Question

I’m using C++, and I have the following structures:

struct ArrayOfThese {
  int a;
  int b;
};

struct DataPoint {
  int a;
  int b;
  int c;
};

In memory, I want to have 1 or more ArrayOfThese elements at the end of each DataPoint. There are not always the same number of ArrayOfThese elements per DataPoint.

Because I have a ridiculous number of DataPoints to assemble and then stream across a network, I want all my DataPoints and their ArrayOfThese elements to be contiguous. Wasting space for a fixed number of the ArrayOfThese elements is unacceptable.

In C, I would have made an element at the end of DataPoint that was declared as ArrayOfThese d[0];, allocated a DataPoint plus enough extra bytes for however many ArrayOfThese elements I had, and used the dummy array to index into them. (Of course, the number of ArrayOfThese elements would have to be in a field of DataPoint.)

In C++, is using placement new and the same 0-length array hack the correct approach? If so, does placement new guarantee that subsequent calls to new from the same memory pool will allocate contiguously?

Answer 1

Since your structs are PODs you might as well do it just as you would in C. The only thing you’ll need is a cast. Assuming n is the number of things to allocate:

DataPoint *p=static_cast<DataPoint *>(malloc(sizeof(DataPoint)+n*sizeof(ArrayOfThese)));

Placement new does come into this sort of thing, if your objects have a a non-trivial constructor. It guarantees nothing about any allocations though, for it does no allocating itself and requires the memory to have been already allocated somehow. Instead, it treats the block of memory passed in as space for the as-yet-unconstructed object, then calls the right constructor to construct it. If you were to use it, the code might go like this. Assume DataPoint has the ArrayOfThese arr[0] member you suggest:

void *p=malloc(sizeof(DataPoint)+n*sizeof(ArrayOfThese));
DataPoint *dp=new(p) DataPoint;
for(size_t i=0;i<n;++i)
    new(&dp->arr[i]) ArrayOfThese;

What gets constructed must get destructed so if you do this you should sort out the call of the destructor too.

(Personally I recommend using PODs in this sort of situation, because it removes any need to call constructors and destructors, but this sort of thing can be done reasonably safely if you are careful.)