如何用c给txt文本每行前添加行号？

#include <fstream>
#include <string>
using namespace std;
int main(int argc, char *argv[]){
if (argc != 2) exit(1);
string infilename(argv[1]);
string outfilename = infilename+".out"
ifstream infile(argv[1]);
if (!infile) {cerr << "cannot open file:" << infilename << endl; exit(1);}
ofstream outfile(outfilename.c_str());
if (!outfile) {cerr << "cannot create file:" << outfilename << endl; exit(1);}
string line;
int line_num = 1;
while(getline(infile, line))
outfile << setw(8) << line_num++ << line << endl;
}