moving std::vector to std::array

Question

std::vector and std::array store their data in contiguous blocks of memory. Suppose I have

class T;    // A movable class
T t;
std::vector<T>  vec;
vec.push_back(t);

std::array<T,1> arr;

Assuming like above that I am guaranteed to know that vec and arr have the same length. Is there a way that I can move vec to array instead of copy it with std::copy(vec.begin(), vec.end(), arr.begin())?

This migth be an XY problem. Could you specify what problem you are trying to solve with that. — Daniel Langr
– Daniel Langr, Commented Jun 30, 2021 at 11:47
Note that you cannot move objects in C++. You can only move their content. An array will always have its own objects-elements and a vector also its own, and both will be distinct. You cannot make the vector elements to be owned/managed by an array object. Span might be a solution as suggested (but we don't know what is the use case). — Daniel Langr
– Daniel Langr, Commented Jun 30, 2021 at 11:51

eerorika · Accepted Answer · 2021-06-30 11:30:47Z

11

You cannot move the vector. But you can move each element of the vector:

std::move(vec.begin(), vec.end(), arr.begin());

answered Jun 30, 2021 at 11:30

eerorika

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4 Comments

Bathsheba Over a year ago

Yes, true indeed. And useful if T is big. Have an upvote.

Reimundo Heluani Over a year ago

But if elements are T as in my example instead of T* or T& how would that work without copying?

ni1ight Over a year ago

"But if elements are T as in my example instead of T* or T& how would that work without copying?" I think that if T does have a move contructor / operator implemented properly, then it should avoid copying.

rawrex Over a year ago

@ReimundoHeluani you can't have a container of references. Pointers are moved/copied like other objects.

Bathsheba · Accepted Answer · 2021-06-30 11:30:23Z

4

Alas not. Neither std::vector nor std::array have a mechanism to attach a block of memory, or to detach a block of memory.

std::copy is possibly the most reliable method.

answered Jun 30, 2021 at 11:30

Bathsheba

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2 Comments

Hatted Rooster Over a year ago

std::copy this C++ standard draft.

463035818_is_not_an_ai Over a year ago

i think it is more than that. A simple picture of a std::vector is just a pointer to dynamically allocated memory, while a std::array is an array. You also cannot assign contents of a vector to a T[N]

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