matrix.cc

#include<iostream>
#include<vector>
 
using std::vector;
using std::cout;
using std::endl;
using std::cin;
 
 
int main(void)
{
    int rows, cols;
    cin >> rows >> cols;
    cout << "rows: " << rows << ", cols: " << cols << endl;

    vector<vector<int>> matrix;
    vector<int> row;

    for (int row_idx = 0; row_idx < rows; ++row_idx) {
        for (int col_jdx = 0; col_jdx < cols; ++col_jdx) {
            int tmp;
            cin >> tmp;
            row.push_back(tmp);
        }
        matrix.push_back(row);
        row.clear();
    }

    for (auto row : matrix) {
        for (auto elt : row) {
            cout << elt << " ";
        }
        cout << endl;
    }
     
    return 0;
}

Sample data

2 3
1 2 3
4 5 6

$ ./a.out < input.txt