C ++这个指针总是const

Question

For a non const member function of class X, this pointer is of type X* const.

Then, this pointer for a member function is always const.

Then, do we always need to const cast as in:

void foo::p() {                        
      const_cast <int&> (member) = 1;    
}

Am I missing something here ?

Answer 1

For a non const member function of class X, this pointer is of type X* const.

No, the type of the this pointer inside a non-const member function is just X* (and it is an rvalue). But even if this were X* const , that would still not prevent you from changing data members.

Your code example doesn't match your question. It shows a const member function. Inside a const member function, the type of this is const X* . You cannot change any data members inside a const member function (with the exception of data members declared as mutable ). It is a contradiction to declare a member function as const if your goal is to change data members. Make up your mind.

Answer 2

For a non const member function of class X, this pointer is of type X* const.

Then, this pointer for a member function is always const.

More or less, it's an rvalue, but can seen as a const pointer, but note: The pointer is const, but not the pointee. That means, you cannot change which object this points to, but you can change the contents of the object.

In your example, the function is declared const:

void foo::p() const { 
              // ^---- here!

So in this case, this is of type X const * const (it's exactly an rvalue of type X const* but that does not matter here) - ie the pointer and the pointee are const. The latter is what really matters.

So if you want to modify members, just don't use const for the method:

void foo::p() {    
           //^------ no const!
      member = 1;       // perfectly legal, because *this is not const
      this->member = 1; // the same
      this = new foo;   // still illegal, this cannot be assigned to
}

Answer 3

the const is after the star symbol, meaning "X* const", and not "const X* or X const*" (which are the same). therefore it means you cannot make "this" point to different object than the one he is pointing at, but you can change the object that is being pointed, which makes sense. "const X* or X const*" means you cannot change the object that is being pointed at the moment, but you can make the pointer point to different objects. hope it helps(: