Can placement new for arrays be used in a portable way?

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Personally I’d go with the option of not using placement new on the array and instead use placement new on each item in the array individually. For example:

int main(int argc, char* argv[])
{
  const int NUMELEMENTS=20;

  char *pBuffer = new char[NUMELEMENTS*sizeof(A)];
  A *pA = (A*)pBuffer;

  for(int i = 0; i < NUMELEMENTS; ++i)
  {
    pA[i] = new (pA + i) A();
  }

  printf("Buffer address: %x, Array address: %x\n", pBuffer, pA);

  // dont forget to destroy!
  for(int i = 0; i < NUMELEMENTS; ++i)
  {
    pA[i].~A();
  }    

  delete[] pBuffer;

  return 0;
}

Regardless of the method you use, make sure you manually destroy each of those items in the array before you delete pBuffer, as you could end up with leaks 😉

Note: I haven’t compiled this, but I think it should work (I’m on a machine that doesn’t have a C++ compiler installed). It still indicates the point 🙂 Hope it helps in some way!

Edit:

The reason it needs to keep track of the number of elements is so that it can iterate through them when you call delete on the array and make sure the destructors are called on each of the objects. If it doesn’t know how many there are it wouldn’t be able to do this.

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