operator new(n) versus new unsigned char[n] for placement new

Question

I'm allocating memory that will later be used for constructing objects with placement new. Should I be using operator new(n), or should I be using new unsigned char[n]? Why?

Answer 1

Factors:

• new[] must be matched with delete[] / new() with delete
• They communicate different things. operator new(n) is a request for memory for unspecified purposes, whereas new unsigned char[n] loosely implies intent to store characters there.

The array form may be slightly worse performance / efficiency wise - exact details depending on your implementation:

5.3.4/12 new T[5] results in a call of operator new where x is a non-neagtive unspecified value representing array allocation overhead: the result of the new-expression will be offset by this amount from the value returned by operator new[]....

BTW - neither are initialised:

• operator new() returns a void* to uninitialised memory: see 3.7.4.1/2 "There are no constraints on the contents of the allocated storage on return from the allocation function", whereas 5.3.4/15 says "a new-expression that creates an object of type T initializes that object as follows: if the new-initializer is ommitted, the object is default-initialized (8.5)"; 8.5/6 says only class types default constructors provide initialisation.

Answer 2

The former returns a pointer to some storage area. The latter returns a pointer to the first element of an array with some objects in it. If you need storage, use the one that gives storage and not objects.