Operator new allocates memory from the heap, on which an object is
constructed. Standard C++ also supports placement new operator, which
constructs an object on a pre-allocated buffer. This is useful when
building a memory pool, a garbage collector or simply when performance
and exception safety are paramount (there's no danger of allocation
failure since the memory has already been allocated, and constructing
an object on a pre-allocated buffer takes less time):
void placement() {
char *buf = new char[1000]; //pre-allocated buffer
string *p = new (buf) string("hi"); //placement new
string *q = new string("hi"); //ordinary heap allocation
cout<<
c_str()
<
<c_str();
}
：
placement new 表达式只是定位，不存在与其相对应的delete，如果delete则选择
delete[] buf。
文章来源:
http://www.cnblogs.com/michael-gao/archive/2008/06/05/1214239.html