[Patch] PR11504: -Wcast-qual and general constness issues with stl_tree.h

[Gabriel Dos Reis]

Gawain Bolton <gbolton@free.fr> writes:

[...]

| >| +      _M_root() { return reinterpret_cast<_Link_type&>(this->_M_header._M_parent); }
| >| +
| >| +      _const_Link_type
| >| +      _M_root() const { return static_cast<_const_Link_type>(this->_M_header._M_parent); }
| >
| >I do not understand why we would need 'reinterpret_cast' here.
| >If the issue is to safely cast away const-ness, the use const_cast<>.
| >
| No, reinterpret_cast does not cast away constness!  Only const_cast
| can cast away constness.

I know that reinterpret_cast<> cannot cast away constness.
(Please, assume I have some working understanding of C++).
The point I was making is that, if the intent is to cast away
const-ness then use const_cast<>. If the intent is to cast from Base
to Derived, then use static_cast (in this particular case).  In
particular, there should not be a need for reinterpret_cast.

| There's something I don't understand here.  You say reinterpret_cast
| is a "red alert" but don't seem to mind casting away constness. 

I think you get that wrong.

| All casting is ugly since it assumes you "know what you're doing" but to

No, you're mistaken.

| me the most dishonest cast is const_cast not reinterpret_cast.

Then, we should agree to disagree.  The point has nothing to do with being
honest or dishonest.

| The reason I think const_casts are the most dishonest is that you have
| "lied" to the compiler about a variable be const. 

No, nobody lied to the compiler.  You should understand that,
const_cast<> is sometimes needed because, something gets accidentally
"const", without the pointed object being effectivelly const.  In those
cases, const_cast<> is safe.  It has nothing to do with lie.

| I see the other
| types of casts as telling the compiler something like "trust me, I
| know what I'm doing, you can't see that this variable is also this
| type but it is".

Perhaps, but then that applies too to "const_cast".

But, there are great differences between reinterpret_cast<> and
static_cast<>.  And here, reinterpret_cast<> is the wrong tool.

Please, do change your patch to take those comments into account.

-- Gaby