Big values in C++ - c++

Question

I'm doing a fibonacci and I want to save all the numbers in a .txt, i have done the code but when the cicles gets big values the program express it as an exponential, and then gets bigger as .INF.
How I can save the entry number?
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
ofstream File;
File.open("fibo.txt");
double val0 = 0, val1 = 1, i = 0, n, out;
cout << "Enter n of Fibonacci(n): ";
cin >> n;
while (i < n) {
if (n == 0) {
File << i << "- " << 0 << endl;
i++;
}
else {
out = val0 + val1;
val0 = val1;
val1 = out;
i++;
File << i << "- " << val0 << endl;
}
}
File.close();
return 0;
}

Answer 1

You can use boost::multiprecision, it's very intuitive and easy to use. Your code can be modified by changing (only) a couple of lines:
#include <boost/multiprecision/cpp_int.hpp> // need this
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
using namespace boost::multiprecision;
int main()
{
ofstream File;
File.open("fibo.txt");
cpp_int val0 = 0, val1 = 1, out; // arbitrary precision integers
int i = 0, n;
cout << "Enter n of Fibonacci(n): ";
cin >> n;
while (i < n) {
if (n == 0) {
File << i << "- " << 0 << endl;
i++;
}
else {
out = val0 + val1;
val0 = val1;
val1 = out;
i++;
File << i << "- " << val0 << endl;
}
}
File.close();
return 0;
}