Std Container of varying number of elements but no dynamic memory allocation

Question

Not quite the same as Stack-Overflow Question: Constant Size Vector.

I need a container for size_t with the following properties:

push_back, contains, find, and erase; as for vector<size_t>
prescribed fixed memory footprint (I know the maximum number of elements at compile time as a constexpr).

The array class doesn't do the trick because array always contains the given number of elements. So I need a container that distinguishes between the memory it reserves and the items that it actually contains yet.

Who knows a suitable container in the c++2020 standard?

Use [boost static vector](https://www.boost.org/doc/libs/1_81_0/doc/html/boost/container/static_vector.html). — PaulMcKenzie, Mar 30 '23 at 12:59
i dont think it exists out of the box, but you can use `std::vector` with custom allocator. Or wrap an `std::array` and then use its memory to store the elements. — 463035818_is_not_an_ai, Mar 30 '23 at 13:02
wait a sec... the elements are `size_t` ? and no dynamic allocation? Then there is almost no difference between having the element and having only the memory ready to place an element. Keep it simple and use `std::array` — 463035818_is_not_an_ai, Mar 30 '23 at 13:06
how is it not the same as the liked question? Just use `reserve`. What's the problem with that? — Jeffrey, Mar 30 '23 at 13:10
reserve uses "new" once. This is incompatible with certain targets. @Jeffrey — fuse tee, Mar 30 '23 at 13:27

score 2 · Accepted Answer · answered Mar 30 '23 at 18:49

The definition of vector is:

template<class T, class Allocator = std::allocator<T>> class vector;

https://en.cppreference.com/w/cpp/container/vector

Notice the Allocator part. :-)

So, if you were, say, in a high-code-maturity environment with a linter that forbids the use of new, you could provide an allocator that allocates from a fixed static pool. This allocator could enforce the needed constraints.

Std Container of varying number of elements but no dynamic memory allocation

1 Answers1