int main(int argc, char *argv[])
{
  char a = '1';
  const char b = a;

  char * a2 = "12345";
  const char * b2 = a2;

  char** a3 = NULL;

  //const char** b3 = a3; //error
   char** const c3 = a3; //ok
   char* const * d3 = a3; //ok
}

class Foo {
public:
 Foo(){
   i = 1;
 }
 void modify(){// make some modification to the this object
   i = 2;
 } 
 void print() const {
   cout << "Foo_i:" << i << endl;
 }
private:
 int i;
};

//演示潜在的危险  
//error: invalid conversion from `Foo**' to `const Foo**'
/////////////////////////////////////////////////////////
int main(int argc, char *argv[])
{
  const Foo x;
  Foo* p;

  //const Foo ** q = &p; //q now points to p; this is (fortunately!) an error
  const Foo ** q = const_cast<const Foo **>(&p); 
  *q = &x; // p now points to x
  p->modify(); // Ouch: modifies a const Foo!! 
  x.print(); // print: Foo_i:2
  return 0;
}