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

There are a few reasons. One is the added complexity for handling arbitrary-sized numbers and the performance hit this gives because the compiler can no longer optimize based on the assumption that every int is exactly X bytes long.

A second one is that storing simple types this way means they need an additional byte to hold the length. So, a value of 255 or less actually needs two bytes in this new system, not one, and in the worst case you now need 5 bytes instead of 4. This means that the performance win in terms of memory used is less than you might think and in some edge cases might actually be a net loss.

A third reason is that computer memory is generally addressable in words, not bytes. (But see footnote). Words are a multiple of bytes, usually 4 on 32-bit systems and 8 on 64 bit systems. You usually can't read an individual byte, you read a word and extract the nth byte from that word. This means both that extracting individual bytes from a word takes a bit more effort than just reading the entire word and that it is very efficient if the entire memory is evenly divided in word-sized (ie, 4-byte sized) chunks. Because, if you have arbitrary sized integers floating around, you might end up with one part of the integer being in one word, and another in the next word, necessitating two reads to get the full integer.

Footnote: To be more precise, while you addressed in bytes, most systems ignored the 'uneven' bytes. Ie, address 0, 1, 2 and 3 all read the same word, 4, 5, 6 and 7 read the next word, and so on.

On an unreleated note, this is also why 32-bit systems had a max of 4 GB memory. The registers used to address locations in memory are usually large enough to hold a word, ie 4 bytes, which has a max value of (2^32)-1 = 4294967295. 4294967296 bytes is 4 GB.