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Possible Duplicate:
C++'s “placement new”

What is an in-place constructor in C++?

e.g. Datatype *x = new(y) Datatype();

peterh
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Akhil
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    This is also called _placement new_ and you might have better chances finding information for that name. – sbi Sep 21 '10 at 19:48

6 Answers6

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This is called the placement new operator. It allows you to supply the memory the data will be allocated in without having the new operator allocate it. For example:

Foo * f = new Foo();

The above will allocate memory for you.

void * fm = malloc(sizeof(Foo));
Foo *f = new (fm) Foo();

The above will use the memory allocated by the call to malloc. new will not allocate any more. You are not, however, limited to classes. You can use a placement new operator for any type you would allocate with a call to new.

A 'gotcha' for placement new is that you should not release the memory allocated by a call to the placement new operator using the delete keyword. You will destroy the object by calling the destructor directly.

f->~Foo();

After the destructor is manually called, the memory can then be freed as expected.

free(fm);
linuxuser27
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    Yep. See also [this answer](http://stackoverflow.com/questions/2697892/what-is-return-type-of-new-in-c/2697929#2697929) for the difference between a _new expression_ and actual memory allocation. – sbi Sep 21 '10 at 19:50
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    "`new` will not allocate any more." - for the object as a fixed-sized thing - sure but of course it will allocate dynamic memory if the object requires it - strings, framebuffers, databases, whatever. – Euri Pinhollow Dec 30 '16 at 20:37
  • Is the destructor allowed to use free() to deallocate the memory allocated to the object through malloc? I am assuming the answer is yes. However, Is there a catch to this? – thegreatcoder Oct 07 '18 at 18:47
  • @thegreatcoder Yes. that is what should happen. I will update the answer. – linuxuser27 May 04 '19 at 22:31
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    Make sure to include header ``! – Czipperz Dec 12 '19 at 18:27
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The short answer is that your code constructs an object in the space pointed to by y. The long answer is best covered by the C++ FAQ.

Michael Kristofik
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This is more commonly known as 'placement new' and is discussed pretty well by the C++ FAQ (in the 'Destructors' area):

It allows you to construct objects in raw memory, which can be useful in certain specialized situations, such as when you might want to allocate an array for a large number of possible objects, but want to construct then as needed because you often might not need anywhere near the maximum, or because you want or need to use a custom memory allocator.

Michael Burr
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I'm rusty on this one but it allows you to write the object to a memory block you have already allocated. It also needs a reciprocal delete statement to clear it from memory.

wheaties
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If you use a memory pool, then you need to use the in place constructor to initialize your object as they are allocated from the pool.

stonemetal
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It's a way to call a constructor without allocating memory. Your y has to be a pointer poniting to enough memory for a new Datatype object. Also, don't call delete, call ~DataType().

Fozi
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