Read matrix with unknown sizes into vectors

read_matrix.cpp

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

int main() {
    std::ifstream file("data.txt");
    if(!file) {
        std::cerr << "Failed to open the file";
        return 1;
    }

    std::vector<std::vector<int>> mat;

    while(!file.eof()) {
        // New row
        mat.push_back(std::vector<int>());

        // Read columns
        int tmp;
        while(file >> tmp) {
            mat.back().push_back(tmp);
            std::cout << mat.back().back();

            if(file.peek() == '\n') break;
        }

        std::cout << std::endl;
    }

    file.close();

    return 0;
}