如何使用ifstream从文件中读取行？

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! :)

PS: 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:

1. You are returning a local variable. The statement return buffer; results in a dangling pointer.

2. 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