Firstly, you probably intended to call it "reserve" and to pass C by
reference in your reference implementation.

Secondly, it doesn't feel very "ranges-like" to fall back onto doing
nothing rather than falling back onto a slower version of the same thing.
For example, ranges::distance will painstakingly advance forward iterators
to compute distances if need be, and ranges::size falls back onto taking
iterator differences, or is simply ill-formed.

What you're proposing is very novel and exotic because it silently does
nothing even if you do something incredibly nonsensical like calling
ranges::reserve with a std::array. Perhaps it should be called
ranges::try_reserve then.

Anyway, I can see the utility in this idea; I'm just not sure how it would
fit into the surrounding design.


On Wed, 15 Jul 2026 at 20:50, Yexuan Xiao via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Where contiguous storage is unnecessary, deque can be used as an
> alternative to vector in C++. This usage is employed by a large amount of
> code. However, vector supports reserve while deque does not, which forces
> users and library authors to determine this property on their own using
> SFINAE or requires expressions. I performed an exact search on GitHub using
> regular expressions, and this pattern appeared 483 times(
> https://github.com/search?q=%2Fif%5Cs%2Bconstexpr%5Cs*%5C%28%5B%5E%29%5D*reserve%5B%5E%29%5D*%5C%29%2F&type=code);
> the vast majority of them use requires expressions, and these code snippets
> represent the needs of the most advanced C++ users. Martin Leitner-Ankerl's
> well-known library unordered_dense also does this and supports C++17 (
> https://github.com/martinus/unordered_dense/blob/e5b9441ecf193f3e1e8f954527fc76edee20d7eb/include/ankerl/unordered_dense.h#L2094).
> I believe the C++ standard library needs to provide this functionality to
> prevent users from reinventing it themselves.
>
> Reference implementation:
>
> template<class C>
> requires (!std::ranges::view<C>) && std::ranges::sized_range<C>
> constexpr void to(C c, std::ranges::range_size_t<C> n)
> {
> if constexpr (requires { c.reserve(n); }) {
> c.reserve(n);
> }
> }
>
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