const member and const method

Hello,
today I learnt this:

Code:

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class A {
  int* _value;
public:
  A (int value) : _value (new int(value)) {}
  ~A() { delete _value; }
   void redouble () const { *_value *= 2; } // const method can modified the value pointed by _value
};

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And it isn't what I want: is there a way treat this thing, please

Keep a reference instead of a pointer.

Quote:

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Originally Posted by wysota

Keep a reference instead of a pointer.

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Why do you suggest a reference? Because a pointer like this isn't good (then I can take a simple int) !?

Code:

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int & _value
  A (int value = 0) : _value (value) {}

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An if I had the sistuation here above (where the constructor cant take nothing) ? here, what I have to assign in the constructor? ('0' for pointers means 'nothing', then for "references" what means "nothing"?)

Quote:

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Originally Posted by mickey

An if I had the sistuation here above (where the constructor cant take nothing) ? here, what I have to assign in the constructor? ('0' for pointers means 'nothing', then for "references" what means "nothing"?)

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You can't do that. References don't allow reseating, and thus no late initialization.

If you want to keep using pointers remember that

Code:

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const int* _value;

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means a const pointer to a non const object. I.e. you can change the pointee, but not the pointer. If you want a pointer, where you can't change the value, you need to declare it:

Code:

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int const *_value;

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But then you can't change the value in a non const method either.

A solution might be to have a wrapper object around the pointer that would only allow accessing the pointed object in a non-const way. Something like a shared data pointer.

I don't understand if this behaviour of C++ (const methods on pointer data, I mean), is right or it's something similar to an accepted bug...
In general if I had a situation like that isn't dangerous to have a method const that isn't really const at all?

The behaviour you observe is correct. Object pointed to by a pointer is not part of the object containing the pointer, thus changing it from a const method of the object is allowed. It's exactly the same as you would change a global (or any other external) object.

Quote:

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Originally Posted by wysota

The behaviour you observe is correct. Object pointed to by a pointer is not part of the object containing the pointer, thus changing it from a const method of the object is allowed. It's exactly the same as you would change a global (or any other external) object.

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And are these two statics part of class?

Code:

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class A {
   static const int _size = 100; //1
   static int rom; //2
}
int A::rom = 10;

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Part of class - yes. Part of object - no. Thus probably (not sure about it) changing "rom" from a const method is allowed (the other variable is const, so you won't be able to modify it).