')要写成Screen& Screen::reSize(int h,int w,char bkground)
Screen& Screen::clear(char bkground='#')要写成Screen& Screen::clear(char bkground)
这是为什么呢？

[珍惜生命远离CPP]

声明处可以加默认值,实现的地方不能有默认值.

[skineffect]

确实是需要checkRange函数也得声明为const,这是为什么呢？
在checkRange函数声明为const之后还有问题，问题提示如下：
33 E:\源代码\C++源代码\C++Primer_and_CSDN\class\Screen.h invalid initialization of reference of type 'Screen&' from expression of type 'const Screen'
哪位大牛给详细解释下，谢谢了。

[ZhangZhongbao001]

别在技术区发广告回复。

[珍惜生命远离CPP]

move的返回值也得是const类型.

[skineffect]

说得没错，但因为我想在Screen.cpp文件中使用这样的表达式myScreen.clear().move(2,2).set('*').display();所以才让这些函数的返回类型为Screen&。
如果将move的返回类型设为const Screen& 的话，后面的set和display肯定会报错的，我想让move变成const的函数，因为只是光标移动，不会改变Screen中的内容，而Screen又不想变成const Screen,有没有折中的办法呢？
PS:解答时说得详细点吧，谢谢了。

[珍惜生命远离CPP]

最简单的最发就是都去掉const
不然的话你就得按照规定的来.

[wjb_yd]

const成员函数只能调用const成员函数
又因为const成员函数的this指针是const，所以只能返回const 类型