Re: change a constant by pointer
On 2008-11-08 12:52, want.to.be.professer wrote:
#include <iostream>
using namespace std;
// Check if N is constant
template <int N>
class ValidateConstInt{};
int main()
{
const int constValue = 12;
const int* pConstValue = &constValue;
int* pValue = const_cast <int*> ( &constValue );
ValidateConstInt <constValue> (); // Check if constValue is
constant
cout << "------------------------------------ " << endl;
cout << " begin: " << endl;
cout << "\t const int constValue = 12; " << endl;
cout << "\t const int* pConstValue = &constValue;"<< endl;
cout << "\t int* pValue = const_cast <int*> (&ConstValue);"<<
endl;
cout << endl;
cout << "------------------------------------ " << endl;
cout << " constValue = " << constValue << endl;
cout << " &constValue = " << &constValue << endl;
cout << " *pConstValue = " << *pConstValue << endl;
cout << " pConstValue = " << pConstValue << endl;
cout << " *pValue = " << *pValue << endl;
cout << " pValue = " << pValue << endl;
cout << endl;
*pValue = 1; // Change the value
cout << "------------------------------------- After *pValue = 1
" << endl;
cout << endl;
cout << " constValue = " << constValue << endl;
cout << " &constValue = " << &constValue << endl;
cout << " *pConstValue = " << *pConstValue << endl;
cout << " pConstValue = " << pConstValue << endl;
cout << " *pValue = " << *pValue << endl;
cout << " pValue = " << pValue << endl;
cout << "------------------------------------ " << endl;
return 0;
}
// ------------- Result:
------------------------------------
begin:
const int constValue = 12;
const int* pConstValue = &constValue;
int* pValue = const_cast <int*> (&ConstValue);
------------------------------------
constValue = 12
&constValue = 0012FF6C
*pConstValue = 12
pConstValue = 0012FF6C
*pValue = 12
pValue = 0012FF6C
------------------------------------- After *pValue = 1
constValue = 12 // not changed
&constValue = 0012FF6C
*pConstValue = 1 // have changed
pConstValue = 0012FF6C
*pValue = 1 // have changed
pValue = 0012FF6C
------------------------------------
The pConstValue and pValue both point to the ConstValue, and the value
access by the point is changed, but constValue has never changed ( and
it shoule not be a constant ). I can not explain, Can somebody help
me ?
One of the reasons we have constants is that they allow for certain
optimisations. Since the compiler knows the value of constValue it can
skip a lot of stuff it would have to do if it was a variable. If you had
not taken its address the compiler would probably not allocated the
memory for it at all and used the value of 12 wherever constValue was
used. But since you took the its address it had to allocate some memory
for it, but since it knew it was constant it still didn't bother to use
it and did the optimisations anyway.
Notice also that what you did (taking the address of a constant, casting
away the const-ness and using the pointer to change the value) is not
allowed, and with other settings / compiler / OS your program would have
crashed instead because constValue would have been places in read only
memory. In sort, if you do not follow the rules of the languages
anything can happen and no guarantees are given.
--
Erik Wikstr??m