Set bit X of an integer to bit Y of another integer without branching?

Question

Can the copy_bit function below be simplified to something like out[out_bit] = in[in_bit]? (i.e. Not using an if statement)

template< typename T >
inline void copy_bit( T& out, const T in, const std::size_t out_bit, const std::size_t in_bit )
{
    if ( (in & (1 << in_bit)) != 0 )
    {
        out |= (1 << out_bit); // Set bit
    }
    else
    {
        out &= ~(1 << out_bit); // Clear bit
    }
}

// Set bit 4 in x to bit 11 in y
copy_bit( x, y, 4, 11 );

Update: Just to be clear, this isn't homework or an XY problem where suggesting std::bitset answers the question.

Answer 1

you can do it like that like this:

//Change the bit if and only if they are not equal:
out ^= (((out >> out_bit) ^ (in >> in_bit)) & 1) << out_bit;

(Shift both values so that the required bits are in the least significant position with >>, select with & only the lower bit of the result of the ^ operation; then shift the result into position of an otherwise zero-value to ^ with the original destination. The result is the same as copying bit in_bit of in to bit out_bit of out.)

Answer 2

One way to do so in one line would be to first reset the output bit to zero, and then OR it with whatever bit the in number has:

(out &= ~(1 << out_bit)) |= (((in >> in_bit) & 1) << out_bit)