Why are types always a certain size no matter its value?

Question

Implementations might differ between the actual sizes of types, but on most, types like unsigned int and float are always 4 bytes. But why does a type always occupy a certai

Answer 1

The short answer is: Because the C++ standard says so.

The long answer is: What you can do on a computer is ultimately limited by hardware. It is, of course, possible to encode an integer into a variable number of bytes for storage, but then reading it would either require special CPU instructions to be performant, or you could implement it in software, but then it would be awfully slow. Fixed-size operations are available in the CPU for loading values of predefined widths, there are none for variable widths.

Another point to consider is how computer memory works. Let's say your integer type could take up anywhere between 1 to 4 bytes of storage. Suppose you store the value 42 into your integer: it takes up 1 byte, and you place it at memory address X. Then you store your next variable at location X+1 (I'm not considering alignment at this point) and so on. Later you decide to change your value to 6424.

But this doesn't fit into a single byte! So what do you do? Where do you put the rest? You already have something at X+1, so can't place it there. Somewhere else? How will you know later where? Computer memory does not support insert semantics: you can't just place something at a location and push everything after it aside to make room!

Aside: What you're talking about is really the area of data compression. Compression algorithms exist to pack everything tighter, so at least some of them will consider not using more space for your integer than it needs. However, compressed data is not easy to modify (if possible at all) and just ends up being recompressed every time you make any changes to it.