how to insert int value to string object?

[peripheral]

when I use getline(ifs,str).

I need to add a int value (int line;)to every line read from ifs object(ifstream ifs("intput.txt");).

how can I Assignment "line"into "str".and put it in the front of "str";

[hamsterman]

#include <sstream>
#include <string>

int main(){
    std::stringstream s;
    std::string line = "line";
    int l = 5;

    s << l << line;
    std::string result = s.str();
    return 0;
}

[peripheral]

I think you mistake me.

I want to copy a text file.in the new ffile. I want to show line numeber in the front of each line.

for example:
......
23 string......
24 string .........
25 string ...
.....

line is a integer not a string.

main{
........
ifstream ifs("intput.txt");
int line=1;
string str;
while(getline(ifs,str))
{line++;
//how to add line to str
........

[hamsterman]

I don't see how am I wrong. Maybe if I put this into function, it will be more clear.

string append(int i, string str){
    stringstream s;
    s << i << " " << str;
    return s.str();
}

Then in main

while(getline(ifs,str)){//str == "hello world"
    line++;//line == 1
    str = append(line, str);//str == "1 hello world"
}