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I just learned about the C++ construct called "placement new". It allows you to exactly control where a pointer points to in memory. It looks like this:
#include <new> // Must #include this to use "placement new"
#include "Fred.h" // Declaration of class Fred
void someCode()
{
char memory[sizeof(Fred)];
void* place = memory;
Fred* f = new(place) Fred(); // Create a pointer to a Fred(),
// stored at "place"
// The pointers f and place will be equal
...
}
(example from C++ FAQ Lite)
In this example, the this pointer of Fred will be equal to place.
I've seen it used in our team's code once or twice. In your experience, what does this construct enable? Do other pointer languages have similar constructs? To me, it seems reminiscent of equivalence in FORTRAN, which allows disparate variables to occupy the same location in memory.
216
Placement new is a variation new operator in C++. Normal new operator does two things : (1) Allocates memory (2) Constructs an object in allocated memory. Placement new allows us to separate above two things. In placement new, we can pass a preallocated memory and construct an object in the passed memory.
Because placement new does not allocate memory, you should not use delete to deallocate objects created with the placement syntax. You can only delete the entire memory pool ( delete whole ). In the example, you can keep the memory buffer but destroy the object stored in it by explicitly calling a destructor.
std::vector<T,Allocator>::emplace_back Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container.
A placement new expression first calls the placement operator new function, then calls the constructor of the object upon the raw storage returned from the allocator function.
It allows you to do your own memory management. Usually this will get you at best marginally improved performance, but sometimes it's a big win. For example, if your program is using a large number of standard-sized objects, you might well want to make a pool with one large memory allocation.
This sort of thing was also done in C, but since there are no constructors in C it didn't require any language support.
179
It is also used for embedded programming, where IO devices are often mapped to specific memory addresses
31
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