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Expertise: Advanced
Language: C++

October 7, 1998

Placement-New Requires Heap-Allocated Buffers

The placement-new operator constructs an object on a pre-allocated buffer. The pre-allocated buffer has to be allocated on the heap.

 
char *pbuff = new char[1024]; // heap allocation using plain new
Person *p = new (pbuff) Person; // placement new uses a pre-allocated buffer

You may be tempted to use a buffer allocated on the stack to avoid the need of explicitly freeing it:

 
char pbuff [1024];  //bad idea: stack allocation instead of heap
Person *p = new ( pbuff ) Person;  //undefined behavior

However, the pre-allocated buffer must comply with certain alignment constraints required by C++. Memory allocated on the heap is guaranteed to comply with these requirements. Stack memory, on the other hand, may not be properly aligned, and hence using it for that purpose yields undefined behavior.