[cfe-dev] clang issue with 'mutable' ?

[Matthieu Monrocq]

I tried looking into the standard (n3485) and in 7.1.1 [dcl.stc] I got:

10/ The mutable specifier can be applied only to names of class data
members (9.2) and cannot be applied to names declared const or static, and
cannot be applied to reference members. [Example:

    class X {
        mutable const int* p; // OK
        mutable int* const q; // ill-formed
    };

-- end example]


As you mentioned, though, this applies to "names declared const", and it is
not clear whether it should cover parameterised types. Specifically I am
confused by:

5.2.5 [expr.ref]

4/ If E2 is declared to have type "reference to T," then E1.E2 is an
lvalue; the type of E1.E2 it T. Otherwise, one of the following rules
applies.

[...]

    - If E2 is a non-static data member and the type of E1 is "cq1 vq1 X",
and the type of E2 is "cq2 vq2 T", the expression designates the named
member of the object designated by the first expression. If E1 is an
lvalue, then E1.E2 is an lvalue; if E1 is an xvalue, then E1.E2 is an
xvalue; otherwise, it is a prvalue. Let the notation vq12 stand ofor the
"union" of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is
volatile. Similarly, let the notation cq12 stand for union cq1 and cq2,
that is, if cq1 or cq2 is const, then cq12 is const. If E2 is declared to
be a mutable member, then the type of E1.E2 is "vq12 T". If E2 is not
declared to be a mutable member, then the type of E1.E2 is "cq12 vq12 T".

[...]

Of interest, contrast: _the type of E2 is "cq2 vq2 T"_ versus _If E2 is
declared to be a mutable member, then the type of E1.E2 is "vq12 T"_ and
note that no special note is made (there) than E2 should not evaluate to
"const" something.

Interesting tidbit.

-- Matthieu



On Mon, Oct 21, 2013 at 6:54 PM, Steve McCoy <mccoyst at gmail.com> wrote:

> Clang is right; gcc is wrong. [dcl.stc] of the standard says, "The mutable
> specifier … cannot be applied to names declared const or static, and cannot
> be applied to reference members."
>
>
> On Mon, Oct 21, 2013 at 12:50 PM, Stefan Seefeld <stefan at seefeld.name>wrote:
>
>> On 2013-10-21 12:35, David Blaikie wrote:
>> > Interestingly GCC ToT (g++-tot (GCC) 4.9.0 20130829 (experimental))
>> > rejects this the same way Clang does, but 4.8.1 does not (it accepts
>> it).
>>
>> Interesting.  Unfortunately I still don't entirely understand why the
>> code is rejected. Is there any wording in the C++ spec that suggests
>> that the "wrapper" template may not be instantiated with some const type
>> ? In my reading, a "mutable" member is simply a statement about it not
>> being part of the "logically const" state of the containing object,
>> which is entirely orthogonal to the question of whether the member
>> itself is immutable. Obviously combining "const" and "mutable" in a
>> single declaration is meaningless. But as in my code, both qualifiers
>> can still end up in the same spot through parametrization, which seems
>> entirely reasonable to me. Am I wrong ?
>>
>> While I can think of some workarounds, I'd rather understand what the
>> issue is before making any attempt at working around it.
>>
>> Many thanks,
>>         Stefan
>>
>> --
>>
>>       ...ich hab' noch einen Koffer in Berlin...
>>
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>
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