Ridiculously Large Number Multiplication (with C++ Strings)

HugeNumberMultiplication.cpp

#include <iostream>
#include <string>
#define OVERFLOW 2
#define ROW b_len
#define COL a_len+b_len+OVERFLOW

using namespace std;

int getCarry(int num) {
    int carry = 0;
    if(num>=10) {
        while(num!=0) {
            carry = num %10;
            num = num/10;
        }
    }
    else carry = 0;
    return carry;
}

int num(char a) {
    return int(a)-48;
}

string mult(string a, string b) {
        string ret;
        int a_len = a.length();
        int b_len = b.length();
        int mat[ROW][COL];
        for(int i =0; i<ROW; ++i) {
            for(int j=0; j<COL; ++j) {
                mat[i][j] = 0;

            }
        }

        int carry=0, n,x=a_len-1,y=b_len-1;
        for(int i=0; i<ROW; ++i) {
            x=a_len-1;
            carry = 0;
            for(int j=(COL-1)-i; j>=0; --j) {
                if((x>=0)&&(y>=0))  {
                    n = (num(a[x])*num(b[y]))+carry;
                    mat[i][j] = n%10;
                    carry = getCarry(n);
                }
                else if((x>=-1)&&(y>=-1)) mat[i][j] = carry;
                x=x-1;
            }
            y=y-1;
        }

        carry = 0;
        int sum_arr[COL];
        for(int i =0; i<COL; ++i) sum_arr[i] = 0;
        for(int i=0; i<ROW; ++i) {
            for(int j=COL-1; j>=0; --j) {
                sum_arr[j] += (mat[i][j]);
            }
        }
        int temp;
        for(int i=COL-1; i>=0; --i) {
            sum_arr[i] += carry;
            temp = sum_arr[i];
            sum_arr[i] = sum_arr[i]%10;
            carry = getCarry(temp);
        }

        for(int i=0; i<COL; ++i) {
            ret.push_back(char(sum_arr[i]+48));
        }

        while(ret[0]=='0'){
            ret = ret.substr(1,ret.length()-1);
        }
        return ret;
}

void printhuge(string a) {
    cout<<"\n";
    for(string::iterator i = a.begin(); i!=a.end(); ++i) {
        cout<<*i;
    }
}

int main() {
    string a,b;
    cin>>a>>b;
    printhuge(mult(a,b));
    return 0;
}

Thank you...saved a few hours!...c++ should just integrate this into standard iostream like python and java....but I'm too lazy to email them and see what their response is...

Your code does not work properly. It has some bug.
I tested your code for the following two numbers:
3141592653589793238462643383279502884197169399375105820974944592
2718281828459045235360287471352662497757247093699959574966967627

Expected answer:
8539734222673567065463550869546574495034888535765114961879601127067743044893204848617875072216249073013374895871952806582723184
(result from https://defuse.ca/big-number-calculator.htm and cross checked on many other sites)

Your answer:
8539734222673567053353449726313700648237079222442681415309610037168088680449950537516095083561704540812373905871952806582723184