On Thu, Apr 20, 2017 at 10:18:09AM +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 (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.
Can't the support or lack thereof depend on -pedantic/-pedantic-errors? I mean, with -pedantic-errors we already error on void * arighmetics or function pointer arithmetics. If std::atomic<void*> would use the void * arithmetics, it would also reject it. Or does it use integer arithmetics instead? Jakub