...
Failure to overload the corresponding dynamic storage function is likely to violate rules such as MEM51-CPP. Properly deallocate dynamically allocated resources. For instance, if an overloaded allocation function uses a private heap to perform its allocations, passing a pointer returned by it to the default deallocation function will likely cause undefined behavior. Even in situations in which the allocation function ultimately calls through to uses the default allocator to obtain a pointer to memory, failing to overload a corresponding deallocation function may leave the program in an unexpected state by not updating internal data for the custom allocator state.
It is acceptable to define a deleted allocation or deallocation function without its corresponding free store function. For instance, it is a common practice to define a deleted non-placement allocation or deallocation function as a class member function when the class also defines a placement new function. This prevents accidental allocation via calls to new for that class type, or deallocation via calls to delete on pointers to an object of that class type. It is acceptable to declare, but not define, a private allocation or deallocation function without its corresponding free store function for similar reasons. However, a definition must not be provided as that still allows access to the free store function within a class member function.
...