C++ Breaking DWORD into characters

Question

I have a four byte DWORD that I need to split into four different characters. I thought I knew how to accomplish this but am getting bizarre numbers every time. Here is my code:

    // The color memory
    int32 col = color_mem[i];

    // The four destination characters
    char r, g, b, a;

    // Copy them in advancing by one byte every time
    memcpy(&r, &col, 1);
    memcpy(&g, &col + 1, 1);
    memcpy(&b, &col + 2, 1);
    memcpy(&a, &col + 3, 1);

Answer 1

Ditch the memcpy and use bit manipulation:

r = (col & 0x000000ff);
g = (col & 0x0000ff00) >>  8;
b = (col & 0x00ff0000) >> 16;
a = (col & 0xff000000) >> 24;

ff in the hexadecimal numbers represents a byte of all 1 bits. This and the & - bitwise AND - will make the bytes that you're not interested in - at each position - 0, and keeping the bits that you are interested in.

The >> shifts in zeros from the left, putting the byte that we want in the most significant position, for the actual assignment. A shift of 8 shifts by a width of one byte, 16 is two bytes, and 24 is three bytes.

Visually, looking at ff, you can imagine that we're walking the byte indices towards the left.

Answer 2

When you do pointer arithmetic, the amount of the increment or decrement is multiplied by the size of the type being pointed to. Cast the int32 pointer to a char pointer in order to access char sized parts at some offset from the base address of col.

This technique can be fragile due to differences in endian-ness on different platforms, so I recommend using the more portable bitmask operations supplied in another answer.

// The color memory  
int32 col = color_mem[i];  

// The four destination characters  
char r, g, b, a;  

// Copy them in advancing by one byte every time  
memcpy(&r, (char*)&col, 1);  
memcpy(&g, ((char*)&col) + 1, 1);  
memcpy(&b, ((char*)&col) + 2, 1);  
memcpy(&a, ((char*)&col) + 3, 1);