C++ Calculate string mod string

Question

I have a 2 very large numbers that are too large to be use in unsigned long so I put them in strings.

I want to make an operation of mod between these strings and to get a string that represent the result.

#include <string>

void main()
{
    string num1 = "123...";
    string num2 = "123...";

    string result = modStrings(num1, num2);
}

Is anything like that possible? Thanks.