How Can Placement New Be Used Safely for Arrays in Portable Code?

Dealing with Placement New for Arrays in Portable Code

Placement new provides a way to allocate memory for objects at a specific memory location. While its use with arrays seems straightforward, achieving portability can present challenges.

The Problem

Consider the following example:

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

Here, pBuffer contains a buffer for the array allocated by new[]. However, new(pBuffer) A[NUMELEMENTS] may not return the same address as pBuffer, potentially leading to memory corruption.

Addressing the Issue

One approach is to manually place each array element individually using placement new:

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

This ensures that each element is allocated at the correct location within the buffer.

Handling Destructors

When deleting the array, it's essential to manually invoke the destructors for each element before deleting the buffer:

for(int i = 0; i < NUMELEMENTS; ++i)
{
  pA[i].~A();
}    
delete[] pBuffer;

This approach guarantees proper object cleanup and memory deallocation.

Conclusion

Placement new for arrays poses portability challenges due to potential misalignment between allocated and returned addresses. By manually placing each array element and handling destructors, developers can overcome these challenges and leverage placement new safely in portable code.

The above is the detailed content of How Can Placement New Be Used Safely for Arrays in Portable Code?. For more information, please follow other related articles on the PHP Chinese website!