C++ iteration, file i/o - c++

Question

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <stdlib.h>
using namespace std;
int main()
{
//Input .txt file
ifstream inputFile ("input.txt");
try
{
int i = 1; //Line iterator
int vertices = 0;
int faces = 0;
string line;
while (getline(inputFile, line))
{
//Take number from line 4, set as variable "vertices"
if (i == 3)
{
getline (inputFile,line);
size_t last_index = line.find_last_not_of("0123456789");
string str = line.substr(last_index);
vertices = atoi(str.c_str()); //Convert to int
cout << "vertices " + str << endl;
}
//Take number from line 11, set as variable "triangles"
if (i == 11)
{
getline (inputFile,line);
size_t last_index = line.find_last_not_of("0123456789");
string str = line.substr(last_index);
faces = atoi(str.c_str()); //Convert to int
cout << "faces " + str << endl;
}
if (i == 13)
{
i++;
break;
}
cout << "line: " + i << endl; //Prints line number
i++;
}
} catch(const char* error) {
cout << "Cannot read file, please try again." << error;
}
return 0;
}
This program is simply trying to read file, take a number from a couple of lines and for each line print "line: " with respective line number. It looks like C++ iterates differently to Java?
For some reason this program outputs:
*ine:
ne:
vertices 752
e:
:
Cannot read mesh, please try again.
faces r
annot read mesh, please try again.
nnot read mesh, please try again.*
I have no idea why.

Answer 1

This line:
cout << "line: " + i << endl;
should be:
cout << "line: " << i << endl;
Your + is adding i to the string constant "line: ", which has the effect of knocking one character off the front each time round the loop (and eventually going off the end, leading to undefined behaviour).
You can't add objects to strings in C++ in the way you're attempting, but you can send multiple objects to cout by repeated use of <<.
You then have the same problem here:
cout << "vertices " + str << endl;
and here:
cout << "faces " + str << endl;