C++ Cheat Sheet: Quick Reference Guide for Programmers

Need a quick refresher on C++ syntax and concepts? This C++ cheat sheet provides a concise and easily accessible guide to the essentials, perfect for both beginners and experienced programmers. Keep this page bookmarked for quick reference.

C++ is a powerful, high-level programming language used for various applications. Developed in 1983 by Bjarne Stroustrup at Bell Labs, C++ remains a staple for software development. Let's dive into the core elements.

Your First C++ Program: A Simple Start

Here's a basic "Hello World!" program to get you started:

#include 
using namespace std;

int main() {
  cout << "Hello World!";
  return 0;
}

Output:

Hello World!

Don't worry if you don't understand everything right away! We'll break down the syntax.

Understanding the Basic C++ Syntax

Every C++ program follows a basic structure:

// Header files - Includes libraries 
#include 

// Using namespace to avoid std:: prefixes
using namespace std;

// Main function - Entry point of the program
int main() {
  // Code statements go here
  return 0;
}

These components are the building blocks of any C++ application. The #include directive, for example, brings in the input/output stream library. using namespace std; avoids having to type std:: before elements like cout.

Mastering Comments in C++: Documenting Your Code

Comments are essential for explaining your code and making it more readable. They are ignored by the compiler. Here are two types:

• Single-line comments: Use // for comments on a single line.
  • // This is a single-line comment
• Multi-line comments: Use /* to start and */ to end comments that span multiple lines.
  • /* This is a multi-line comment.It can span multiple lines. */

Variables in C++: Storing Data

Variables are named storage locations that hold values.

• They store data of a specific type (e.g., integer, character, etc.).
• You must declare variables before using them.

// Declaring a single variable
data_type var_name;

// Declaring multiple variables
data_type var1_name, var2_name, var3_name;

Identifiers and Constants: Naming Conventions

• Identifiers: Unique names for variables, functions, etc., must start with a letter or underscore.
• Constants: Fixed values that cannot be changed during program execution.

Data Types in C++: Defining Variable Types

Data types specify the kind of data a variable can hold.

1. Integer (int): Stores whole numbers. Takes 4 bytes.
   • int var = 123;
2. Character (char): Stores single characters. Takes 1 byte.
   • char var = 'a';
3. Floating Point (float): Stores single-precision decimal numbers. Takes 4 bytes.
   • float num = 1.23;
4. Double (double): Stores double-precision decimal numbers. Takes 8 bytes.
   • double num = 1.2345;
5. Boolean (bool): Stores true or false values.
   • bool b = false;
6. String (string): Stores sequences of characters (text). Include ``.
   • string str = "GeeksforGeeks";

Input and Output in C++: Interacting with the User

C++ uses cin for input and cout for output, both found in the `` library.

1. Input: Use cin >> variable; to read input from the user.
   • int var; cin >> var;
2. Output: Use cout << "text" << variable; to display output.
   • cout << "Hello World!";

New Lines

Insert new lines in your output using \n or endl.

• cout << "Hello World!\n";
• cout << "Hello World!" << endl;

Conditional Statements in C++: Controlling Program Flow

Conditional statements execute different code blocks based on conditions.

1. if statement: Executes code if a condition is true.

   if (condition) {
     // Code to execute if condition is true
   }
   

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2. Nested if statement: An if statement inside another if statement.

   if (condition1) {
     // Code to execute if condition1 is true
     if (condition2) {
       // Code to execute if both condition1 and condition2 are true
     }
   }
   

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3. if-else statement: Executes one block of code if the condition is true and another if it's false.

   if (condition) {
     // Code to execute if condition is true
   } else {
     // Code to execute if condition is false
   }
   

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4. else if statement: Checks multiple conditions sequentially.

   if (condition1) {
     // Code to execute if condition1 is true
   } else if (condition2) {
     // Code to execute if condition2 is true
   } else {
     // Code to execute if all conditions are false
   }
   

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5. Ternary Operator: A shorthand for if-else statements.

   (condition) ? expression1 : expression2;
   

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6. switch statement: Executes different code blocks based on the value of an expression.

   switch (expression) {
     case value1:
       // Code to execute if expression == value1
       break;
     case value2:
       // Code to execute if expression == value2
       break;
     default:
       // Code to execute if expression doesn't match any case
       break;
   }
   

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Break and Default

• break: Exits the switch statement.
• default: Executes if none of the case values match the expression.

Loops in C++: Repeating Code Blocks

Loops repeatedly execute a block of code.

