Load an array of a file in C ++

I'm trying to read a file and load it into an array in c ++ like this:

int main(int argc, char **argv){
if (argc != 2){
    cerr << "Sample call <mat.cc> <filename>" << endl;
}

string filename(argv[1]);
MAT G = readGraph(filename);

My readGraph function does this:

using namespace std;
using Mat = vector<vector<int>>;

vector<vector<int>> readGraph(string fileName){
Mat g;
ifstream infile(fileName);

string line;
while(getline(infile, line)){
    istringstream iss(line);
    if (line[0] == 'p'){
        string s1, s2;
        int nodes;
        iss >> s1 >> s2 >> nodes;
        g.resize(nodes);
        for(int i = 0; i < nodes ; i++){
            g[i].resize(nodes);
        }
        cout << "Graph with " << nodes << " nodes" << endl;
    } else if (line[0] == 'e'){
        char e;
        int s, t, w;
        iss >> e >> s >> t;
        g[ s - 1][t - 1] = 1;
    }
}
return g;
}

When I compile my program g ++ -o mat mat.cc rome99.txt I get the following error:

/usr/bin/ld:rome99.txt: file format not recognized; treating as linker script

/usr/bin/ld:rome99.txt:1: syntax error

collect2: error: ld returned 1 exit status

I need help I have tried all the forms and does not recognize the file, the file contains something like this:

p sp 3353 8870

a 2958 2959 535

a 1494 1573 77

a 1590 1603 184

a 2634 2635 179

a 1984 1985 22

a 1453 1454 33

a 1453 1450 48

a 697 698 50

a 1447 1453 23

a 2036 2034 71

a 564 566 191

The line p indicates nodes and arcs ...