...
In this noncompliant code example, the global operator new(std::size_t) function is replaced by a custom implementation. However, the custom implementation fails to honor the behavior required by the function it replaces, as per the C++ Standard, [new.delete.single], paragraph 3. Specifically, if the custom allocator fails to allocate the requested amount of memory, the replacement function returns a null pointer instead of throwing an exception of type std::bad_alloc. By returning a null pointer instead of throwing, functions relying on the required behavior of operator new(std::size_t) to throw on memory allocations may instead attempt to dereference a null pointer. See EXP34-C. Do not dereference null pointers for more information.
...