Exception handling?

[JoeC]

How do I write a try throw command to see if this array is good?

This is what I have so far:

void bitmap::changeBit(int ac, int dn, BYTE col){
BYTE up = 16-dn;
BYTE num = (up*acc)+ac;

if((num < acc*dwn) || (num > 0)){
if(bits == NULL){MessageBox(NULL, "HELP", "Info", MB_OK);}else{
try{bits[num] = col;}
catch(Exception e){MessageBox(NULL, e.message(), "Info", MB_OK);}

}
}
}