C++ std::move doesn't move anything: A deep dive into Value Categories

Post by masklinn » Sun, Jan 11, 2026, 9:43 AM UTC

stingraycharles wrote: post #5

I don’t think this is particularly insightful, as move semantics and r-values are higher level language semantics, nothing more and nothing less. Rust’s borrow checker doesn’t actually borrow anything either, it’s operating on a similar level of abstraction.

Post by masklinn » Sun, Jan 11, 2026, 9:53 AM UTC

shmerl wrote: post #6

vlovich123 wrote: post #4

Earlier quoted context omitted.

Std move doesn’t move ownership. It simply casts into something that could have its ownership taken. Whether or not that actually happens is impossible to identify statically and the value after ownership is consumed is unspecified - sometimes it’s UB to access the value again, sometimes it’s not.

May be disown would be more descriptive, but the point is that it's intended for transferring of ownership versus copying data.

Post by tsimionescu » Sun, Jan 11, 2026, 9:56 AM UTC

shmerl wrote: post #2

I always understood move as moving ownership, so it's not a misnomer. > std::move is like putting a sign on your object “I’m done with this, you can take its stuff.” Which exactly is moving ownership.

Post by tsimionescu » Sun, Jan 11, 2026, 10:03 AM UTC

cocoto wrote: post #3

shmerl wrote: post #2

I always understood move as moving ownership, so it's not a misnomer. > std::move is like putting a sign on your object “I’m done with this, you can take its stuff.” Which exactly is moving ownership.

Personally I see std::move more like removing ownership because it’s not explicit from its call where the ownership is transferred.

  std::string abc = "abc";
  std::move(abc); //doesn't remove ownership or do anything really
  std::print(abc); //guaranteed to print "abc"

Post by masklinn » Sun, Jan 11, 2026, 10:20 AM UTC

tsimionescu wrote: post #16

adrianN wrote: post #14

Earlier quoted context omitted.

My function can choose to move or not to move from an object based on io input.

Can you show an example of what you mean? My claim is that, if I call `foo(std::move(myObj))`, it is statically knowable if `foo` receives a copy of `myObj` or whether it is moved to it. Of course, `foo` can choose to further copy or move the data it receives, but it can't choose later on if it's copied or not. Now, if I give `foo` a pointer to myObj, it could of course choose to copy or move from it later and based…

    void foo(std::unique_ptr&& p) {
        std::random_device rdev {};
        auto dist = std::uniform_int_distribution(0, 1);
        if (dist(rdev)) {
            auto pp = std::move(p);
        }
    }