Code Review C++

#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <sstream>
#include "sub.h"

int main()
{
    std::ifstream file;
    file.open(...);

    if (!file) {
        std::cerr << "Error opening file." << std::endl;
        return EXIT_FAILURE;
    }

    int n;
    std::string firstLine;
    getline(file, firstLine);
    std::stringstream(firstLine) >> n;

    int a,b,c;
    std::vector<int> A1,B1,C1;
    while (file >> a >> b >> c) {
        A1.push_back(a);
        B1.push_back(b);
        C1.push_back(c);
    }

    file.close();

    std::ofstream output;
    output.open(...);

    output << n << "\n";

    size_t size = A1.size();
    for (size_t i=0; i<size; ++i) {
        output << A1[i] << " " << B1[i] << " " << C1[i] << std::endl;
    }

    //call sub method with pointer to beginning of vectors
    //I have no control over sub method, its signature is (int, int*, int*, int*)
    int n1 = sub(n, &A1[0], &B1[0], &C1[0]);

    //output result and close file
    output << n1;
    output.close();
}