std::assignable_from(3)

🌸 NAME 🌸

std::assignable_from — bestie, it's the concept that checks whether you can assign one thing to another 💅

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✨ SYNOPSIS ✨

Okay so first things first, this lives in the header <concepts> — that's the include you want 🎀

template< class LHS, class RHS >
concept assignable_from =
    std::is_lvalue_reference_v<LHS> &&
    std::common_reference_with<
        const std::remove_reference_t<LHS>&,
        const std::remove_reference_t<RHS>&> &&
    requires(LHS lhs, RHS&& rhs) {
        { lhs = std::forward<RHS>(rhs) } -> std::same_as<LHS>;
    };

(since C++20) 💖

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💅 DESCRIPTION 💅

So like, assignable_from<LHS, RHS> is the concept that says: an expression of the type and value category that RHS specifies can be assigned to an lvalue expression whose type is LHS. That's it, that's the vibe ✨

Like, lhs = std::forward<RHS>(rhs) has to actually work and hand you back an LHS, no cap 🫶

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💀 NOTES 💀

Not me about to tell you the most important part 😭 Assignment does not have to be a total function. Translation for the girlies: sometimes x = y just isn't valid, and that's fine, it's not a bug.

Specifically — if assigning to some object x can cause some other object y to get modified, then x = y is likely not in the domain of =. 🙈

When does that happen? Typically when the right operand is owned, directly or indirectly, by the left operand. For example: smart pointers to nodes in a node-based data structure, or something like std::vector<std::any>. Iconic, but also a little cursed ngl 🔥

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👀 SEE ALSO 👀

• std::is_assignable — checks if a type has an assignment operator for a specific argument (class template, C++11) 💜
• std::is_trivially_assignable — same job, checking if a type has an assignment operator for a specific argument (class template, C++11) 🦋
• std::is_nothrow_assignable — again with the "checks if a type has an assignment operator for a specific argument" energy (class template, C++11) 🌈

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Ok that's the whole page bestie, go assign some things 🎀 source vibes straight from http://cppreference.com, version 2024.06.10 — mwah, slay, periodt 💖✨👑