Settobox – Assigning dimensions of a box to a length register

Commands to assist the reuse of boxes (set up by \sbox or by the lrbox environment); the \settobox... commands behave similarly to the \settowidth (etc.) commands. For example:

        \newsavebox{\mybox}
        \newlength{\mylength}
        \sbox{\mybox}{Hello World}
        \settoboxwidth{\mylength}{\mybox}