Constant pointers in c++
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Hi guys,
Why is a constant pointer only able to point to a non constant value? Any help would be appreciated.
You need a pointer to a const value.
Hope this helps.
const int something = 10;// Ok, pointer points to a const object// Promises not to change the object it points to.const int * somethingptr = &something; // Also ok, pointer points to a const object// Promises not to change the object it points to.// It also promises not to change the pointer itself.const int * const somethingptr = &something;// Not ok. The pointer itself can't change, but what it points to can.// The compiler doesn't allow it, since you're assigning it something which is supposed to be const.int * const somethingptr = &something;Hope this helps.
Because using the pointer you have a read only access to specific data, but that data has the ability to change via code that has the read\write scope of the addressed object.
However, if you wanted to have a pointer to constant data, you'd have to have:
The data 'ptr' is addressing must be labeled as const.
EDIT: Ahh, Deventer beat me to it.
Random tidbit you may not have known, the 'this' pointer is defined as "const T* this;" in the object methods of the containing type. Hence why you can't do "this = other_object;" in some function "void T::foo()", but can change the instance data addressed via 'this' (unless of course the function is declared as "void T::foo() const").
long x = 0;const long* ptr = &x;x = 5;*ptr == 5; // trueHowever, if you wanted to have a pointer to constant data, you'd have to have:
const long x = 1;const long* const ptr = &x;The data 'ptr' is addressing must be labeled as const.
EDIT: Ahh, Deventer beat me to it.
Random tidbit you may not have known, the 'this' pointer is defined as "const T* this;" in the object methods of the containing type. Hence why you can't do "this = other_object;" in some function "void T::foo()", but can change the instance data addressed via 'this' (unless of course the function is declared as "void T::foo() const").
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