Create new C++ object at specific memory address?

Question

Is it possible in C++ to create a new object at a specific memory location? I have a block of shared memory in which I would like to create an object. Is this possible?

Answer 1

On Windows, MapViewOfFileEx and VirtualAllocEx allow one to specify a preferred virtual address. No guarantees though.

Answer 2

Assuming you have a pointer to the memory location you're wanting to place an object at, I believe one can cast the pointer to a new type and then place an object at the location of that pointer. This is a solution which doesn't require new().

Given your memory:

// you would use the pointer you have to your allocation of memory char* mem_start = new char[1024]; // Now we have 1024 bytes to play with

One can cast it to a given type:

CustomType* object_ptr = (CustomType*) mem_start;

Lastly, you can construct an object there:

*(object_ptr) = CustomType();

Answer 3

You want placement new(). It basically calls the constructor using a block of existing memory instead of allocating new memory from the heap.

Edit: make sure that you understand the note about being responsible for calling the destructor explicitly for objects created using placement new() before you use it!

Answer 4

if you want to allocate a lot of fine-grained objects, the best approach will be to use placement new in conjunction with some sort of a ring buffer. otherwise, you will have to keep track of the pointers aside from the object pointers themselves.

Answer 5

Yes. You need to use placement variant of operator new(). For example:

void *pData = ....; // memory segment having enough space to store A object
A *pA = new (pData) A;

Please note that placement new does not throw exception.