twos_compl_abs

// Custom abs(), that correctly handles most-negative values on 2's complement systems.
// Requires T to be either unsigned, or convertible to unsigned.

#include <type_traits>

template<typename T, typename R = std::make_unsigned_t<T>>
R twos_compl_abs(T val) {
    if (val >= 0) { return val; }
    else          { return static_cast<R>(val) * -1; }
}