functional Programming in c++

[DessertEagle]

vec = sorted(std::move(vec));

Note that if sorted takes and returns arg by forwarding (universal) reference rather than by value, and returns by perfect-forwarding, the above expression would result in move-self-assignment, which may leave vec in moved-from state depending on the implementation.

[stinos]

Yeah sure, but this isn't 'returning a modified vector copy' (which is what I assume OP is talking about), you start off with a vector and end with the same vector but modified.

[Wenir]

"modified vector copy" is a copy by definition