Suppose I need to write a function which might take either a constant reference to an lvalue or a temporary value, is there any advantage in terms of performance in writing both overloads, the one taking a const T& and one taking T&&, if we do not want to move from the rvalue?
I was assuming having two overloads (or just writing the function once with universal references) would be beneficial but I can't pin down the exact reason. I even tried a small example: https://godbolt.org/z/53r34x4Mj but I can't really make sense of the generated code.
If you're never going to move from the argument under any circumstance, nor modify it, there is no benefit to writing the version that takes an r-value reference; the
const type&version will accept r-values just fine (keeping them alive until the function returns) with no additional overhead.The only reason to write both versions is if you plan to potentially modify or move-from the argument only when receiving an r-value reference, where the
constreference case never does so, and doesn't copy either, because if it would need to copy anyway, you may as well just accept by value and get the best of all worlds (construct directly into argument for prvalues, move construct for xvalues, and copy for everything else, but without hand-writing the code to handle that).