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}