C / C++ / MFC

// FD.cpp : Defines the entry point for the console application.
//
 
#include <iostream>
 
using namespace std;
 
long Hex2Int(char * pszHex) {
 
    static unsigned char byteLookup[22][2]  =  
    {   '0', 0, '1', 1, '2', 2, '3', 3, '4', 4, '5', 5, '6', 6, '7', 7, 
        '8', 8, '9', 9, 'A', 10,'B', 11,'C', 12,'D', 13,'E', 14,'F', 15,
        'a', 10,'b', 11,'c', 12,'d', 13,'e', 14,'f', 15
    };
 
    register long total = 0;
 
    int  nPosition = 0;
 
    if(*pszHex) {
 
        register int len = strlen(pszHex) - 1;
 
        while (len >= 0) {
            bool bFound = false;
            int i;
            for (i = 0; i < sizeof(byteLookup)/2; ++i) {
                if(byteLookup[i][0] == *(pszHex+len)) {
                    ++bFound;
                    break;
                }
            }
 
            // didn't find the character in the lookup, so throw.
            if(!bFound) throw "Invalid hex number\n";
 
            if(nPosition == 0) {
                total = byteLookup[i][1];
            }
            else {
                // use left shift to multiply the next value
                total += byteLookup[i][1] << (4 * nPosition);
            }
            
            --len;
            ++nPosition;

            // can only handle up to 32 bits (for this implementation anyway) so throw.
            if (nPosition > 8) throw "Too many hex digits\n";
 
        }
        // here, we could have code to extend the high order bits if the string is < 8 chars
        // and begins with a char/lookup '8' or greater.  The way things work now, you need 
        // to pass an 8 char string starting with '8' or better to get a signed result
        // Any takers???  
 
    }
    else {
        // Empty string was passed in, so throw as well, rather than return 0
        throw "Empty string\n";
    }
 
    return total;
}
 
int main(int argc, char* argv[])
{
    char s[255];
    long result = -1;
 
    while (result) {
        cout << endl << "Enter a hex number please, or 0 to exit: ";
        
        cin >> s;
        
        try {
            result = Hex2Int(s);
            cout << endl << "Hex2Int returned " << dec << result << " [" << hex << result << "]" << endl;
        }
        catch (char * e) {
            cout << e;
        }
    }
 
    return 0;
}