What is the most efficient way to represent small values in a struct?

Question

Often I find myself having to represent a structure that consists of very small values. For example, Foo has 4 values, a, b, c, d that, range from

Answer 1

There is no definitive answer, and you haven't given enough information to allow a "right" choice to be made. There are trade-offs.

Your statement that your "primary goal is time efficiency" is insufficient, since you haven't specified whether I/O time (e.g. to read data from file) is more of a concern than computational efficiency (e.g. how long some set of computations take after a user hits a "Go" button).

So it might be appropriate to write the data as a single char (to reduce time to read or write) but unpack it into an array of four int (so subsequent calculations go faster).

Also, there is no guarantee that an int is 32 bits (which you have assumed in your statement that the first packing uses 128 bits). An int can be 16 bits.