为什么我在使用＆这时会出错？

Question

I know that the this pointer is implicitly passed to member functions when they are called. When I try to get the address of this pointer (via &this ), though, I get the compiler error "lvalue required". Why is this?

class st
{    
  int a,b;
public :
  void print()
  {      
    cout << &this; //gives lvalue required... why?

    cout << this; //will print address of object.
  }   
}

Answer 1

this is not an lvalue but an prvalue. From [class.this]:

In the body of a non-static (9.3) member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.

^{Emphasis mine}

& requires an lvalue so you cannot get the address of this .

Answer 2

Because this pointer is a rvalue. this pointer is a constant value, it is passed to the member function like a local variable, so it's value is stored in a memory location that would become invalid when returning from that function.

Answer 3

Presumably you're trying to print out the values in the object. cout doesn't know how to do this, and you have to teach it. cout << *this; might do this if cout knew how to do it, but you can teach it. Here's an example that is more natural c++. (You should also consider a constructor).

    #include <iostream>
    using namespace std;
    class st
    {
        public:
        int a,b;
    };
    std::ostream& operator<<(std::ostream& s, const st& val)
    {
        return s << "a:" << val.a << " b:" << val.b ;
    }
    int main() {
        st foo;
        foo.a = 1;
        foo.b = 2;
        cout << "foo is " << foo << endl;
    }