Why is reading uint8_t as hex not working as expected?

Question

Consider code like this:

#include <iostream>
#include <sstream>

int main()
{
    std::stringstream ss;
    ss << "a0 b1";

    uint8_t byte;
    ss >> std::hex >> byte;    

    std::cout << std::hex << byte << std::endl;

    return 0;
}

Why does this output a instead of a0 even though a0 fits in uint8_t as hex?

Answer 1

Because uint8_t is also (probably) unsigned char, for which special rules exist when you perform formatted extraction from a C++ stream.

Unfortunately this is just an alias, not a distinct type.

Basically it's skipping the "lexically convert to a number" step because it thinks you want to pull out a character. The character 'a'.

I think you'll want to read into an unsigned int then downsize if needed.

If you do downsize to a uint8_t, you're also going then to have to promote it back to a larger int (lol) for much the same reason, to trigger serialisation.

(live demo)

To be honest I'd just avoid the small fixed-width types when dealing with streams (unless you're doing unformatted work with read() and write()). It's too easy to forget about this problem.