std::move_constructible(3)

💅 std::move_constructible — a whole mood (C++ Standard Libary, section 3)

okay so bestie, buckle up 🎀 this one is from the C++ Standard Libary manual, section 3, and it's genuinely one of those concepts that sounds scary but is actually just "can you move this thing into a new home, yes or no" 💖

✨ NAME ✨

std::move_constructible — literally that. std::move_constructible. iconic, no notes 👑

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🌸 SYNOPSIS 🌸

Defined in header <concepts>, and it goes a little something like this:

template< class T >
concept move_constructible = std::constructible_from<T, T> &&
                             std::convertible_to<T, T>;

and this whole thing has been a thing since C++20 ⭐

So here's the vibe check 🦋: the concept move_constructible is satisfied if T is a reference type, or if it's an object type where an object of that type can be constructed from an rvalue of that type — and it has to work in both direct-initialization and copy-initialization contexts, with the usual semantics. both. not one. both. 💀

💜 Semantic requirements 💜

If T is an object type, then move_constructible<T> is modeled only if — and this is the important part, don't skip it 🙈 — given:

• rv, an rvalue of type T, and
• u2, a distinct object of type T equal to rv,

the following are true:

• After the definition T u = rv;, u is equal to u2;
• T(rv) is equal to u2; and
• If T is not const-qualified, then rv's resulting state (after the definition/expression is evaluated in either bullets above) is valid but unspecified; otherwise, it is unchanged.

ngl that last bullet is the one that gets people 😭 "valid but unspecified" is not a free pass to do crimes, it's a very specific kind of "we're not telling you what's in there, but it's legal" situation. respect the unspecified state. 🫶

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🎀 See also 🎀

• is_move_constructible (C++11) — class template
• is_trivially_move_constructible — checks if a type can be constructed from an rvalue (C++11) — class template
• is_nothrow_move_constructible (C++11) — class template
• and a reference for you too, bestie (C++11) ✨

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that's the whole page, we ate and left no crumbs 💅🔥

http://cppreference.com · 2024.06.10 · std::move_constructible(3) — mwah, go move-construct something responsibly 🧚‍♀️💜