[c++] How do you open a file in C++?

I want to open a file for reading, the C++ way. I need to be able to do it for:

  • text files, which would involve some sort of read line function.

  • binary files, which would provide a way to read raw data into a char* buffer.

This question is related to c++ file io

The answer is

You need to use an ifstream if you just want to read (use an ofstream to write, or an fstream for both).

To open a file in text mode, do the following:

ifstream in("filename.ext", ios_base::in); // the in flag is optional

To open a file in binary mode, you just need to add the "binary" flag.

ifstream in2("filename2.ext", ios_base::in | ios_base::binary ); 

Use the ifstream.read() function to read a block of characters (in binary or text mode). Use the getline() function (it's global) to read an entire line.

fstream are great but I will go a little deeper and tell you about RAII.

The problem with a classic example is that you are forced to close the file by yourself, meaning that you will have to bend your architecture to this need. RAII makes use of the automatic destructor call in C++ to close the file for you.

Update: seems that std::fstream already implements RAII so the code below is useless. I'll keep it here for posterity and as an example of RAII.

class FileOpener
    FileOpener(std::fstream& file, const char* fileName): m_file(file)

    std::fstream& m_file;

You can now use this class in your code like this:

int nsize = 10;
char *somedata;
ifstream myfile;
FileOpener opener(myfile, "<path to file>");
// myfile is closed automatically when opener destructor is called

Learning how RAII works can save you some headaches and some major memory management bugs.

#include <fstream>

ifstream infile;
infile.open(**file path**);

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

void main()
    ifstream in_stream; // fstream command to initiate "in_stream" as a command.
    char filename[31]; // variable for "filename".
    cout << "Enter file name to open :: "; // asks user for input for "filename".
    cin.getline(filename, 30); // this gets the line from input for "filename".
    in_stream.open(filename); // this in_stream (fstream) the "filename" to open.
    if (in_stream.fail())
        cout << "Could not open file to read.""\n"; // if the open file fails.
    //.....the rest of the text goes beneath......

Follow the steps,

  1. Include Header files or name space to access File class.
  2. Make File class object Depending on your IDE platform ( i.e, CFile,QFile,fstream).
  3. Now you can easily find that class methods to open/read/close/getline or else of any file.
CFile/QFile/ifstream m_file;
m_file.Open(path,Other parameter/mood to open file);

For reading file you have to make buffer or string to save data and you can pass that variable in read() method.

To open and read a text file line per line, you could use the following:

// define your file name
string file_name = "data.txt";

// attach an input stream to the wanted file
ifstream input_stream(file_name);

// check stream status
if (!input_stream) cerr << "Can't open input file!";

// file contents  
vector<string> text;

// one line
string line;

// extract all the text from the input file
while (getline(input_stream, line)) {

    // store each line in the vector

To open and read a binary file you need to explicitly declare the reading format in your input stream to be binary, and read memory that has no explicit interpretation using stream member function read():

// define your file name
string file_name = "binary_data.bin";

// attach an input stream to the wanted file
ifstream input_stream(file_name, ios::binary);

// check stream status
if (!input_stream) cerr << "Can't open input file!";

// use function that explicitly specifies the amount of block memory read 
int memory_size = 10;

// allocate 10 bytes of memory on heap
char* dynamic_buffer = new char[memory_size];

// read 10 bytes and store in dynamic_buffer
file_name.read(dynamic_buffer, memory_size);

When doing this you'll need to #include the header : <iostream>

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

int main () {
  ofstream file;
  file.open ("codebind.txt");
  file << "Please writr this text to a file.\n this text is written using C++\n";
  return 0;

**#include<fstream> //to use file
#include<string>  //to use getline
using namespace std;
int main(){
ifstream file;
string str;
file.open("path the file" , ios::binary | ios::in);
   getline(file , str);

Examples related to c++

Method Call Chaining; returning a pointer vs a reference? How can I tell if an algorithm is efficient? Difference between opening a file in binary vs text How can compare-and-swap be used for a wait-free mutual exclusion for any shared data structure? Install Qt on Ubuntu #include errors detected in vscode Cannot open include file: 'stdio.h' - Visual Studio Community 2017 - C++ Error How to fix the error "Windows SDK version 8.1" was not found? Visual Studio 2017 errors on standard headers How do I check if a Key is pressed on C++

Examples related to file

Gradle - Move a folder from ABC to XYZ Difference between opening a file in binary vs text Angular: How to download a file from HttpClient? Python error message io.UnsupportedOperation: not readable java.io.FileNotFoundException: class path resource cannot be opened because it does not exist Writing JSON object to a JSON file with fs.writeFileSync How to read/write files in .Net Core? How to write to a CSV line by line? Writing a dictionary to a text file? What are the pros and cons of parquet format compared to other formats?

Examples related to io

Reading file using relative path in python project How to write to a CSV line by line? Getting "java.nio.file.AccessDeniedException" when trying to write to a folder Exception: Unexpected end of ZLIB input stream How to get File Created Date and Modified Date Printing Mongo query output to a file while in the mongo shell Load data from txt with pandas Writing File to Temp Folder How to get resources directory path programmatically ValueError : I/O operation on closed file