Streams and File I/O ·

Table of Contents

cpp - This article is part of a series.

Part 1: This Article

Part 2: C++ for_each Traversal Algorithm

1 Predefined Standard Stream Objects
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In C++, predefined standard stream objects are several global objects provided by the iostream library, representing standard input/output devices (usually the keyboard and screen). These objects are automatically created when the program starts and can be used directly.

Object	Name	Device	Equivalent to C	Class
`cin`	Standard Input	Keyboard	`stdin`	`istream`
`cout`	Standard Output	Screen	`stdout`	`ostream`
`cerr`	Standard Error	Screen	`stderr` Unbuffered	`ostream`
`clog`	Standard Log	Screen	`stderr` Buffered	`ostream`

2 File I/O Implementation
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File I/O is defined in the header file <fstream>

Opening a File: open() and close() are the standard procedures for file operations;
The second parameter of the member function open() specifies the file opening mode, with options including append mode ios::app, etc.;
Pointer Positioning: seekg() seekp() is crucial for random access within a file;
<< overloads the stream insertion and left shift operators, and >> overloads the stream extraction and right shift operators;
C++’s stream mechanism supports various media such as files, strings, and arrays, offering diverse I/O capabilities.

Class	Name	Description
`ifstream`	Input file stream	Used to read information from a file
`ofstream`	Output file stream	Used to create a file and write information to it
`fstream`	File stream	Can read and write files simultaneously

#include <fstream>  // File stream operations
#include <iostream> // Standard input/output
using namespace std;

int main() {
    char data[100];

    // 1. Write to a file
    ofstream outfile; // Create an output file stream object
    outfile.open("afile.dat");  // Open the file, write mode ios::out
    if (!outfile.is_open()) {   // Check if the stream object is associated with a file
        cerr << "The file 'afile.dat' does not exist" << endl;
        return 1;
    }
    cout << "Writing to the file" << endl;

    cout << "Enter your name: ";
    cin.getline(data, 100);     // Read a line from the keyboard, up to 100 characters
    outfile << data << endl;    // Write the name to the file and add a newline

    cout << "Enter your age: ";
    cin >> data;
    cin.ignore();               // Clear the input buffer, eliminating the influence of the previous input on the next input
    outfile << data << endl;    // Write the age to the file

    outfile.close();            // Close the file, ensuring the data is written to disk

    // 2. Read the file
    ifstream infile("afile.dat"); // Open the file in read mode
    if (!infile.is_open()) {
        cerr << "The file 'afile.dat' does not exist" << endl;
        return 1;
    }
    cout << "Reading from the file" << endl;

    while (infile >> data) { 
        cout << data << endl; 
    } 

    infile.close(); 

    return 0;
}

1 Predefined Standard Stream Objects #

2 File I/O Implementation #

1 Predefined Standard Stream Objects
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2 File I/O Implementation
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