Question

Disclaimer: this question is directly related to my programming homework.

My C++ assignment consists of opening a .txt file, performing a bunch of operations on it, and then saving the .txt file. Problem is, I’m having a hard time just grasping the basic concepts of reading and writing files.

My code:

#include <iostream>
#include <fstream>
using namespace std;

int main () {
    ifstream inData;
    ofstream outData;

    // is it necessary to open datalist.txt for both the in and out streams?
    inData.open ("datalist.txt");
    outData.open("datalist.txt");

    if (inData.is_open()) { 
        cout << "yay, i opened it\n"; // this outputs as expected

        char fileData[100]; // have to use char arrays as per instructor. no strings
        inData >> fileData; // store text from datalist.txt in fileData char array
        cout << fileData; // nothing happens here... why?

        outData << "changing file text cause I can"; // this works just fine.
    }
    else {
        cout << "boo, i couldn't open it";
    }

    inData.close();
    outData.close();

    return 0;
}

The main issue I’m encountering is that I don’t understand how to read the data in a file at even a basic level, let alone parse the file into relevant information (the purpose of the program is to read, write, and manipulate information in a semi-colon delimited list).

In addition to this question, I also am a little confused on two other things. First, is it necessary to open datalist.txt for both the in and out streams, for some reason it just feels weird that I have to open the same file twice. Second, my instructor doesn’t want us to use the string class, and instead use char arrays. I don’t understand the logic behind this and was hoping someone could elaborate why (or perhaps give a counter argument to why) strings are bad.

Answer 1

You don’t open a file for reading and writing at the same time. Well, not with two different objects that don’t know about each other at any rate. You either use a std::fstream (which can do simultaneous reading and writing), or you read first, close the file, process the data, then write it.

Also:

//have to use char arrays as per instructor. no strings

I think you may want to get a better instructor. The use of a naked, stack-based char* array is not something that any C++ teacher worth their salt should endorse.

This is where buffer overruns come from.