Fixing of a -Wconversion GCC warning for a loop variable

Question

I want to fix compiler warnings in existing code and came across the following free function:

std::uint16_t calculate_crc16(std::vector<unsigned char> const& kData) {
  std::int32_t result{0};

  // This nested loop iterates over each bit of the input data.
  for (unsigned char const& kByte : kData) {
    // TODO(wolters): Refactor to eliminate [-Wconversion] compiler warning.
    for (unsigned char j{1}; j; j <<= 1) {
      if (((result & 1) ? 1 : 0) ^ ((kByte & j) ? 1 : 0)) {
        result >>= 1;
        result ^= 0x9299;
      } else {
        result >>= 1;
      }
    }
  }

  return static_cast<std::uint16_t>(result);
}

The line below the TODO comment raises the following GCC v4.7.1 warning:

warning: conversion to 'unsigned char' from 'int' may alter its value [-Wconversion]

How can I refactor that code, to avoid the warning? I failed replacing unsigned char with int in the loop, since that changes the behavior.

Answer 1

You might replace the j <<= 1 with

 j = static_cast<unsigned char> (j << 1)

or perhaps even

 j = static_cast<unsigned char> ((j << 1)&UCHAR_MAX)

However, I find your code not very readable anyway.... Perhaps you might put some comment...