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


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 (https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html).
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.

Personally I'd be in favour of removing the extension for built-in
types too, but I doubt that would get consensus :-)


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