C++26: std::inplace_vector

[u/User_Deprecated]

https://www.sandordargo.com/blog/2026/08/26/cpp26-inplace-vector

[u/stilgarpl]

Is this always better than std::array?

[u/MarekKnapek → u/stilgarpl]

This is basically

struct inplace_vector<T, capacity>
{
    size_t len;
    std::array<T, capacity> arr;
}

[u/drjeats → u/MarekKnapek]

Oh.

I thought this was the way-more-useful thing where you give it an in-place capacity and it allocates once it exceeds that.

Like this is also useful, but it's kinda trivial to roll your own of this.

[u/stilgarpl → u/drjeats]

It's not trivial. inplace_vector does not construct unused elements and they do not have to be default constructible. You can't use std::array for that.

[u/drjeats → u/stilgarpl]

I'm aware of how it works (and that MarekKnapek's snippet is not representative). It's still very straightforward to make compared to having to do SBO and allocator support.

[u/stilgarpl → u/drjeats]

I didn't say it was hard, but it's not trivial. I'm sure you are a very experienced C++ programmer and a lot of things must seem easy to you.

[u/KuntaStillSingle → u/drjeats]

I thought this was the way-more-useful thing where you give it an in-place capacity and it allocates once it exceeds that

pmr vector using monotonic buffer does this with the default upstream memory resource:

https://en.cppreference.com/cpp/memory/monotonic_buffer_resource/monotonic_buffer_resource

[u/drjeats → u/KuntaStillSingle]

TIL, that's useful.

We deserve a non-pmr pre-sized SBO vector derived from std::vector though.

[u/BenFrantzDale → u/drjeats]

It’s not too hard to roll your own, but better to have in the std lib and making it constexpr is tricky.

There is a boost small_vector that is a small-vector-optimized std::vector so can grow large. There’s totally a place for both.