Best practices for circular shift (rotate) operations in C++

Question

Left and right shift operators (<< and >>) are already available in C++. However, I couldn\'t find out how I could perform circular shift or rotate operations.

Answer 1

Below is a slightly improved version of Dídac Pérez's answer, with both directions implemented, along with a demo of these functions' usages using unsigned char and unsigned long long values. Several notes:

1. The functions are inlined for compiler optimizations
2. I used a cout << +value trick for tersely outputting an unsigned char numerically that I found here: https://stackoverflow.com/a/28414758/1599699
3. I recommend using the explicit syntax for clarity and safety.
4. I used unsigned char for the shiftNum parameter because of what I found in the Additional Details section here:

The result of a shift operation is undefined if additive-expression is negative or if additive-expression is greater than or equal to the number of bits in the (promoted) shift-expression.

Here's the code I'm using:

#include 

using namespace std;

template 
inline T rotateAndCarryLeft(T rotateMe, unsigned char shiftNum)
{
    static const unsigned char TBitCount = sizeof(T) * 8U;

    return (rotateMe << shiftNum) | (rotateMe >> (TBitCount - shiftNum));
}

template 
inline T rotateAndCarryRight(T rotateMe, unsigned char shiftNum)
{
    static const unsigned char TBitCount = sizeof(T) * 8U;

    return (rotateMe >> shiftNum) | (rotateMe << (TBitCount - shiftNum));
}

void main()
{
    //00010100 == (unsigned char)20U
    //00000101 == (unsigned char)5U == rotateAndCarryLeft(20U, 6U)
    //01010000 == (unsigned char)80U == rotateAndCarryRight(20U, 6U)

    cout << "unsigned char " << 20U << " rotated left by 6 bits == " << +rotateAndCarryLeft(20U, 6U) << "\n";
    cout << "unsigned char " << 20U << " rotated right by 6 bits == " << +rotateAndCarryRight(20U, 6U) << "\n";

    cout << "\n";


    for (unsigned char shiftNum = 0U; shiftNum <= sizeof(unsigned char) * 8U; ++shiftNum)
    {
        cout << "unsigned char " << 21U << " rotated left by " << +shiftNum << " bit(s) == " << +rotateAndCarryLeft(21U, shiftNum) << "\n";
    }

    cout << "\n";

    for (unsigned char shiftNum = 0U; shiftNum <= sizeof(unsigned char) * 8U; ++shiftNum)
    {
        cout << "unsigned char " << 21U << " rotated right by " << +shiftNum << " bit(s) == " << +rotateAndCarryRight(21U, shiftNum) << "\n";
    }


    cout << "\n";

    for (unsigned char shiftNum = 0U; shiftNum <= sizeof(unsigned long long) * 8U; ++shiftNum)
    {
        cout << "unsigned long long " << 3457347ULL << " rotated left by " << +shiftNum << " bit(s) == " << rotateAndCarryLeft(3457347ULL, shiftNum) << "\n";
    }

    cout << "\n";

    for (unsigned char shiftNum = 0U; shiftNum <= sizeof(unsigned long long) * 8U; ++shiftNum)
    {
        cout << "unsigned long long " << 3457347ULL << " rotated right by " << +shiftNum << " bit(s) == " << rotateAndCarryRight(3457347ULL, shiftNum) << "\n";
    }

    cout << "\n\n";
    system("pause");
}