Cast pointer to variable on creation

Question

Today I saw a strange initialization of a pointer. It looks like this:

struct A
{
  void* data;
  int bufLen;
  ...
}

void fun(A* a, int* result)
{
  SomeClass* b = new (a->data) SomeClass();
}

It's completely fine, it does compile, it does work, but I don't know why. I'd like to know what's going on with the initialization of the variable b. Is it a cast? Is it a copy of the variable a->data in a new memory slot?

Possible duplicate of What uses are there for "placement new"? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jul 4, 2016 at 9:48
Thank you @Biffen. One question: for example, can I use the SomeClass variable to pass information to another thread? If I can, do I need anyway to call the descrutor in the fun function? — Astinog
– Astinog, Commented Jul 4, 2016 at 12:24
1 Yes. 2 It depends. This has nothing to do with placement new, though. — Biffen
– Biffen, Commented Jul 4, 2016 at 12:26
In the webpage you linked it says that I need to call explicitly the destructor in this case. My first thought was that if I deallocate that specific part of memory then this means that it can be used for other purpose. But I need to read that part of memory from another thread before it can be used again by some else. — Astinog
– Astinog, Commented Jul 4, 2016 at 12:29

Mattia F. · Accepted Answer · 2016-07-04 10:18:52Z

2

It's called placement new and it's generally used to place an object in a specified address in memory.

In your code, an object of type SomeClass will be placed at the memory location of a->data.

To use it, you can overload the operator new by yourself, or use the ones already defined in the Standard Library, contained in the header file <new>.

answered Jul 4, 2016 at 10:18

Mattia F.

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