This is a constant expression denoting the size (in bytes) of the values of type $⟨$ty$\mathrm{⟩}$. The type of this expression is $\mathrm{⟨}$ty$\mathrm{⟩}$ size and its value is an integer indicating the number of bytes.

The size expressions are usually used to call polymorphic C functions. For example, the following is the import expression that imports memcpy in the C standard library for copying the first element of arrays:

val ’a#unboxed memcpy =
    _import "memcpy" : (’a array, ’a vector, ’a size) -> unit ptr

This function is called in the following way:

fun ’a#unboxed copy (a : ’a array, v) =
    if Array.length a > 0 andalso Vector.length v > 0
    then memcpy (a, v, _sizeof(’a))
    else ()