const_cast VS mutable ? any difference?

Question

From my understanding , mutable cancels the constness of a variable

Class A {
  void foo() const {
  m_a = 5;
}
mutable int m_a;
};

But also const_cast :

void print (char * str)
{
  cout << str << endl;
}

int main () {
  const char * c = "this is a line";
  print ( const_cast<char *> (c) );
  return 0;
}

So , what changes one from the other ?

Thanks

Answer 1

const_cast cannot cancel constness of an object. const_cast can only remove constness from an access path to an object. Access path is a pointer or a reference to an object. Removing the constness from the access path has absolutely no effect on the object itself. Even if you use const_cast to remove the constness of the access path, that still does not necessarily give you the permission to modify the object. Whether you can do it or not still depends on the object itself. If it is const, you are not allowed to modify it and any attempts to do so will result in undefined behavior.

For example, this illustrates the intended use of const_cast

  int i = 5; // non-constant object
  const int *p = &i; // `p` is a const access path to `i`

  // Since we know that `i` is not a const, we can remove constness...
  int *q = const_cast<int *>(p);
  // ... and legally modify `i`
  *q = 10;
  // Now `i` is 10

The only reason the above is legal and valid is the fact that i is actually a non-constant object, and we know about it.

If the original object was really constant, then the above code would produce undefined behavior:

  const int j = 5; // constant object
  const int *p = &j; // `p` is a const access path to `j`

  int *q = const_cast<int *>(p); // `q` is a non-const access path to `j`
  *q = 10; // UNDEFINED BEHAVIOR !!!

C++ language does not allow you to modify constant objects and const_cast is completely powerless here, regardless of how you use it.

mutable is a completely different thing. mutable creates a data filed that can be legally modified even if the containing object is declared const. In that sense mutable does allow you to modify [some designated parts of] constant objects. const_cast, on the other hand, can't do anything like that.

Answer 2

The difference is that const_cast can't cheat, but mutable is an exception to the rules.

On the first snippet m_a is mutable, and thus an exception to the rule that you can't modify data members on const member functions.

On the second snippet, const_cast tries to cheat, but really can't: while the type has changed, actual modification is not allowed: the string is truly const. Attempting to modify it would cause the program to exhibit undefined behaviour.

Answer 3

The difference is semantic - i. e. same generated code, same run-time results (constness is a purely compile-time construct anyway), but the two constructs convey a slightly different meaning.

The idea is that you use mutable for variables that are in the class, but don't constitute the state of the object. The classic example is the current position in a blob object. Navigating in the blob does not count as "modifying" the blob in a way that matters. By using mutable, you're saying "this variable may change, but the object is still the same". You're stating that for this particular class, const-ness does not mean "all variables are frozen".

const_cast, on the other way, means that you're violating existing const correctness and hope to get away with it. Probably because you're working with a 3rd party API that does not respect const (e. g. an old school C-based one).