Different behaviors in strringstream given std::hex, uint8_t vs int

Question

I suppose the fact that uint8_t is really just an alias for char means that when the characters in the stringstream get read back in, they are stored natively, i.e. the ascii code for the character, whereas if the char is input to an int, with std::hex specified, it gets read in as a hex digit converted to base 10.

This is a small example exhibiting the behavior, but the reason I'm asking is because I started out under the assumption that specifying an output vector of unit8_t was the right approach, as I need to later feed this vector as an array to a C function as a byte vector (you pass it in as a void pointer and it figures out what do to with it somehow). So I figured reading in pairs of input chars into a string stream and then out into a uint8_t would help me skip manually bitshifting. Is there a way of keeping the idea of reading into a uint8_t but evoking the handling of an int? Can I cast to int at the point of the read from the stringstream? Or what?

#include <iostream>
#include <string>
#include <sstream>
#include <iostream>
#include <iomanip>

#include <vector>
#include <cstdint>

int main (int argc, char** argv) {
  uint8_t n = 0;
  std::stringstream ss;
  ss << 'F';
  ss >> std::hex >> n;
  std::cout << "uint8:" << (int)n << std::endl;

  int m = 0;
  std::stringstream tt;
  tt << 'F';
  tt >> std::hex >> m;
  std::cout << "int:" <<  std::hex << m << std::endl;
  
  return 0;
}

output

uint8:70
int:f

Since `uint8_t` is usually just a synonym for `unsigned char`, the `>>` operator reads it as a character. You'll find that 70 is the ASCII value of `'F'`. — Mark Ransom, Mar 19 '21 at 20:12

score 2 · Accepted Answer · answered Mar 19 '21 at 20:30

There are operator>> overloads for char, signed char and unsigned char, which all read a single character from the stream. A uint8_t argument selects the unsigned char overload as the best match, reading F as a character (ASCII code 70 or 0x46).

To match an integer extraction operator>> overload, use an argument of type short or bigger (for example, uint16_t):

  uint16_t n = 0;
  std::stringstream ss;
  ss << 'F';
  ss >> std::hex >> n;

Can I cast to int at the point of the read from the stringstream?

No. The argument is taken by reference, which in function calls works as a pointer. Type punning a uint8_t argument as (int&) will invoke undefined behavior. Just use a bigger type instead.

The salient point being not that a bigger type is necessary, just a different type from uint8_t? — Joshua, Mar 19 '21 at 20:33
You could say a different type, yes, but not int8_t or bool (that works differently). — rustyx, Mar 19 '21 at 21:49

Different behaviors in strringstream given std::hex, uint8_t vs int

1 Answers1