Dynamically sized non resizable array

Question

Is there a C++ type that acts like a "dynamically sized non resizable array"? This kind of type can be thought of as one of two things:

vector<T> but without resize, push_back, etc.
array<T,N> but where N is dynamic and not static.

I do not want a solution that only works if the type of the elements within the array is a non-copyable type. I want a generic solution.

Why do you need this? Why cant you use either an vector or an array? — 463035818_is_not_an_ai, Mar 05 '15 at 12:52
I like my types to be as accurate as possible. I plan on using some `vector` but settings its size only once and then only changing its values. I prefer if I could document this via the type system instead of via a comment. — tohava, Mar 05 '15 at 12:53
@Cyber - but I don't want it only for noncopyable value types, I want it for all types. — tohava, Mar 05 '15 at 12:54
What's wrong with using `std::vector<>` and just not using `resize()`, `push_back`? Just because you're not going to use it doesn't mean you need it to be removed from the interface. It's pointless to look for a container that does exactly only the features you plan to use. — tenfour, Mar 05 '15 at 13:01
Why would an array class work "only" if the types are non-copyable? — Neil Kirk, Mar 05 '15 at 13:01
@tenfour: There's no way to prevent those functions from being used, if someone forgets that it's not supposed to be resized. That could be disastrous if you rely on stable iterators and references to elements. — Mike Seymour, Mar 05 '15 at 13:02
As far as I know there was a proposal of `std::dynarray`. But now I do not see it in the C++ Working Draft. — Vlad from Moscow, Mar 05 '15 at 12:53
@Cyber - This is not a duplicate. Note that in the original question the solution advises to use noncopyable elements and I specifically said I want a generic one. — tohava, Mar 08 '15 at 09:46

Angew is no longer proud of SO · Accepted Answer · 2021-12-28T09:32:12.763

28

Yes, there (pretty much) is. std::unique_ptr<T[]>. The primary template has a partial specialisation for this case, which provides the appropriate interface (operator [], no operator * etc.)

Alternatively, you can wrap std::vector in your own class and restrict its interface. You could even do that by deriving a class from std::vector using non-public inheritance and publishing only the relevant parts of its interface:

template <class T, class A = std::allocator<T>>
struct FixedVector : private std::vector<T, A>
{
  using FixedVector::vector::vector;
  using FixedVector::vector::operator=;
  using FixedVector::vector::get_allocator;
  using FixedVector::vector::at;
  using FixedVector::vector::front;
  using FixedVector::vector::back;
  using FixedVector::vector::data;
  using FixedVector::vector::begin;
  using FixedVector::vector::cbegin;
  using FixedVector::vector::end;
  using FixedVector::vector::cend;
  using FixedVector::vector::empty;
  using FixedVector::vector::size;
  using FixedVector::vector::operator[];
};

edited Dec 28 '21 at 09:32

answered Mar 05 '15 at 12:54

Angew is no longer proud of SO

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Correct me if I'm wrong, but this needs to be a `const` because otherwise somebody can replace the pointer. Right? A `const` allows this to really function as a one-time resizable vector and nothing else. – tohava Mar 05 '15 at 12:57
1

Very nice trick with `using` – tohava Mar 05 '15 at 13:06
is seems like using a `const std::vector<>` is the obvious solution – Arvid Mar 06 '15 at 18:23
@Arvid That would prevent you from modifying the elements. – Angew is no longer proud of SO Mar 06 '15 at 18:53
Not being versed in templates, how do you use this? – Victor Eijkhout Oct 31 '16 at 13:01
@VictorEijkhout You use this exactly as you would use `std::vector`, except that there's no way to change its size from outside. You must therefore create it with the size you want. – Angew is no longer proud of SO Oct 31 '16 at 13:27
@Angew "create with the size" using what syntax? – Victor Eijkhout Dec 13 '16 at 21:33
@VictorEijkhout Use one of the [constructors which takes `size_type`](http://en.cppreference.com/w/cpp/container/vector/vector) in your mem-initializer list or wherever you're initialising it. – Angew is no longer proud of SO Dec 14 '16 at 08:51
@Angew It does not behave "exactly as you would std::vector": ``std::vector ext(15); ext[2] = 3;`` this doesn't work with your fixed vector. But I managed to figure that one out. – Victor Eijkhout Dec 15 '16 at 14:08
2

@VictorEijkhout I apparently missed `operator[]` in the list of imported functions. Thanks for pointing it out. – Angew is no longer proud of SO Dec 15 '16 at 14:10
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There's a semicolon `;` missing after `cbegin` in your code block, but unfortunately edits have to be at least 6 characters, so I cannot update it. – leosh Dec 28 '21 at 07:53
Wrapping std::vector would not solve the copyable issue, right? – Benji Altman Mar 09 '23 at 23:02

Dynamically sized non resizable array

1 Answers1