Unsigned int bytes reverse

unsigned_bytes_reverse.cc

// all the cases in the end gonna be reduced by compiler to the latest variant 
// so even, knowing builtin function could save you time, but if you case 
// recognizable by the compiler, don't worry to use your way of doing things.

#include <iostream>
#include <stdint.h>
using namespace std;

// 4 bytes:  3 2 1 0 -> 0 1 2 3
unsigned reverse_bytes(unsigned num) {
    return ((num & 0xFF000000) >> 24) 
            | ((num & 0xFF0000) >> 8) 
            | ((num & 0xFF00) << 8 ) 
            | ((num & 0xFF) << 24);
}

unsigned reverse_bytes1(unsigned num) {
    uint8_t *n1, *n2;
    unsigned reversed;
    n1 = (uint8_t *) &num;
    n2 = (uint8_t *) &reversed;

    n2[0] = n1[3];
    n2[1] = n1[2];
    n2[2] = n1[1]; 
    n2[3] = n1[0];
    return reversed;
}

unsigned reverse_bytes2(unsigned num) {
    unsigned reversed;
    union {
        unsigned n;
        uint8_t nb[4];
    } repr;
    repr.n = num;
    reversed = (repr.nb[0] << 24) | (repr.nb[1] << 16) | (repr.nb[2] << 8) | repr.nb[3];
    return reversed;
}

unsigned reverse_bytes3(unsigned num) {
    return __builtin_bswap32(num);
}

int main()
{
    unsigned num = 12341235;
    /* The compiler inserts an implicit float upcast. */
    cout << "Before " << hex <<  num << endl;
    cout << "After " << hex << reverse_bytes(num) << endl;
    cout << "Before " << hex <<  num << endl;
    cout << "After " << hex << reverse_bytes1(num) << endl;
    cout << "Before " << hex <<  num << endl;
    cout << "After " << hex << reverse_bytes2(num) << endl;
    cout << "Before " << hex <<  num << endl;
    cout << "After " << hex << reverse_bytes3(num) << endl;
    return 0;
}