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Subject: Re: [SG10] On __has_attribute
From: Aaron Ballman (aaron_at_[hidden])
Date: 2014-03-27 15:22:56


> Last Monday we discussed the idea of adding an extensible mechanism to
> detect the presence of an attribute to SG10's recommendations, based on
> Clang's __has_attribute feature. Two important observations:
>
> 1) Implementations have attribute syntaxes other than C++ attributes (both
> for historical reasons and as an ongoing concern to support attributes in
> C), and
> 2) We need a mechanism to determine whether the attribute-detection feature
> is available.
>
> Point (1) implies that we should not reserve the global name
> __has_attribute for *only* C++ attributes, so we considered the idea of a
> macro that takes the syntactic form of the attribute as an argument,
> leading us to:
>
>
> Option A:
> ---------
>
> __has_attribute([[deprecated]])
> __has_attribute(__attribute__((some_gnu_thing)))
> __has_attribute(__declspec(foobar))
> __has_attribute([clsid])
>
> The natural way to address (2) with this model is to use a preprocessor
> defined(__has_attribute) check, matching our recommendation for
> __has_include. However, this presents a problem: that check will succeed
> for existing versions of Clang, but existing versions of Clang do not
> support an attribute-syntax argument to __has_attribute (only the name of a
> GNU __attribute__).
>
> So, Option A would need to use either a feature-test macro (maybe
> __cpp_has_attribute), or a different macro name (maybe
> __has_attribute_syntax), in order to provide a way to portably check for
> the feature.
>
>
> Option B:
> ---------
>
> A simpler approach would be to acknowledge point (1) in the macro name,
> giving:
>
> __has_cpp_attribute ( attribute-token )
>
> Programs would test for the existence of this feature in the preprocessor
> with defined(__has_cpp_attribute), consistent with how they would check for
> __has_include. Vendors could choose to augment this with
> __has_gnu_attribute, __has_declspec_attribute, ... for the other syntaxes.
>
>
> At this point, I think Option B is superior to Option A. Therefore, my
> first draft of a suggested addition to our recommendations is the following:

I agree that Option B is preferable.

>
>
> Testing for the presence of an attribute: __has_cpp_attribute
>
> [some introductory text]
>
> *Syntax*
>
> *has-attribute-expression:*
> __has_cpp_attribute ( *attribute-scoped-token* )
>
> *Semantics*
>
> A *has-attribute-expression* shall appear only in the controlling constant
> expression of a #if or #elif directive ([cpp.cond] 16.1). The
> *has-attribute-expression* is replaced by the pp-number 1 if the
> implementation supports an attribute with the specified name, and by the
> p-number 0 otherwise.

Do we want to consider any ideas for attribute versioning? For
instance, say there was a desire to allow [[noreturn]] on a lambda.
With this proposal, there's no way to differentiate between versions
of the [[noreturn]] attribute. We could use a feature test macro, but
that's what this proposal really is.

We could support this by changing the pp-number replacement (0 means
not found, > 0 supported, but you could check the constant value to
see what version is supported).

>
> The #ifdef and #ifndef directives, and the defined conditional inclusion
> operator, shall treat __has_cpp_attribute as if it were the name of a
> defined macro. The identifier __has_cpp_attribute shall not appear in any
> context not mentioned in this section.
>
> An implementation should only claim to support an *attribute-token* with no
> *attribute-namespace* if it follows the behavior specified by a draft of
> the C++ standard or of a technical specification produced by ISO/IEC
> JTC1/SC22/WG21. An implementation should only claim to support an
> *attribute-token* with an *attribute-namespace* if it follows the behavior
> specified by the vendor identified by the *attribute-namespace*.
>
> [OPEN QUESTION: Do we want to provide recommendations like this last
> paragraph at all? If so, should we list the currently-know
> attribute-namespaces? Having a centralized list of them is useful, and this
> seems like a good place to maintain that list. Would it be in-scope for the
> features SG to maintain a list of the known vendor extension attributes?]

I agree that this paragraph can go.

>
> *Example*
>
> This demonstrates a way to use the attribute [[deprecated]] only if it is
> available.
>
> #ifdef __has_cpp_attribute
> # if __has_cpp_attribute(deprecated)
> # define ATTR_DEPRECATED(msg) [[deprecated(msg)]]
> # endif
> #endif

~Aaron


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