Does Placement New on Arrays Guarantee Portability in C ?

Can Placement New for Arrays Ensure Portability?

While placement new provides a means to initialize arrays in C , its use for arrays introduces potential portability issues. Specifically, the pointer obtained from new[] may deviate from the provided address, hindering buffer allocation for the array.

The standard's 5.3.4, note 12, acknowledges this discrepancy, making it challenging to allocate a buffer of the appropriate size for the array. An example highlights the issue:

int main() {
  const int NUMELEMENTS = 20;
  char *pBuffer = new char[NUMELEMENTS * sizeof(A)];
  A *pA = new(pBuffer) A[NUMELEMENTS];

  // With Visual Studio, pA will be four bytes higher than pBuffer
  printf("Buffer address: %x, Array address: %x\n", pBuffer, pA);
}

In this example, the compiler appears to store a count of array elements in the buffer's first four bytes. As a result, memory corruption occurs because the buffer is allocated with only sizeof(A) * NUMELEMENTS bytes of space.

Avoiding Portability Concerns:

To mitigate these portability issues, consider the following approach:

• Manual Placement New: Rather than using placement new on the array, use it to initialize each array element individually. For instance:

int main() {
  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);
}

• Manual Destruction: Manually destroy each array element before deleting the buffer to prevent memory leaks.

It's important to note that the additional overhead for placement new[] can vary depending on the implementation and class definition. Nonetheless, this manual approach ensures portability across different compilers and eliminates the need to ascertain the overhead dynamically.

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