1. for Loop: Executes a block of code a fixed number of times.

   for (initialization; condition; update) {
     // Code to repeat
   }
   

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2. while Loop: Executes a block of code as long as a condition is true.

   while (condition) {
     // Code to repeat
     update_condition;
   }
   

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3. do-while Loop: Executes a block of code at least once, then repeats as long as a condition is true.

   do {
     // Code to repeat
   } while (condition);
   

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Arrays in C++: Storing Collections of Data

Arrays store a fixed-size sequence of elements of the same data type.

dataType array_name[size];

Example

#include 
#include 
using namespace std;

int main() {
  // Declare and initialize an array of strings
  string fruits[] = {"Apple", "Banana", "Orange", "Grapes"};

  // Access and print each element of the array
  for (int i = 0; i < 4; i++) {
    cout << "Fruit at index " << i << ": " << fruits[i] << endl;
  }

  return 0;
}

Multi-Dimensional Arrays

Arrays can have multiple dimensions. The most common is a 2-dimensional array.

data_type array_name[size1][size2]....[sizeN];

Example:

#include 
using namespace std;

int main() {
  // Declaration and initialization of a 2D array
  int arr[3][4] = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};

  // Accessing elements in the 2D array
  // Output: 1
  cout << "Element at arr[0][0]: " << arr[0][0] << endl;

  return 0;
}

Vectors in C++: Dynamic Arrays

Vectors are dynamic arrays that can grow or shrink in size. You need to include the `` header file.

vector vector_name;

Commonly Used Vector Functions

• push_back(): Adds an element to the end.
• pop_back(): Removes the last element.
• clear(): Removes all elements.
• empty(): Checks if the vector is empty.
• at(i): Accesses the element at index i.
• front(): Accesses the first element.
• back(): Accesses the last element.
• erase(): Removes an element at a specified position.

Example

#include 
#include 
using namespace std;

int main() {
  // Create an empty vector
  vector numbers;

  // push_back()
  numbers.push_back(10);
  numbers.push_back(20);
  numbers.push_back(30);

  // Accessing elements using at()
  // Output: 10
  cout << "Element at index 0: " << numbers.at(0) << endl;

  return 0;
}

Pointers and References in C++: Working with Memory

References

A reference is an alias for an existing variable.

int var = 12;
// A reference variable to var
int& ref = var;

ref is now another name for var.

Pointers

A pointer is a variable that stores the memory address of another variable. It uses the * operator to declare a pointer and the & operator to get the address of a variable.

int i = 3;
// A pointer to variable i or “stores the address of i”
int *ptr = &i;

Functions in C++: Reusable Code Blocks

Functions are reusable blocks of code that perform specific tasks.

// Function declaration
return_type function_name(parameters);

// Function definition
return_type function_name(parameters) {
  // Function body
  // Code to be executed
  return value; // If applicable
}

Example: Adding Two Numbers

#include 
using namespace std;

// Function declaration
int sum(int a, int b);

// Function definition
int sum(int a, int b) {
  return a + b;
}

int main() {
  // Function call
  int result = sum(3, 4);
  cout << "Sum: " << result << endl;
  return 0;
}

String Functions in C++: Manipulating Text

C++ provides several built-in functions for string manipulation. Include the `` header.

1. length() Function: Returns the length of the string.

   string str = "GeeksforGeeks";
   cout << "The length of the string is: " << str.length();
   

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2. substr() Function: Extracts a substring from a string.

   string substr(size_t pos, size_t len) const;
   

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   #include 
   #include 
   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;
   }
   

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3. append() Function: Appends one string to another.

   #include 
   using namespace std;
   
   int main() {
     string str = "Geeksfor";
     str.append("Geeks");
     cout << "Appended string: " << str << endl;
     return 0;
   }
   

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4. compare() Function: Compares two strings.

   #include 
   using namespace std;
   
   int main() {
     string str1 = "Geeks";
     string str2 = "for";
     string str3 = "Geeks";
   
     int result1 = str1.compare(str2);
     cout << "Comparison result: " << result1 << endl;
     return 0;
   }
   

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5. empty() Function: Checks if a string is empty.

   #include 
   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;
   }
   

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Object-Oriented Programming (OOP) Concepts in C++

1. Class

A class is a blueprint for creating objects, defining their properties (data members) and behaviors (member functions).

2. Object

An object is an instance of a class. It is a real-world entity that has properties and behaviors.

3. Polymorphism

Polymorphism means "many forms." It allows objects of different classes to respond differently to the same function call.

Types of Polymorphism

• Compile-time Polymorphism
• Runtime Polymorphism

Compile-time Polymorphism can be achieved using:

• Operator overloading
• Function overloading

Runtime Polymorphism can be achieved using:

• Function overriding
• Virtual Functions

4. Inheritance

Inheritance allows a class (child class) to inherit properties and behaviors from another class (parent class). This promotes code reusability.

Types of Inheritance:

• Single Inheritance: A derived class inherits from a single base class.
• Multiple Inheritance: A derived class inherits from multiple base classes.
• Multilevel Inheritance: A derived class inherits from another derived class.
• Hierarchical Inheritance: When