<iostream> Reference

The <iostream> library in C++ is essential for handling input and output operations. Whether you're reading from the keyboard, writing to the screen, or logging errors, understanding how to use this library effectively is crucial for any C++ programmer.

In this tutorial, we'll explore various components of the <iostream> library, including cout, cin, cerr, clog, endl, flush, getline, put, and get. We'll also delve into formatted I/O using the <iomanip> library. By the end of this tutorial, you'll have a solid understanding of how to perform input and output operations in C++.

Core Content

1. Basic Output: cout

cout is used for standard output, typically the screen. It's part of the std namespace and is commonly used with the insertion operator (<<).

1#include <iostream>
2 
3int main() {
4  std::cout << "Hello, World!" << std::endl;
5  return 0;
6}

Hello, World!

2. Basic Input: cin

cin is used for standard input, typically from the keyboard. It's also part of the std namespace and is commonly used with the extraction operator (>>).

1#include <iostream>
2 
3int main() {
4  int number;
5  std::cout << "Enter a number: ";
6  std::cin >> number;
7  std::cout << "You entered: " << number << std::endl;
8  return 0;
9}

Enter a number: 42
You entered: 42

3. Error Output: cerr

cerr is used for error messages. Unlike cout, it does not use buffering, ensuring that error messages are displayed immediately.

1#include <iostream>
2 
3int main() {
4  std::cerr << "This is an error message!" << std::endl;
5  return 0;
6}

This is an error message!

4. Logging Output: clog

clog is similar to cout, but it also does not use buffering, making it suitable for logging purposes.

1#include <iostream>
2 
3int main() {
4  std::clog << "This is a log message!" << std::endl;
5  return 0;
6}

This is a log message!

5. Manipulators: endl and flush

endl inserts a newline character and flushes the output buffer, ensuring that all data is written to the destination.

1#include <iostream>
2 
3int main() {
4  std::cout << "Line 1";
5  std::cout << std::endl;
6  std::cout << "Line 2";
7  return 0;
8}

Line 1
Line 2

flush also flushes the output buffer without inserting a newline character.

1#include <iostream>
2 
3int main() {
4  std::cout << "Part 1";
5  std::cout.flush();
6  std::cout << " Part 2";
7  return 0;
8}

Part 1 Part 2

6. Reading a Line: getline

getline reads an entire line of input, including spaces, until it encounters a newline character.

1#include <iostream>
2#include <string>
3 
4int main() {
5  std::string name;
6  std::cout << "Enter your full name: ";
7  std::getline(std::cin, name);
8  std::cout << "Hello, " << name << "!" << std::endl;
9  return 0;
10}

Enter your full name: John Doe
Hello, John Doe!

7. Writing a Single Character: put

put writes a single character to the output stream.

1#include <iostream>
2 
3int main() {
4  std::cout.put('H');
5  std::cout.put('e');
6  std::cout.put('l');
7  std::cout.put('l');
8  std::cout.put('o');
9  std::cout.put('!');
10  return 0;
11}

Hello!

8. Reading a Single Character: get

get reads a single character from the input stream.

1#include <iostream>
2 
3int main() {
4  char ch;
5  std::cout << "Enter a character: ";
6  std::cin.get(ch);
7  std::cout << "You entered: " << ch << std::endl;
8  return 0;
9}

Enter a character: A
You entered: A

9. Formatted I/O with <iomanip>

The <iomanip> library provides manipulators for formatting input and output operations.

a. Setting Width

setw sets the field width for subsequent output operations.

1#include <iostream>
2#include <iomanip>
3 
4int main() {
5  int num = 42;
6  std::cout << "Number: " << std::setw(10) << num << std::endl;
7  return 0;
8}

Number:         42

b. Setting Precision

setprecision sets the number of digits to be displayed in floating-point numbers.

1#include <iostream>
2#include <iomanip>
3 
4int main() {
5  double pi = 3.141592653589793;
6  std::cout << "PI: " << std::setprecision(5) << pi << std::endl;
7  return 0;
8}

PI: 3.1416

c. Fixed and Scientific Notation

fixed and scientific control the notation used for floating-point numbers.

1#include <iostream>
2#include <iomanip>
3 
4int main() {
5  double pi = 3.141592653589793;
6  std::cout << "Fixed: " << std::fixed << pi << std::endl;
7  std::cout << "Scientific: " << std::scientific << pi << std::endl;
8  return 0;
9}

Fixed: 3.141593
Scientific: 3.141593e+00

Practical Example

Let's create a complete program that demonstrates various input and output operations, including formatted I/O.

1#include <iostream>
2#include <iomanip>
3#include <string>
4 
5int main() {
6  std::string name;
7  int age;
8  double height;
9 
10  std::cout << "Enter your full name: ";
11  std::getline(std::cin, name);
12 
13  std::cout << "Enter your age: ";
14  std::cin >> age;
15 
16  std::cout << "Enter your height (in meters): ";
17  std::cin >> height;
18 
19  std::cout << "
20--- User Information ---
21";
22  std::cout << "Name: " << name << std::endl;
23  std::cout << "Age: " << age << std::endl;
24  std::cout << "Height: " << std::fixed << std::setprecision(2) << height << " meters" << std::endl;
25 
26  return 0;
27}

Expected Output

Enter your full name: John Doe Enter your age: 30 Enter your height (in meters): 1.75

--- User Information --- Name: John Doe Age: 30 Height: 1.75 meters

Summary

What's Next?

Now that you have a comprehensive understanding of input and output operations using the <iostream> library, it's time to explore file I/O with the <fstream> library. The next tutorial will cover how to read from and write to files in C++. Stay tuned!