getline causing a segmentaion fault in C++? - c++

Question

I am currently trying to learn C++. I am given a .txt file that contains a different individuals data on each line. I want to read that data into an array of strings. I don't see anything wrong with this function and I have done the same thing before, but for some reason I am receiving a segmentation fault.
#include <string>
#include <iostream>
#include <fstream>
void readFile(std::istream&, std::string*);
int lineCount(std::istream&);
int main(){
std::ifstream inFile("input.txt");
int numLines = lineCount(inFile);
std::string data[numLines];
inFile.close();
inFile.open("input.txt");
readFile(inFile, data);
inFile.close();
return 0;
}
int lineCount(std::istream& inFile){
std::string line;
int numLines = 0;
while(std::getline(inFile, line)){
numLines++;
}
return numLines;
}
void readFile(std::istream& inFile, std::string *data){
int i = 0;
while(std::getline(inFile, data[i])){
std::cout << i << "\n"; //testing values
std::cout << data[i] << "\n"; //testing values
i++;
}
}
Here is the output of the above code.
//Output
//Note, these are fictional people
0
Florence,Forrest,1843 Glenview Drive,,Corpus Christi,TX,78401,10/12/1992,5/14/2012,3.215,127/11/1234,2.5,50
1
Casey,Roberta,3668 Thunder Road,,Palo Alto,CA,94306,2/13/1983,5/14/2014,2.978,95
2
Koch,Sandra,2707 Waterview Lane,Apt 302,Las Vegas,NM,87701,6/6/1972,12/14/2015,2.546,69
Segmentation fault //occurs in while condition
Any help would be appreciated

Answer 1

I feel sick that I didn't see this right away.
int numLines = lineCount(inFile);
returns the correct number of lines in the file. Bug's not here, man.
std::string data[numLines];
Is not kosher C++, but will create an array with an element for every line in the file if supported. Your program is running, so it's supported. Still, prefer to use Library Containers.
Meanwhile in readFile...
while(std::getline(inFile, data[i]))
Will try to read a line into data[i]. Whether the read succeeds or not, there must be a data[i] to read into. There won't be for the last try.
The logic goes
read in line 1. Successful, so
read in line 2. Successful, so
read in line 3. Successful, so
read in line 4. Fail. But this does not keep getline from looking off the end of data for a string and going boom (specifically undefined behaviour that manifested as going boom) because there isn't one.
The right solution
int main(){
std::ifstream inFile("input.txt");
// no longer need. Vector keeps track for us
// int numLines = lineCount(inFile);
std::vector<std::string> data;
// read nothing from file. Don't need to rewind
readFile(inFile, data);
// note: files close themselves when they are destroyed.
//inFile.close();
return 0;
}
void readFile(std::istream& inFile, std::vector<std::string> & data){
int i = 0;
std::string line; // line to read into. Always there, so we don't have to worry.
while(std::getline(inFile, line)){
std::cout << i << "\n"; //testing values
std::cout << line << "\n"; //testing values
data.push_back(line); // stuff line into vector.
i++;
}
}
The No vector Allowed Solution
int main(){
std::ifstream inFile("input.txt");
int numLines = lineCount(inFile);
// legal in every C++, but prefer container may want some extra armour
// here to protect from numlines 0.
std::string * data = new std::string[numlines];
// the following is a faster way to rewind a file than closing and re-opening
inFile.clear(); // clear the EOF flag
inFile.seekg(0, ios::beg); // rewind file.
readFile(inFile, data);
inFile.close();
return 0;
}
void readFile(std::istream& inFile, std::string * data){
int i = 0;
std::string line; // same as above. line is here even if data[i] isn't
while(std::getline(inFile, line)){
std::cout << i << "\n"; //testing values
std::cout << line << "\n"; //testing values
data[i] = line; // stuff line into array. Smart compiler may realize it can move
//if not, c++11 adds a formal std::move to force it.
i++;
}
}