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

Something simple which most answers seem to miss:

because it suits the design goals of C++.

Being able to work out a type's size at compile time allows a huge number of simplifying assumptions to be made by the compiler and the programmer, which bring a lot of benefits, particularly with regards to performance. Of course, fixed-size types have concomitant pitfalls like integer overflow. This is why different languages make different design decisions. (For instance, Python integers are essentially variable-size.)

Probably the main reason C++ leans so strongly to fixed-size types is its goal of C compatibility. However, since C++ is a statically-typed language which tries to generate very efficient code, and avoids adding things not explicitly specified by the programmer, fixed-size types still make a lot of sense.

So why did C opt for fixed-size types in the first place? Simple. It was designed to write '70s-era operating systems, server software, and utilities; things which provided infrastructure (such as memory management) for other software. At such a low level, performance is critical, and so is the compiler doing precisely what you tell it to.