c++ string to boost::multiprecision::cpp_int

How would I convert a string to "boost::multiprecision::cpp_int"?

Additionally, I have a .txt file with 100 numbers each of 50 digits and I use ifstream to read them line by line into a string array. How can I convert each string from the array into a cpp_int, then add all the 100 numbers and get the sum?

To convert a single string, use the cpp_intconstructor: cpp_int tmp("123");.

For the text file case, read each number in a loop as a std::string via std::getline, then emplace back in a std::vector<cpp_int>. Then use the latter to compute your sum. Example:

#include <fstream>
#include <iostream>
#include <string>
#include <vector>

#include <boost/multiprecision/cpp_int.hpp>

using namespace boost::multiprecision;

int main()
{    
    std::vector<cpp_int> v;
    std::fstream fin("in.txt");

    std::string num;
    while(std::getline(fin, num))
    {
        v.emplace_back(num);
    }
    cpp_int sum = 0;
    for(auto&& elem: v)
    {
        std::cout << elem << std::endl; // just to make sure we read correctly
        sum += elem;
    }
    std::cout << "Sum: " << sum << std::endl;
}

PS: you may do it without a std::vector, via a temporary cpp_int that you construct inside the loop and assign it to sum:

std::string num;
cpp_int sum = 0;
while(std::getline(fin, num))
{
    cpp_int tmp(num);
    sum += tmp;
}
std::cout << "Sum: " << sum << std::endl;