6.63 Built-in functions for C++ allocations and deallocations ¶

Calling these C++ built-in functions is similar to calling ::operator new or ::operator delete with the same arguments, except that it is an error if the selected ::operator new or ::operator delete overload is not a replaceable global operator and for optimization purposes calls to pairs of these functions can be omitted if access to the allocation is optimized out, or could be replaced with implementation provided buffer on the stack, or multiple allocation calls can be merged into a single allocation. In C++ such optimizations are normally allowed just for calls to such replaceable global operators from new and delete expressions.

void foo () {
  int *a = new int;
  delete a; // This pair of allocation/deallocation operators can be omitted
	    // or replaced with int _temp; int *a = &_temp; etc.
  void *b = ::operator new (32);
  ::operator delete (b); // This one cannnot.
  void *c = __builtin_operator_new (32);
  __builtin_operator_delete (c); // This one can.
}