Describe the “placement new" and why would i use it?, C/C++ Programming

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Describe the “placement new" and why would I use it?


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3/15/2013 5:37:27 AM

A: There are several uses of placement new. The simplest use is to place an object at a specific location in memory. It is done by supplying the place as a pointer parameter to the new part of a new expression:

#include // Must #include this to use "placement new"

#include "Fred.h" // Declaration of class Fred

void someCode()

{

char memory[sizeof(Fred)]; // Line #1 void* place = memory; // Line #2

Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)

// The pointers f & place will be equal

...

}

Line #1 creates an array of sizeof(Fred) bytes of memory, that is adequate to hold a Fred object. Line #2 creates a pointer place which points to the first byte of this memory (experienced C programmers will note  down that this step was needless; it''s there just to make the code more obvious). Line #3 in essence just calls the constructor Fred::Fred(). The this pointer in the Fred constructor will be equal to place. Therefore the returned pointer f will be equal to place.

ADVICE: Don''t use this "placement new" syntax if not you have to. Use it only while you really care that an object is placed at a specific location in memory. For instance, while your hardware has a memory-mapped I/O timer device, and you wished to place a Clock object at that memory location.

DANGER: You are taking solitary job that the pointer you pass to the "placement new" operator points to a region of memory which is sufficient and is appropriately aligned for the object type which you''re creating. Neither the compiler nor run-time system makes any effort to check whether you did this right. If your Fred class requires to be aligned on a 4 byte boundary however you supplied a location which isn''t properly aligned, you can have serious disaster on your hands You have been warned. You are also exclusively responsible for destructing the placed object. It is done by explicitly calling the destructor:

void someCode()

{

char memory[sizeof(Fred)];

void* p = memory; Fred* f = new(p) Fred();

...

f->~Fred(); // Explicitly call the destructor for the placed object

}

It is about the only time you ever explicitly call a destructor.

Note: there is a much apparent but more sophisticated way of handling the destruction / deletion situation.

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