const_cast

Xiangbuilder · 10-07-2003, 01:20 AM

I guess I can const_cast a const variable to nonconst, so I can change its value. But why the program below output *j=0xbffff2e4 instead of *j=3?

Code:

#include<iostream>
using namespace std;

int main ()
{
   const int i=0;
   cout<<"i="<<i<<endl;
   int* j=const_cast<int*>(&i); //preferred
   *j=3;
   cout<<"i="<<i<<endl;
   cout<<"*j="<<j<<endl;
   j=(int*)&i; //deprecated form
}
//: c03:const_cast.cpp
[root@localhost b_const]# g++ const_cast_a.cpp -o const_cast_a
[root@localhost b_const]# ./const_cast_a
i=0
i=0
*j=0xbffff2e4
[root@localhost b_const]#

thank you.

dimm_coder · 10-07-2003, 05:42 AM

Well, this problem caused by compiler optimization.
While compiling, it puts the value of i to all places of its output (it
thinks that it is a constant, like it is declared, so for optimization
purposes it makes so).
But try the next sample:
const_cast<int&>(i) = 1;
cout << const_cast<int&>(i) << endl;

So U'll get the value = 1 on output.

Nevertheless there is a more important possible problem
with const_cast using.
If const data is placed by compiler in section where writing is denied (f.e. in code section)
and U can try to use such type of code:

struct x { int a; };

const x a = {0};

int main()
{
const_cast<x&>(a).a = 1;
return 0;
}

Bang!!! Segmenattion fault!

So, as result, U need to take advanced care while using const_cast or
donot use it simply.

Xiangbuilder · 10-07-2003, 08:15 AM

Thank you.

abrb220 · 10-07-2003, 09:07 AM

Er...
Looks to me like a typo...the line

cout<<"*j="<<j<<endl;

should be

cout<<"*j="<<*j<<endl;

You were actually displaying the pointer and not the contents.

dimm_coder · 10-07-2003, 09:26 AM

Quote:

Originally posted by abrb220
Er...
Looks to me like a typo...the line

cout<<"*j="<<j<<endl;

should be

cout<<"*j="<<*j<<endl;

You were actually displaying the pointer and not the contents.

Yep, but this error doesn't give any big changes to this problem. The main problem doesn't depends on this.

Xiangbuilder · 10-07-2003, 09:36 AM

sorry for my simple mistake.