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Re: Deprecating arithmetic on std::atomic<void*>

On 04/20/2017 11:22 AM, Jonathan Wakely wrote:
On 20/04/17 10:18 +0100, Jonathan Wakely wrote:
On 20/04/17 08:19 +0200, Florian Weimer wrote:
On 04/19/2017 07:07 PM, Jonathan Wakely wrote:
I know it's a bit late, but I'd like to propose deprecating the
libstdc++ extension that allows arithmetic on std::atomic<void*>.
Currently we make it behave like arithmetic on void*, which is also a
GNU extension (
We also allow arithmetic on types such as std::atomic<void(*)()> which
is probably not useful (PR 69769).

Why is it acceptable to have the extension for built-in types, but not for library types wrapping them? Why be inconsistent about this?

C++17 [atomic.types.pointer] paragraph 4 says:

Requires: T shall be an object type, otherwise the program is
ill-formed. [Note: Pointer arithmetic on void* or function pointers
is ill-formed. — end note]

That doesn't give us any leeway to support it.

If we can ignore the Note, we can also ignore the requirement …

I suppose we could support it without -Wpedantic, although there's no
way for the library to know if the front-end is being pedantic or not.

Could you use SFINAE to detect whether the compiler supports arithmetic on void *?


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