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Thinking in C++
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mutable: bitwise vs. logical const

What if you want to create a const member function, but you’d still like to change some of the data in the object? This is sometimes referred to as the difference between bitwise const and logical const (also sometimes called memberwise const). Bitwise const means that every bit in the object is permanent, so a bit image of the object will never change. Logical const means that, although the entire object is conceptually constant, there may be changes on a member-by-member basis. However, if the compiler is told that an object is const, it will jealously guard that object to ensure bitwise constness. To effect logical constness, there are two ways to change a data member from within a const member function.

The first approach is the historical one and is called casting away constness. It is performed in a rather odd fashion. You take this (the keyword that produces the address of the current object) and cast it to a pointer to an object of the current type. It would seem that this is already such a pointer. However, inside a const member function it’s actually a const pointer, so by casting it to an ordinary pointer, you remove the constness for that operation. Here’s an example:

//: C08:Castaway.cpp
// "Casting away" constness

class Y {
  int i;
  void f() const;

Y::Y() { i = 0; }

void Y::f() const {
//!  i++; // Error -- const member function
  ((Y*)this)->i++; // OK: cast away const-ness
  // Better: use C++ explicit cast syntax:

int main() {
  const Y yy;
  yy.f(); // Actually changes it!
} ///:~

This approach works and you’ll see it used in legacy code, but it is not the preferred technique. The problem is that this lack of constness is hidden away in a member function definition, and you have no clue from the class interface that the data of the object is actually being modified unless you have access to the source code (and you must suspect that constness is being cast away, and look for the cast). To put everything out in the open, you should use the mutable keyword in the class declaration to specify that a particular data member may be changed inside a const object:

//: C08:Mutable.cpp
// The "mutable" keyword

class Z {
  int i;
  mutable int j;
  void f() const;

Z::Z() : i(0), j(0) {}

void Z::f() const {
//! i++; // Error -- const member function
    j++; // OK: mutable

int main() {
  const Z zz;
  zz.f(); // Actually changes it!
} ///:~

This way, the user of the class can see from the declaration which members are likely to be modified in a const member function.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire