This C++ cheat sheet is a quick reference guide for beginners and intermediates. It helps you learn or revise key C++ concepts without switching between multiple websites.
C++ is a high-level, general-purpose programming language created by Bjarne Stroustrup at Bell Labs in 1983. It is widely used for building software, games, operating systems, and performance-critical applications.
Basic Structure of C++ Program
C++
// Header file for input and output
#include <iostream>
// To avoid using std:: before cout and cin
using namespace std;
// Main function - entry point of every C++ program
int main()
{
// Display output on the screen
cout << "Hello, World!" << endl;
// Take input from the user
int num;
cout << "Enter a number: ";
cin >> num;
// Display the entered number
cout << "You entered: " << num << endl;
// Indicates successful program execution
return 0;
}
OutputHello, World!
Enter a number: You entered: 0
- Comments are used for providing an explanation of the code that makes it easier for others to understand the functionality of the code.
- They are not executed by the compiler.
- Comments can also be used to temporarily disable specific statements of code without deleting them.
- Single-line: starts with //
- Multi-line: starts with /* and ends with */.
C++
#include <iostream>
using namespace std;
int main()
{
// Single-line comment: This prints a greeting message
cout << "Hello, World!" << endl;
/*
Multi-line comment:
The next lines print some example numbers and messages.
This shows how multi-line comments work in C++.
*/
cout << "Number 1: 10" << endl;
cout << "Number 2: 20" << endl;
return 0; // End of the program
}
OutputHello, World!
Number 1: 10
Number 2: 20
Variables
- A variable is a container used to store data values and must be declared before they can be used.
- You can declare multiple variables at the same time.
- A variable name can contain letters, digits, and underscores (_), and it must start with a letter or an underscore.
Identifiers: Unique names given to variables so they can be recognized individually in a program.
Constants: Values that do not change during program execution.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring variables
int age = 20;
double height = 5.7;
char grade = 'A';
string name = "John";
// Printing variable values
cout << "Name: " << name << endl;
cout << "Age: " << age << endl;
cout << "Height: " << height << endl;
cout << "Grade: " << grade << endl;
return 0;
}
OutputName: John
Age: 20
Height: 5.7
Grade: A
Data Types
- Data types define the type of data a variable can store. Every variable in C++ must have a data type.
- C++ is a statically typed language, which means the data type of a variable is fixed at compile time and cannot change later.
Types of Data Types in C++
1. Integer (int)
- Stores whole numbers (e.g., 10, -5, 0). Can be used for counting, indexing, or arithmetic operations.
- Integers take 4 bytes of memory.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring an integer variable
int age = 25;
// Printing the value of the variable
cout << "My age is: " << age << endl;
return 0;
}
2. Floating Point (float)
- Stores decimal numbers with single precision (e.g., 3.14). Useful for measurements, fractions, and approximate calculations.
- It takes 4 bytes of memory.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring a float variable
float pi = 3.14;
// Printing the value of the variable
cout << "Value of pi: " << pi << endl;
// Performing a simple arithmetic operation
float radius = 2.0;
float area = 3.14 * radius * radius;
cout << "Area of circle with radius " << radius << " is: " << area << endl;
return 0;
}
OutputValue of pi: 3.14
Area of circle with radius 2 is: 12.56
3. Double (double)
- Stores decimal numbers with double precision (more accurate than float, e.g., 3.14159). Used when more precision is required in calculations.
- It takes 8 bytes of memory.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring a double variable
double price = 99.99;
// Printing the value of the variable
cout << "Price: " << price << endl;
// Performing a simple calculation
double discount = 10.5;
double finalPrice = price - discount;
cout << "Final Price after discount: " << finalPrice << endl;
return 0;
}
OutputPrice: 99.99
Final Price after discount: 89.49
4. Character (char)
- Stores a single character (e.g., 'A', 'b', '9'). Usually enclosed in single quotes and can be used in text processing.
- It takes 1 byte of memory.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring a char variable
char grade = 'A';
// Printing the value of the variable
cout << "My grade is: " << grade << endl;
return 0;
}
5. Boolean (bool)
- Stores only true or false values. Commonly used in conditions and logical operations.
- It takes 1 byte of memory.
C++
#include <iostream>
using namespace std;
int main()
{
// Declaring a bool variable
bool isStudent = true;
// Printing the value of the variable
cout << "Is the person a student? " << isStudent << endl;
// Changing the value
isStudent = false;
cout << "Is the person a student now? " << isStudent << endl;
return 0;
}
6. String (string)
- Stores a sequence of characters (e.g., "Hello"). Used for text, names, messages, or sentences.
- We have to include <string> header file for using string class.
C++
#include <iostream>
#include <string>
using namespace std;
int main()
{
// Declaring a string variable
string name = "Alice";
// Printing the string
cout << "Name: " << name << endl;
// Concatenating strings
string greeting = "Hello, " + name + "!";
cout << greeting << endl;
return 0;
}
OutputName: Alice
Hello, Alice!
Input and Output in C++ (cin and cout) are part of the iostream library, used to read data from the user (cin) and display data on the screen (cout).
- Used to take input from the user via the keyboard.
- Uses the extraction operator >> to read data into a variable.
2. Output (cout)
- Used to display information on the screen.
- Uses the insertion operator << to send data to the output.
C++
#include <iostream>
using namespace std;
int main()
{
// Declare variables
string name;
int age;
// Taking input from the user
cout << "Enter your name: ";
cin >> name;
cout << "Enter your age: ";
cin >> age;
// Displaying the input back to the user
cout << "Hello, " << name << "! You are " << age << " years old." << endl;
return 0;
}
Output:
Enter your name: Alice
Enter your age: 20
Hello, Alice! You are 20 years old.
Conditional Statements
- Conditional statements allow us to control the flow of the program based on certain conditions.
- It helps us to run a specific section of code based on a condition.
Types of conditional statements
1. if statement
- Executes a block of code only if the condition is true.
- The condition is usually a boolean expression. If the condition is false, the code inside the if block is skipped.
C++
#include <iostream>
using namespace std;
int main()
{
int i = 10;
// If statement
if (i < 15)
{
cout << "10 is less than 15";
}
return 0;
}
2. if-else statement
- Executes one block if the condition is true and another block if it is false.
- Allows two possible paths of execution.
C++
#include <iostream>
using namespace std;
int main()
{
int i = 10;
// If statement
if (i < 15)
{
cout << "10 is less than 15";
}
// Else statement with the above if
else
{
cout << "10 is not less than 15";
}
return 0;
}
3. else if ladder
- else if Checks multiple conditions sequentially.
- Executes the block of the first true condition and skips the rest.
- Useful when there are more than two possible outcomes.
C++
#include <stdio.h>
int main()
{
int marks = 85;
// Assign grade based on marks
if (marks >= 90)
{
printf("A\n");
}
else if (marks >= 80)
{
printf("B\n");
}
else if (marks >= 70)
{
printf("C\n");
}
else if (marks >= 60)
{
printf("D\n");
}
else
{
printf("F\n");
}
return 0;
}
4. switch statement
- Executes one block out of many based on a variable’s value.
- Each case must have a break to stop execution after a match.
- Useful when there are fixed known options.
C++
#include <iostream>
using namespace std;
int main()
{
// Variable to the used as switch expression
char x = 'A';
// Switch statement with three cases
switch (x)
{
case 'A':
cout << "A";
break;
case 'B':
cout << "B";
break;
default:
cout << "Other than A and B";
break;
}
return 0;
}
5. Shorthand if else (Ternary Operator)
- Ternary operator (?:) is a short-hand for simple if-else statements.
- It evaluates a condition and returns one value if true and another if false.
C++
#include <iostream>
using namespace std;
int main()
{
int x = 10, y = 20;
// Using ternary operator
int max_val = (x > y) ? x : y;
cout << "The maximum value is " << max_val;
return 0;
}
OutputThe maximum value is 20
Loops
- Loopsare used to repeat a block of code multiple times until a condition is met.
- They save time and effort by avoiding writing the same code again and again.
Types of Loops
1. For Loop
- for loop repeats a block of code a fixed number of times.
- Usually has initialization, condition, and increment/decrement in one line.
- Best used when the number of iterations is known.
C++
#include <bits/stdc++.h>
using namespace std;
int main()
{
// for loop to print "Hi" 5 times
for (int i = 5; i < 10; i++)
{
cout << "Hi" << endl;
}
return 0;
}
2. While Loop
- It repeats a block of code as long as the condition is true.
- The condition is checked before executing the code.
- Best used when the number of iterations is not known in advance.
C++
#include <iostream>
using namespace std;
int main()
{
// while loop to print numbers from 1 to 5
int i = 0;
while (i < 5)
{
cout << "Hi" << endl;
i++;
}
return 0;
}
3. Do-While Loop
- It is similar to while loop, but executes the code at least once.
- The condition is checked after executing the block.
- Useful when you want the code to run once before checking the condition.
C++
#include <iostream>
using namespace std;
int main()
{
// do-while loop to print "Hi" 5 times
int i = 0;
do
{
cout << "Hi" << endl;
i++;
} while (i < 5);
return 0;
}
Arrays
- An array is a collection of multiple values of the same type stored under a single name.
- Each value in the array can be accessed using an index (starting from 0).
- Arrays are useful to store and manage a list of similar items efficiently.
C++
#include <iostream>
using namespace std;
int main()
{
// declaring and initializing an array of size 5
int arr[5] = {2, 4, 8, 12, 16};
// printing array elements
for (int i = 0; i < 5; i++)
{
cout << arr[i] << " ";
}
return 0;
}
Multi-Dimensional Arrays
- A multi-dimensional array is an array of arrays.
- The most common type is a 2D array, which can be thought of as a table or matrix with rows and columns.
- Values are accessed using multiple indices, e.g., array[row][column].
C++
#include <iostream>
using namespace std;
int main()
{
// declaring and initializing a 2D array
// with 3 rows and 4 columns
int matrix[3][4] = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};
// printing the elements of the 2D array
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 4; j++)
{
cout << matrix[i][j] << " ";
}
cout << endl;
}
return 0;
}
Output1 2 3 4
5 6 7 8
9 10 11 12
Vectors
- A vector is a dynamic array that can grow or shrink in size automatically.
- It is part of the C++ Standard Template Library (STL), so you need #include <vector>.
- Elements in a vector are accessed using indices just like arrays.
- Vectors provide useful functions like push_back(), pop_back(), size(), and clear().
C++
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// Declares an empty vector
vector<int> v1;
// Declares vector with given size
// and fills it with a value
vector<int> v2(3, 5);
// Print items of v2
for (int x : v2)
{
cout << x << " ";
}
cout << endl;
// Initializes vector using initializer list.
vector<int> v3 = {1, 2, 3};
// Print items of v3
for (int x : v3)
{
cout << x << " ";
}
return 0;
}
References
- A reference is an alias for another variable, meaning it refers to the same memory location.
- Once a reference is initialized, it cannot be changed to refer to another variable.
C++
#include <iostream>
using namespace std;
int main()
{
int x = 10;
// ref is a reference to x.
int &ref = x;
// printing value using ref
cout << ref << endl;
// Changing the value and printing again
ref = 22;
cout << ref;
return 0;
}
Pointers
- A pointer is a variable that stores the memory address of another variable.
- Pointers can be reassigned to point to different variables.
C++
#include <iostream>
using namespace std;
int main()
{
int var = 10;
// declare pointer and store address of x
int *ptr = &var;
// print value and address
cout << "Value of x: " << var << endl;
cout << "Address of x: " << &var << endl;
cout << "Value stored in pointer ptr: " << ptr << endl;
cout << "Value pointed to by ptr: " << *ptr << endl;
return 0;
}
OutputValue of x: 10
Address of x: 0x7ffed805557c
Value stored in pointer ptr: 0x7ffed805557c
Value pointed to by ptr: 10
Functions
- A function is a block of code designed to perform a specific task.
- It help avoid repeating code and make programs organized and readable.
- A function can take inputs (parameters) and return a value, but both are optional.
- Functions are called/invoked whenever you need to execute that task.
C++
#include <iostream>
using namespace std;
// Function declaration and definition
void greet()
{
cout << "Hello, World!" << endl;
}
int main()
{
// Calling the function
greet();
return 0;
}
String Functions
There are several string functions present in Standard Template Library in C++ that are used to perform operations on strings. Some of the commonly used string functions are:
1. length()
- Returns the length of a string.
C++
#include <iostream>
#include <string>
using namespace std;
int main()
{
// Declare a string
string name = "GeeksforGeeks";
// Find and print the length of the string
cout << "The length of the name is: " << name.length() << endl;
return 0;
}
OutputThe length of the name is: 13
2. substr()
- It is used to extract a substring from a given string.
C++
#include <iostream>
#include <string>
using namespace std;
int main()
{
string str = "GeeksforGeeks";
// Extracts a substring starting from
// index 1 with a length of 5
string sub = str.substr(1, 5);
cout << "Substring: " << sub << endl;
return 0;
}
3. append()
- Appends a string at the end of the given string.
