Added references vs pointers under day 11 (#293)

* Added references vs pointers under day 11

* changed position on sidebar

docs/day-11/reference-vs-pointers.md

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+---
+
+sidebar_position: 4
+title: "References vs Pointers in C++"
+description: "In this tutorial, we will learn about the differences between references and pointers in C++ with the help of examples. Understanding references and pointers is crucial for effective C++ programming."
+sidebar_label: "References vs Pointers"
+slug: references-vs-pointers-in-cpp
+---
+
+## 1. What are References and Pointers in C++?
+
+In C++, both references and pointers are used to refer to memory locations, but they have different properties and uses. 
+
+- **References**: An alias for another variable.
+- **Pointers**: A variable that holds the memory address of another variable.
+
+![References vs Pointers in C++](../../static/img/day-11/references-vs-pointers.png)
+
+## 2. Declaring References and Pointers
+
+### Declaring References
+
+A reference is declared using the `&` operator.
+
+**Syntax:**
+
+```cpp
+datatype &referenceName = variableName;
+```
+
+**Example:**
+
+```cpp
+#include <iostream>
+using namespace std;
+
+int main() {
+    int a = 10;
+    int &ref = a;  // ref is a reference to a
+    cout << "a: " << a << endl;    // Output: 10
+    cout << "ref: " << ref << endl;  // Output: 10
+    return 0;
+}
+```
+
+### Declaring Pointers
+
+A pointer is declared using the `*` operator.
+
+**Syntax:**
+
+```cpp
+datatype *pointerName;
+```
+
+**Example:**
+
+```cpp
+#include <iostream>
+using namespace std;
+
+int main() {
+    int a = 10;
+    int *ptr = &a;  // ptr holds the address of a
+    cout << "a: " << a << endl;      // Output: 10
+    cout << "ptr: " << ptr << endl;  // Output: address of a
+    cout << "*ptr: " << *ptr << endl;  // Output: 10
+    return 0;
+}
+```
+
+## 3. Differences Between References and Pointers
+
+### 1. Syntax and Usage
+
+- **References**: Use the `&` operator for declaration and do not require dereferencing.
+- **Pointers**: Use the `*` operator for declaration and require dereferencing with `*`.
+
+**Example:**
+
+```cpp
+int a = 5;
+int &ref = a;  // Reference
+int *ptr = &a;  // Pointer
+```
+
+### 2. Initialization
+
+- **References**: Must be initialized when declared.
+- **Pointers**: Can be declared without initialization and assigned later.
+
+**Example:**
+
+```cpp
+int a = 5;
+int &ref = a;  // Must be initialized
+int *ptr;     // Can be declared without initialization
+ptr = &a;     // Assigned later
+```
+
+### 3. Reassignment
+
+- **References**: Cannot be reassigned to refer to another variable.
+- **Pointers**: Can be reassigned to point to different variables.
+
+**Example:**
+
+```cpp
+int a = 5, b = 10;
+int &ref = a;  // Reference to a
+// ref = b;   // Error: Cannot reassign a reference
+
+int *ptr = &a;  // Pointer to a
+ptr = &b;       // Pointer reassigned to b
+```
+
+### 4. Null Values
+
+- **References**: Cannot be null.
+- **Pointers**: Can be null (useful to indicate that they are not pointing to any valid memory).
+
+**Example:**
+
+```cpp
+int a = 5;
+int &ref = a;  // Reference cannot be null
+
+int *ptr = nullptr;  // Pointer can be null
+ptr = &a;            // Pointer assigned to a valid address
+```
+
+### 5. Memory Address
+
+- **References**: Implicitly provide the memory address of the referenced variable.
+- **Pointers**: Explicitly hold and can be used to manipulate memory addresses.
+
+**Example:**
+
+```cpp
+int a = 5;
+int &ref = a;  // Reference to a
+int *ptr = &a; // Pointer to a
+
+cout << "Address of a: " << &a << endl;
+cout << "Address using ref: " << &ref << endl;
+cout << "Address using ptr: " << ptr << endl;
+```
+
+## 4. When to Use References and Pointers
+
+### Use References:
+- When you need an alias for another variable.
+- When you want to avoid null checks and pointer arithmetic.
+- For pass-by-reference in function arguments to avoid copying large objects.
+
+### Use Pointers:
+- When you need to allocate memory dynamically.
+- When you need to implement data structures like linked lists, trees, etc.
+- When you need to use null values to indicate "no object".
+
+## 5. Examples
+
+### Example 1: Using References
+
+```cpp
+#include <iostream>
+using namespace std;
+
+void increment(int &ref) {
+    ref++;
+}
+
+int main() {
+    int a = 10;
+    increment(a);
+    cout << "a: " << a << endl;  // Output: 11
+    return 0;
+}
+```
+
+### Example 2: Using Pointers
+
+```cpp
+#include <iostream>
+using namespace std;
+
+void increment(int *ptr) {
+    (*ptr)++;
+}
+
+int main() {
+    int a = 10;
+    increment(&a);
+    cout << "a: " << a << endl;  // Output: 11
+    return 0;
+}
+```
+
+## 6. Conclusion
+
+Both references and pointers are powerful features in C++ that allow you to manipulate memory and create more efficient programs. Understanding the differences between them and knowing when to use each is crucial for effective C++ programming.