Re: [boost] [Review] Type Traits Introspection library by Edward Diener starts tomorrow Friday 1

Subject: Re: [boost] [Review] Type Traits Introspection library by Edward Diener starts tomorrow Friday 1
From: John Bytheway (jbytheway+boost_at_[hidden])
Date: 2011-07-02 11:41:46

On 02/07/11 13:57, Edward Diener wrote:
> On 7/2/2011 4:44 AM, John Maddock wrote:
>>>> I mean things like
>>>>
>>>> struct A
>>>> {
>>>> static const int value = 42;
>>>> };
>>>
>>> That works correctly in the library for gcc and VC++. Evidently the
>>> code that tests, which uses an expression of &A::value, is accepted by
>>> those compilers. Is it illegal to take the address of a static const
>>> data member in C++ ?
>>
>> No, but you *may* need an out-of-line definition for the member "value"
>> if the compiler considers the address to be "used": my guess is you're
>> only using it within the context of another metafunction, so probably
>> this should be OK.... probably ;)
> 
> Is there a relevant section in the C++ standard about this which you can
> cite offhand ?

Don't have a copy of the current standard, but I can quote C++0x (N3290)
[class.static.data] (9.4.2) p3:

"The member shall still be defined in a namespace scope if it is
odr-used (3.2) in the program and the namespace scope definition shall
not contain an initializer."

The implication being that no namespace-scope definition is required
when it is not odr-used.

> In my quick test of this I did not specify an out-of-line definition but
> then again I am not "using" the const value in any other way then to
> test against it.

The definition of odr-used ([basic.def.odr], 3.2) is quite long, but
you're fine if it's an "unevaluated operand".  In particular, see [expr]
(5) p7:

"In some contexts, unevaluated operands appear (5.2.8, 5.3.3, 5.3.7,
7.1.6.2). An unevaluated operand is not evaluated."

These four references are typeid, sizeof, noexcept, and decltype.  Is
your use inside such an expression?  I was guessing it would be in sizeof().

John Bytheway