$ cat foo.cpp 
#include <cstdio>

class foo {
private:
  mutable int dawut;
public:
  foo() : dawut(42) {
  }

  void showme() const {
    printf("foo.dawut = %d\n", dawut);
  }

  void bar() const {
    dawut = 23;
  }
};

int main() {
  const foo foo;
  foo.showme();
  foo.bar();
  foo.showme();
  return 0;
}

$ gcc -std=c++11 -O2 -Wall foo.cpp -o foo
$ ./foo 
foo.dawut = 42
foo.dawut = 23