If you cast references between two integers of different sizes, say using reinterpret_cast, you start sinking into dangerous undefined behavior, right?

uint16_t originalInt = 5000;

uint8_t &smallerThanOriginalRef = reinterpret_cast<uint8_t&>(originalInt);
uint32_t &largerThanOriginalRef = reinterpret_cast<uint32_t&>(originalInt);

Would assigning to 'smallerThanOriginalRef' only set the first byte of 'originalInt'?

So this:

smallerThanOriginalRef = 123;

Wouldn't first clear the higher bytes of 'originalInt', and so 'originalInt' wouldn't be guaranteed to be 123, right?

And assigning to 'largerThanOriginalRef' would accidentally write on bytes outside of 'originalInt', which might write on memory that is used by other variables?

(in practice, those ints might be internally represented as a 32 bit or 64 bit integer anyway... but that's not guaranteed by the standard)

Yup, undefined behavior territory. In practice what would happen depends on what endianness the platform is. Assigning to smallerThanOriginalRef might end up clearing either the higher or lower byte of originalInt.

Also you don't have to use reinterpret_cast, takes longer to type out than simply using v=(uint32_t)variable, which will do the exact same thing.

My original use had to do with passing a uint32_t to a function wanting a uint8_t&, but then I realized the all problems involved with that.

Interestingly, GCC 4.8 does not even warn about that code of yours although it's arguably in violation of the standard which says "A reference shall be initialized to refer to a valid object or function" (8.3.2) with "valid" being the important bit.

Since originalInt is not of a type that the new reference type can accomodate, it isn't a valid object (well, originalInt itself is a valid object, but result of the cast which the reference is initialized with isn't). You would think that this is obvious to the compiler, too. But maybe it's because of the cast operation. Probably the compiler assumes "programmer said cast, so he knows what he's doing".

It would be the same as if pointers were used.

uint16_t originalInt = 5000;
uint8_t &smallerThanOriginalRef = reinterpret_cast<uint8_t&>(originalInt);
uint32_t &largerThanOriginalRef = reinterpret_cast<uint32_t&>(originalInt);
 
smallerThanOriginalRef = 123;
largerThanOriginalRef = 1000000;

uint16_t originalInt = 5000;
uint8_t *smallerThanOriginalPtr = reinterpret_cast<uint8_t*>(&originalInt);
uint32_t *largerThanOriginalPtr = reinterpret_cast<uint32_t*>(&originalInt);
 
*smallerThanOriginalPtr = 123;
*largerThanOriginalPtr = 1000000;