Re: Why is Ada a good choice for an ambitious beginner to programming

[Hyman Rosen <hyrosen@mail.com>]

Bill Findlay wrote:
> ... specifically to create local (i.e. not heap) dynamic arrays.
> As I said, I don't know whether C++ beginners need to do that.

I've never actually seen placement new used for this. After all,
in C++, how would you get the local storage in which to place the
object? There are two common uses for placement new that I'm aware
of. One looks like new(nothrow)Type, and selects a storage allocator
which returns null if it can not allocate the requested memory (the
default one throws an exception when this happens). The other looks
like new(pointer)Type, and that allocator simply returns its argument,
which must be properly aligned and sized storage for holding the
object (and this latter one is like Ada's for x'address use).

That second form decouples allocation of storage from initialization
of objects. It's not used often in user code, but figures heavily in
some library implementation code. For example, std::vector maintains
a size and a capacity. The former represents the number of elements
being held in the vector, and the latter represents the amount of
storage available. While capacity exceeds storage, new elements are
appended to the vector by using placement new to put them in the
unused space.