C++ allocator<X>::deallocate(NULL,1) allowed?

Question

Both free(NULL) and ::operator delete(NULL) are allowed. Does the allocator concept (e.g. std::allocator also allow deallocate(NULL,1), or is it required to put your own guard around it?

Answer 1

You'll need to add your own check.

According to §20.4.​1.1/8, deallocate requires:

p shall be a pointer value obtained from allocate(). n shall equal the value passed as the first argument to the invocation of allocate which returned p.

allocate throws an exception when storage can't be given (§20.4.​1.1/7). In other words, allocate never returns 0, and therefore deallocate should never get a 0. Passing a 0 would lead to undefined behavior.