C++ - Standard Input Stream and Output sStream.

Standard Input Stream/Output Stream in C++

Introduction

C++ provides mechanisms to interact with the user and external systems through standard input and output streams. These streams allow data to be read from input devices (like the keyboard) and written to output devices (like the screen). The standard input stream is typically used to read data, and the standard output stream is used to display data.

1. Standard Input Stream (cin)

The standard input stream in C++ is used to take input from the user or from files. The object cin is an instance of the istream class and is used to read data from the standard input device, usually the keyboard.

Syntax

cin >> variable;

Example

#include 
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";
    cin >> age;  // Reading input from the user
    cout << "Your age is " << age << endl;
    return 0;
}

Common Operations with cin

• Reading integers: cin >> variable;
• Reading strings: To read a single word or string (excluding spaces), use cin >> string;.
• Reading entire lines: To read a full line with spaces, use getline(cin, string);.

Handling Input Failure

If an input operation fails (e.g., if the user enters a letter instead of a number), C++ sets the input stream's fail state. To handle this, you can use the cin.fail() function to check the input status.

if(cin.fail()) {
    cout << "Invalid input!" << endl;
    cin.clear();  // Clear the error flag
    cin.ignore(numeric_limits::max(), '\n');  // Ignore incorrect input
}

2. Standard Output Stream (cout)

The standard output stream in C++ is used to display output on the screen. The object cout is an instance of the ostream class and is used to send data to the output device, usually the console or terminal.

Syntax

cout << expression;

Example

#include 
using namespace std;

int main() {
    int age = 25;
    cout << "Your age is " << age << endl;  // Displaying output
    return 0;
}

Common Operations with cout

• Displaying text: Use cout << "text"; to display a string of text.
• Displaying variables: Use cout << variable; to display the value of a variable.
• Formatting output: You can use manipulators such as endl for new lines, setw to set the width of output, fixed, and setprecision to control the number of decimal places in floating-point numbers.

Example with Formatting

#include 
#include 
using namespace std;

int main() {
    double pi = 3.14159;
    cout << fixed << setprecision(2);  // Set precision to 2 decimal places
    cout << "Value of Pi: " << pi << endl;
    return 0;
}

3. Input and Output with Files

In addition to reading from and writing to the console, C++ also provides the capability to perform input and output with files using the ifstream (input file stream) and ofstream (output file stream) classes.

Example of Writing to a File

#include 
#include 
using namespace std;

int main() {
    ofstream outFile("output.txt");  // Create or open a file
    outFile << "Hello, file!" << endl;  // Write to the file
    outFile.close();  // Close the file
    return 0;
}

Example of Reading from a File

#include 
#include 
using namespace std;

int main() {
    ifstream inFile("output.txt");  // Open the file for reading
    string content;
    getline(inFile, content);  // Read the first line from the file
    cout << "File content: " << content << endl;
    inFile.close();  // Close the file
    return 0;
}

4. Error Handling with Streams

When working with input and output streams, it's essential to handle errors that might occur during I/O operations. C++ provides several mechanisms for checking the state of input/output streams and handling failures.

Stream States

• fail: Set when an I/O operation fails (e.g., reading a non-numeric value into an integer).
• eof: Set when the end of a file is reached during input operations.
• bad: Set when a serious error occurs, such as a hardware failure.
• good: Indicates that no errors have occurred and the stream is in a good state.

Checking Stream State

if (cin.fail()) {
    cout << "Error: Input failure!" << endl;
}

Clear and Ignore Stream Errors

In case of errors, you can clear the error flag using cin.clear() and ignore invalid input using cin.ignore().

Example

if (cin.fail()) {
    cout << "Invalid input!" << endl;
    cin.clear();  // Clear error flag
    cin.ignore(numeric_limits::max(), '\n');  // Ignore incorrect input
}

The standard input and output streams in C++ provide a convenient way to interact with the user and external systems. Understanding how to use cin and cout, along with file I/O operations, stream states, and error handling, is essential for writing robust and efficient programs in C++.