Reading Files in C++ [duplicate]

Question

I'm looking for a way to way to read files in C++ I can write files, but here's where I'm stuck at:

ifstream readfile;
readfile.open("C:/Users/Crazy/Desktop/Useless.txt")

I 've seen people do things such as:

#include <iostream>
#include <fstream>

using namespace std;

int main() {

    ifstream myReadFile;
    myReadFile.open("text.txt");
    char output[100];
    if (myReadFile.is_open()) {
        while (!myReadFile.eof()) {

            myReadFile >> output;
            cout << output;

        }
    }
    myReadFile.close();
    return 0;
}

But in

char output[100];

I want the whole thing read.

Also, I just want to only read it, not to check if it's already open, not to, check for errors. I just want to read the whole thing, and the whole thing only.

For what it's worth, the people writing that code are wrong. — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Mar 20, 2018 at 22:32
I don't understand. If you don't want to check if the file is already open, what do you want to happen if it isn't? If you don't want to check for errors, what do you expect to happen if there is an error? You can't get around these possible eventualities by putting your fingers in your ears and singing "la la la la"... — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Mar 20, 2018 at 22:33
Lightness Races in Orbit, I'll open it the first thing I do, and close it at the end. If I have an error, well, I test every possible thing I can think of, and if I have an error fix it, but if I do have an error, I can work how to work around it. — Katido 622
– Katido 622, Commented Mar 20, 2018 at 22:44
@Katido622: LRO means an error like "file does not exist", not "bug in your program". You have to decide how to handle run time errors. — Martin Bonner supports Monica
– Martin Bonner supports Monica, Commented Mar 21, 2018 at 10:11

Thomas Matthews · Accepted Answer · 2018-03-20 23:04:22Z

1

If you want to read the entire file into a variable you'll need to:
1. Determine size of file in characters.
2. Use std::vector and declare a vector of that size,
or use the new operator and dynamically allocate a char array.
3. Use ifstream::read to read in the entire file.
4. Close the ifstream.
5. Remember to delete the char buffer.

I recommend using an OS API to determine the file length.

Edit 1: Example

#include <iostream>
#include <fstream>
#include <vector>
std::ifstream my_file("my_data");
my_file.seekg(0, std::ios_base::end); // Seek to end of file.
const unsigned int file_length = my_file.tellg();
my_file.seekg(0);
std::vector<char> file_data(file_length);
my_file.read(&file_data[0], file_length);

edited Mar 20, 2018 at 23:04

answered Mar 20, 2018 at 22:29

Thomas Matthews

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13 Comments

Thomas Matthews Over a year ago

Always ask yourself, "Do I really need to read in the entire file or can I read a line at a time or a chunk at a time?", before reading an entire file into memory.

Katido 622 Over a year ago

Okay, I think I'll use ifstream::read, but how do I use it?

Thomas Matthews Over a year ago

Why the downvote? This is a standard process for reading in files.

Katido 622 Over a year ago

If you're talking to me, I didn't downvote, I upvoted.

Thomas Matthews Over a year ago

@Katido622: Search references on how to use ifstream::read

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