const修饰成员函数
常对象只能调用const修饰的成员函数
返回值也能用const修饰

#include <iostream>
using namespace std;
class B{
private:
    mutable int x ;
public:

    const int& f(int x ) const;
    B(){
        x = 2;
    }
    void f() const;
};
const int& B::f(int xx) const{
    x = xx;
    cout << x << endl;
    return x;
}
void B::f() const{
    cout << x << endl;
}

int main(){
    const B b;
    const int &a = b.f(1);
    //a = 3;
    cout  << a <<endl;
    b.f();

}