new int[0] is permitted in C++ but is std::allocator<int>().allocate(0) well defined?
More general, must all Allocators accept 0 as a parameter to allocate?
Edit:
After reading the answers I tested Visual Studio's std::allocator: allocate(0) gives nullptr
deallocate(nullptr, anything) is a nop.
So using nullptr is a good suggestion, but the standard do not demand that deallocate(nullptr, 0) is a nop, see C++ allocator::deallocate(NULL,1) allowed?
Table 34 — Cpp17Allocator requirements
Memory is allocated for n objects of type T but objects are not constructed. allocate may throw an appropriate exception.174 [Note: If n == 0, the return value is unspecified. —end note]
I would read that as "The allocator shall/should handle n == 0, not throw and return a value that might be a valid pointer or be nullptr."
Indeed new int[0] is well-defined. Note that you are required to call delete[] on the returned pointer, and the behavior on dereferencing the pointer is undefined.
Similar rules apply to std::allocator().allocate(0): you can't dereference the returned pointer, and you need to clean up memory in the normal way by calling std::allocator::deallocate.
An allocator is allowed to throw a std::bad_alloc exception; you could deal with the 0 parameter case by doing that, without contravening any requirements laid down by the standard. Returning nullptr is an alternative.
