Can we use `const_cast` to modify a constant variable? [duplicate]

Answer 1

It is undefined behaviour to modify a constant value. Don't do it. If you need to modify the value, don't declare it as const.

Answer 2

The reason why you see 10 printed for ia is most likely the compiler optimization: it sees a const object, decides that it's not going to change, and replaces the last printout with this:

cout<< 10 <<"  "<<*ppa<<endl;

In other words, the generated code has the value of the const "baked into" the binary.

Casting away the const-ness of an object that has originally been declared as const and writing to that object is undefined behavior:

$5.2.11/7 - Note: Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier68) may produce undefined behavior (7.1.5.1).

Depending on the platform, const objects may be placed in a protected region of memory, to which you cannot write. Working around the const-ness in the type system may help your program compile, but you may see random results or even crashes.