How to read lines from a file using the ifstream? - c++

Question

I have a text file with the following information in it:
2B,410,AER,2965,KZN,2990,,0,CR2
2B,410,ASF,2966,KZN,2990,,0,CR2
2B,410,ASF,2966,MRV,2962,,0,CR2
2B,410,CEK,2968,KZN,2990,,0,CR2
2B,410,CEK,2968,OVB,4078,,0,CR2
2B,410,DME,4029,KZN,2990,,0,CR2
2B,410,DME,4029,NBC,6969,,0,CR2
2B,410,DME,4029,TGK,\N,,0,CR2
(it is airline route info)
I'm trying to loop through the file and extract each line into a char* - simple right?
Well, yes, it's simple but not when you've completely forgotten how to write successful i/o operations! :)
My code goes a little like:
char * FSXController::readLine(int offset, FileLookupFlag flag)
{
// Storage Buffer
char buffer[50];
std::streampos sPos(offset);
try
{
// Init stream
if (!m_ifs.is_open())
m_ifs.open(".\\Assets\\routes.txt", std::fstream::in);
}
catch (int errorCode)
{
showException(errorCode);
return nullptr;
}
// Set stream to read input line
m_ifs.getline(buffer, 50);
// Close stream if no multiple selection required
if (flag == FileLookupFlag::single)
m_ifs.close();
return buffer;
}
Where m_ifs is my ifStream object.
The problem is that when I breakpoint my code after the getline() operation, I notice that 'buffer' has not changed?
I know it is something simple, but please could someone shed some light onto this - I'm tearing my forgetful hair out! :)
P.S: I never finished writing the exception handling so it is pretty useless right now!
Thanks

Answer 1

Here is a fix with some important c++ libraries you may want to learn, and what I believe a better solution. Since you just need your final result to be strings:
// A program to read a file to a vector of strings
// - Each line is a string element of a vector container
#include <fstream>
#include <string>
#include <vector>
// ..
std::vector<std::string> ReadTheWholeFile()
{
std::vector<std::string> MyVector;
std::string JustPlaceHolderString;
std::ifstream InFile;
InFile.open("YourText.txt"); // or the full path of a text file
if (InFile.is_open())
while (std::getline(InFile, PlaceHolderStr));
MyVector.push_back(PlaceHolderStr);
InFile.close(); // we usually finish what we start - but not needed
return MyVector;
}
int main()
{
// result
std::vector<std::string> MyResult = ReadTheWholeFile();
return 0;
}

Answer 2

There are two basic problems with your code:
You are returning a local variable. The statement return buffer; results in a dangling pointer.
You are using a char buffer. C-style strings are discouraged in c++, you should always prefer std::string instead.
A far better approach is this:
string FSXController::readLine(int offset, FileLookupFlag flag) {
string line;
//your code here
getline(m_ifs, line) //or while(getline(my_ifs, line)){ //code here } to read multiple lines
//rest of your code
return line;
}
More information about std::string can be found here