const成员函数返回*this

 #include <iostream>
 using namespace std;
 class A{
 public:
     A &set(char);
     const A &display();
 //private:
     char a1 = 'l';
 };
 inline A &A::set(char c)
 {
     a1 = c;
     return *this;
 }

 inline const A &A::display()
 {
     cout << a1 << endl;
     return *this;
 }

 int main()
 {
 A screen;
 screen.display().set('a');   //错误,不能修改const对象;
 screen.a1 = 'z';
 //.set('l');
 return ;
 }