Consider the following program:

program1.cpp

#include <iostream>
int main() {
    const int bar = 5;
    int&& i = const_cast<int&&>(bar);
    i = 0;
    std::cout << bar;

    return 0;
}

When I compiled and ran this with

g++ program1.cpp -Wall -Wextra -Wpedantic -o program1
./program1

the output was 5.

But when I compiled and ran this program, which I also expected to output 5:

program2.cpp

#include <iostream>
class example{
public:
    const int bar;
    example(const int i) : bar(i){}
    void foo() const{
        int&& i = const_cast<int&&>(bar);
        i = 0;
    }
};
int main() {
    example e(5);
    e.foo();
    std::cout << e.bar;

    return 0;
}

g++ program2.cpp -Wall -Wextra -Wpedantic -o program2
./program2

The output was 0.

Why is the result different when the const variable is in a class?