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What is placement new?

It's a way to pass parameters to the allocator rather than just to the constructor.
Allocating an object from the heap, such as new Fred(5,7), is a two-step process: first an appropriately sized and aligned block of uninitialized memory is allocated from the heap, then the constructor is called with the this pointer pointing to that block of memory. Parameters are often passed to the constructor (for example, the above example passes (5,7)), but occasionally parameters also must be passed to the allocation step. For example, if there was a special allocator that used a particular pool of memory, it might be necessary to pass a reference to that pool of memory to the allocation step, that is, to new itself: new(myPool) Fred(5,7).
Another common reason to pass a parameter for the allocation step is to pass a pointer to a particular preallocated region of memory. For example, if pointer p is a void* that points to a pile of memory that is at least sizeof(Fred) bytes long and is appropriately aligned, one could say new(p) Fred(5,7). This would construct a Fred object at the location pointed to by p (that is, it would pass p as the this pointer to Fred's constructor) and would ultimately return a Fred* that would point to the same location that void* p points to. For example,
#include // Must #include this to use placement new
using namespace std;
#include "Fred.hpp" // Declaration of class Fred

void sample() throw()
{
char memory[sizeof(Fred)]; // Line 1
void* p = memory; // Line 2
Fred* f = new(p) Fred(); // Line 3 (be careful here!)
// The pointers f and p will be equal
// ...
}
Line 1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred object. Line 2 creates a pointer p that points to the first byte of this memory (experienced C programmers will note that this step is unnecessary; it's there only to make the code more obvious). Line 3 essentially calls the constructor Fred::Fred(). The this pointer in the Fred constructor is equal to p. The returned pointer f is equal to p.
Passing a void* pointer with the placement new syntax should be used only when it is essential to place an object at a particular location. For example, when writing an operating system, the placement new syntax could be used to place a Clock object at a particular memory-mapped I/O timer device. Neither the compiler nor the runtime system make any attempt to check whether the passed pointer points to a region of memory that is big enough and is properly aligned for the object being created. For example, if Fred objects need to be aligned on a 4-byte boundary but the supplied pointer p isn't properly aligned, it could be a serious (and subtle) disaster. You have been warned.
Also, the programmer takes sole responsibility to deallocate the object when the placement new syntax is used. This is done by explicitly calling the destructor, which is one of the few times a destructor should be called explicitly:
void sample2() throw()
{
char memory[sizeof(Fred)];
void* p = memory;
Fred* f = new(p) Fred();
// ...
f->~Fred();
}
(1) Explicitly call the destructor for the placed object
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