C++ :: Sort Text File By First Number Of The Line

I have a text file like this

0 1 2 C
10 1 2 S
7 1 2 C
11 1 2 S
9 3 43 C
10 3 43 S
1 3 43 C
101 3 43 S

with this code

ifstream in("fout2.txt");
if (in) {
vector<string> lines;
string line;
while (getline(in, line))

[Code] ....

I obtain this result

0 1 2 C
1 3 43 C
10 1 2 S
10 3 43 S
11 1 2 S
101 3 43 S
7 1 2 C
9 3 43 C

but i want a result like this :

0 1 2 C
1 3 43 C
7 1 2 C
9 3 43 C
10 1 2 S
10 3 43 S
11 1 2 S
101 3 43 S