C++
#include <iostream>
#include <string>
using namespace std;
int main() {
string str = "Geeksfor";
str.append("Geeks");
cout << "Appended string: " << str << endl;
return 0;
}
OutputAppended string: GeeksforGeeks
4. compare()
- It is used to compare two strings lexicographically.
C++
#include <iostream>
#include <string>
using namespace std;
int main()
{
string str1 = "Geeks";
string str2 = "for";
string str3 = "Geeks";
int result1 = str1.compare(str2);
cout << "Comparison result: " << result1 << endl;
int result2 = str1.compare(str3);
cout << "Comparison result: " << result2 << endl;
return 0;
}
OutputComparison result: -31
Comparison result: 0
5. empty()
- It is used to check if a string is empty.
C++
#include <iostream>
using namespace std;
int main()
{
string str1 = "GeeksforGeeks";
string str2 = "";
if (str1.empty())
cout << "str1 is empty" << endl;
else
cout << "str1 is not empty" << endl;
if (str2.empty())
cout << "str2 is empty" << endl;
else
cout << "str2 is not empty" << endl;
return 0;
}
Outputstr1 is not empty
str2 is empty
Math Functions
| Function | Description | Example |
|---|
| min(x, y) | Returns the minimum value of x and y. | cout << min(10, 20); |
|---|
| max(x, y) | Returns the maximum value of x and y. | cout << max(10, 20); |
|---|
| sqrt(x) | Returns the square root of x. | cout << sqrt(25); |
|---|
| ceil(x) | It rounds up the value x to its nearest integer. | double ceilX = ceil(3.14159); |
|---|
| floor(x) | It rounds the value of x downwards to the nearest integer. | double floorX = floor(3.14159); |
|---|
| pow(x,n) | It returns the value x raised to the power of y | double result = pow(3.0, 2.0); |
|---|
Object-Oriented Programming
Object-oriented programming generally means storing data in the form of classes and objects.
Class and Objects
- Class: A class is a user-defined data type that contains its data members and member functions. A class is a blueprint for objects having similar attributes and behavior.
- Objects: An object is an instance or a variable of the class.
Pillars of OOPs
1. Encapsulation
- Encapsulation means bundling data (variables) and functions (methods) together in a class.
- It hides the internal details of an object from the outside world (data hiding).
- Access to data is provided through public functions (getters/setters).
- Helps in preventing accidental modification of important data.
2. Abstraction
- Abstraction is hiding complex implementation details and showing only the necessary features.
- It helps in simplifying the program for the user.
- Achieved in C++ using abstract classes and pure virtual functions.
- Example: A car driver knows how to drive, but doesn’t need to know the engine details.
3. Inheritance
- Inheritance allows a class (child/derived class) to acquire properties and behavior of another class (parent/base class).
- Promotes code reusability.
- Types in C++: single, multiple, multilevel, hierarchical, hybrid.
- Example: A Car class can inherit from a Vehicle class to reuse common features.
4. Polymorphism
- Polymorphism means one entity can take many forms.
- Two types: Compile-time (function overloading, operator overloading) , Run-time (virtual functions)
- Helps in using the same interface for different types of objects.
- Example: A print() function can behave differently for int, float, or string.
File Handling
- File handling means reading data from a file and manipulating the data of a file.
File Handling Operations
- 1. Open a file: We can use open() member function of ofstream class to open a file.
- 2. Read a file: We can use getline() member function of ifstream class to read a file.
- 3. Write to a file: We can use << operator to write to a file after opening a file with the object of ofstream class.
C++
#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main()
{
ofstream outputFile("example.txt");
// Open the file for writing
outputFile.open("example.txt");
if (outputFile.is_open()) {
// Write data to the file
outputFile << "Hello, World!" << endl;
outputFile << 42 << endl;
outputFile.close(); // Close the file
}
else {
// Failed to open the file
cout << "Error opening the file for writing."
<< endl;
return 1;
}
// Reading from a file
ifstream inputFile("example.txt");
if (inputFile.is_open()) {
string line;
while (getline(inputFile, line)) {
// Print each line
cout << line << endl;
}
// Close the file
inputFile.close();
}
else {
// Failed to open the file
cout << "Error opening the file for reading."
<< endl;
return 1;
}
return 0;
}
This C++ cheat sheet can serve as a reference guide for programmers that provides quick access to concepts of C++.
