How to implement big int in C++

Question

I\'d like to implement a big int class in C++ as a programming exercise—a class that can handle numbers bigger than a long int. I know that there are several open sou

Answer 1

I'm not convinced using a string is the right way to go -- though I've never written code myself, I think that using an array of a base numeric type might be a better solution. The idea is that you'd simply extend what you've already got the same way the CPU extends a single bit into an integer.

For example, if you have a structure

typedef struct {
    int high, low;
} BiggerInt;

You can then manually perform native operations on each of the "digits" (high and low, in this case), being mindful of overflow conditions:

BiggerInt add( const BiggerInt *lhs, const BiggerInt *rhs ) {
    BiggerInt ret;

    /* Ideally, you'd want a better way to check for overflow conditions */
    if ( rhs->high < INT_MAX - lhs->high ) {
        /* With a variable-length (a real) BigInt, you'd allocate some more room here */
    }

    ret.high = lhs->high + rhs->high;

    if ( rhs->low < INT_MAX - lhs->low ) {
        /* No overflow */
        ret.low = lhs->low + rhs->low;
    }
    else {
        /* Overflow */
        ret.high += 1;
        ret.low = lhs->low - ( INT_MAX - rhs->low ); /* Right? */
    }

    return ret;
}

It's a bit of a simplistic example, but it should be fairly obvious how to extend to a structure that had a variable number of whatever base numeric class you're using.