Copying starting at offset to beginning of structure

Question

So I have a structure set up like so

EDIT: PLAYERCOORDS = 1, it is defined at the beginning of my code

 struct PLAYERCOORDINATESSTRUCT 
{
    int type, x, y;
    bool isalive;
    bool isconnected;
    PLAYERCOORDINATESSTRUCT(int customx, int customy, bool customisalive, bool customisconnected)
    {
        type=(PLAYERCOORDS);
        x = customx;
        y = customy;
        isalive = customisalive;
        isconnected = customisconnected;
    }
    PLAYERCOORDINATESSTRUCT()
    {
        type=(PLAYERCOORDS);
    }
};

char* bufferwithstructdata = new char[sizeof(PLAYERCOORDINATESSTRUCT)-4]; 
//-4 since it's missing first int
PLAYERCOORDINATESSTRUCT testdata;
memcpy(&testdata,bufferwithstructdata,sizeof(PLAYERCOORDINATESSTRUCT)-4)

I have a char buffer that is holding all of the information for one of these structs, except for the int type (the first 4 bytes of the struct)

I am trying to figure out how to properly memcpy this to a source with an offset. I found that I need to be 4 bytes off of the structure address to begin at int x. However, I found that when I tried to add 4 to the address by doing (&testdata+4), it instead added 40 to the address! Anyone have a clue as to how I might memcpy with an offset of 4 bytes to the structure, or a way that I could just offset an address by 4 bytes instead of 10 at a time?

Thanks a lot for reading.

Answer 1

What you want is the following:

memcpy(((char*)&testdata)+4, bufferwithstructdata, sizeof(PLAYERCOORDINATESSTRUCT)-4);

The reason is because the + operator applied to pointers adds the integer specified times the size of the thing being pointed to. This is why adding 4 added 40 to the address. Casting to a char* before adding 4 will add 4 byte locations. Also, if you want something a bit more portable, the following would be a nice trick to do what you want:

memcpy(((int*)&testdata)+1, bufferwithstructdata, sizeof(PLAYERCOORDINATESSTRUCT)-4);

This has the benefit of adding one integer size unit to the address, regardless of the size of int.

As Lalaland has pointed out, it would be even better to use offsetof like so:

memcpy(offsetof(PLAYERCOORDINATESSTRUCT, x), bufferwithstructdata, sizeof(PLAYERCOORDINATESSTRUCT)-4);