How does C++ keep the const value? [duplicate]

Question

#include <iostream>
using namespace std;
int main() {
    const int a = 10;
    auto *b = const_cast<int *>(&a);
    *b = 20;
    cout << a << " " << *b;
    cout << endl << &a << " " << b;
}

The output looks like:

10 20
0x7ffeeb1d396c 0x7ffeeb1d396c

The a and *b are at the same address, why do they have different value?

Answer 1

Most probably this is caused by optimization.

As molbdnilo already said in his comment: "Compilers trust you blindly. If you lie to them, strange things can happen."

So when optimization is enabled, the compiler finds the declaration

const int a = 10;

and thinks "ah, this will never change, so we don't need a "real" variable and can replace all appearances of a in the code simply with 10". This behavior is called constant folding.

Now in the next line you "cheat" the compiler:

auto *b = const_cast<int *>(&a);
*b = 20;

and change a, although you have promised not to do so. The result is confusion.

Like a lot of people have already mentioned and Christian Hackl has thoroughly analyzed in his excellent in-depth answer, it's undefined behavior. The compiler is generally allowed to apply constant folding on constants, which are explicitly declared const.

Your question is a very good example (I don't know how anyone can vote it down!) why const_cast is very dangerous (and even more when combined with raw pointers), should be only used if absolutely necessary, and should be at least thoroughly commented why it was used if it was unavoidable. Comments are important in that case because it's not only the compiler you are "cheating":

Also your co-workers will rely on the information const and rely on the information that it won't change. If it DOES change though, and you didn't inform them, and did not comment, and did not exactly know what you were doing, you'll have a bad day at work :)

Try it out: Perhaps your program will even behave different in debug build (not optimized) and release build (optimized), and those bugs are usually pretty annoying to fix.