How can I build a vector whose data is pointed to an array, and is this possible for unified memory?

Question

We know that if we have a vector like

std::vector<int> a_vector;

and this vector has elements, we can have the array of elements from

int *the_array= a_vector.data();

My question is: How can we do the opposite?

int the_array[5]={1,2,3,4,5};

std::vector<int> a_vector;

a_vector.data=the_array; //this does not seem to work

To clarify, I don't wish to copy the values , nor create an array *from * the values but have them in the same memory area.

Also , why am I asking this?

I would like to have this original array, in a managed (unified) area of memory

__device__ __managed__ int the_array[5];

and build the vector to point there.

You could probably do it with a custom allocator, but, why go to the effort? Plus you wouldn't be able to expand the vector. See stackoverflow.com/questions/8049657/… — Den-Jason
– Den-Jason, Commented Jun 18, 2021 at 7:28
Firstly, int *the_array={1,2,3,4,5} is invalid. Second, if you want to control where the data managed by a std::vector resides, use a custom allocator. If you want to create an array of automatic storage duration, and then have a std::vector manage that array, my guess is that an allocator for the job will be a hairy job (lots of possibilities of introducing undefined behaviour in response to innocent operations by other code on the vector). — Peter
– Peter, Commented Jun 18, 2021 at 7:40
In the latest versions of thrust, a thrust vector can be of a universal_vector type, which means that it is using a managed allocator. — Robert Crovella
– Robert Crovella, Commented Jun 18, 2021 at 15:13

eerorika · Accepted Answer · 2021-06-18 07:56:56Z

1

we can have the array of elements from

What we get from std::vector::data is a pointer to the first element.

My question is: How can we do the opposite?
a_vector.data=the_array; //this does not seem to work

No. Only way for a vector to take ownership of an array like that is from another vector of same type.

answered Jun 18, 2021 at 7:56

eerorika

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2 Comments

KansaiRobot Over a year ago

so I guess the question is if I can define vectors in managed memory...

eerorika Over a year ago

@KansaiRobot Perhaps by writing a custom allocator that calls cudaMallocManaged. I don't have experience with that though.

paleonix · Accepted Answer · 2021-07-13 11:09:17Z

1

Being able to construct a vector with already allocated memory would somewhat defeat the RAI design principle which is propably the reason it's isn't part of the std::vector API.

If all you need is a nice modern C++ interface to your allocated memory, span would be the right choice. You don't need to wait for NVIDIA to implement C++20, but could use the gsl-lite implementation of span which provides __host__ __device__ member functions such that you can use their span inside (or as arguments to) kernels as well.

If you want an actual container (not a view), which manages the memory (also allocates it), Thrust would be the obvious choice. While it was possible before to use their device_vector with a custom allocator, nowadays they provide a universal_vector for unified memory.

edited Jul 13, 2021 at 11:09

answered Jun 18, 2021 at 13:04

paleonix

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