C++, Classes, Const, and strange syntax

Question

I was re-reading c++ primer(4th ed.) today - the section on member functions and const references etc, and I came up with this wierd little program:

using st         


        

Answer 1

Your object is not entirely immutable: it's just the reference you created to point to it that's constant.

Answer 2

The real issue isn't the behavior of ConstCheater::getccp() - it's that there's no error on the line:

const ConstCheater cc(7);

which initializes a non-const pointer with what should be a const this pointer. However, constructors cannot be const (9.3.2/5, but a bit of thought should make it obvious why). So the constructor is allowed to initialize a non-const pointer with a pointer to a const object (or an object that's 'about to become' const). That's the hole you're driving though.

As to why it's allowed, I imagine it would be difficult for the standard to try to close the hole since it would have to enumerate all the ways that a constructor's this would have to be treated const and all the ways it would have to be treated non-const when constructing a const object. That seems like a pretty difficult task.

Answer 3

I'd say the answer by Tony that you marked as correct is incorrect, and the answer by Michael Burr is correct.

To put what he said more clearly (for me at least):

There are two potential places for misunderstanding:

1. The way implicit const works
2. The way that this is interpreted during construction of a const object

---

1. Implicit Const

Implicit const (the constification internal to ConstCheater when it is made const) doesn't turn cc into a pointer-to-const but rather a const-pointer, that is to say when you do this:

  const ConstCheater cc(7); 

Internally goes from:

  ConstCheater * ccp;

...to...

  ConstCheater * const ccp;

...rather than the...

  const ConstCheater * ccp;    

that may have been expected.

2. Construction of const object

The stranger thing is though is that this is allowed to be passed to cpp's initializer in the constructor since this, one would think, should be treated as a pointer-to-const, and thus not a valid value to pass to a const-pointer.

That is to say one might expect:

 ...: ccp(this) ... // expected to fail but doesnt

to fail because conceptually you might expect that this was (somewhat) equivalent to:

 const ConstCheater         cc(7);
 const ConstCheater * const this = &cc; // const-pointer-to-const

and thus you would think that:

 ConstCheater * const ccp = this; //expected error!

would fail! But it doesn't because apparently during construction apparently this is treated specially as if it was:

 const ConstCheater * this = &cc; 

and thus the object is effectively not const during construction.

I'm not sure I understand completely the reasoning, but Michael Burr points out there appears to be a logical and technical barrier to providing the expected behavior so the standard seems to carve out the current somewhat odd behavior.

I recently asked a related question which was: Why does C++ not have a const constructor? but thus far haven't really understood completely the reasoning why it would be untenable, though I suppose it would place a burden on C++ developers to have to define an awkward const constructor for any class they'd like to create const object of.

Answer 4

const qualifier restricts to call non-const methods on your object, so the problem is your design that allows you to give non-const reference to a member by a const method. The common approach is

      Member& getccp()       {return *member;}
const Member& getccp() const {return *member;} 

In some cases when logical constness of your object don't suffer from external modification of it's member you can allow

      Member& getccp() const {return *member;}

Another example of difference in logical and formal constness is the mutable members. I.e. mutable can be some term that was computed at the last const method's invoke, if you will get the same input at next invoke you can easily return stored value.

Answer 5

Even though getccp() is a const method it makes no promises what you do with the reference it returns. The method itself does not modify the object and therefore does not break the rules.

If it returned a const ConstCheater& then that would be different.

As your example shows, there is much more complexity to const than just applying it to an object. the C++ FAQ has a section on const correctness and in particular it covers the case you are highlighting here.

Answer 6

What you are making const is the reference to a ConstCheater. Nothing in ConstCheater is making value a const.