Lecture 3: C++ Fundamentals

### Basic Program Structure

## `hello.cpp`
          A minimal C++ program:

```cpp
          #include <iostream>

int main() {
            std::cout << "Hello, World!" << std::endl;
            return 0;
          }
          ```

---

## `hello.cpp`
          A minimal C++ program:

```cpp
          #include <iostream>
          using namespace std;

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

---

### Key Parts Explained

- `#include <iostream>`
            - A **preprocessor directive** that includes the standard input/output stream library.
          - `int main()`
            - The main **function** where program execution begins.
          - `std::cout`
            - The standard **output stream** object, used for printing. `std` is the namespace.
          - `return 0;`
            - A **return statement** that signals successful program termination.

# C++ Operators

---

## Common Operators

| Category | Operators | Example |
          | :--- | :--- | :--- |
          | Arithmetic | `+`, `-`, `*`, `/`, `%` | `5 + 3` |
          | Assignment | `=`, `+=`, `-=` | `x = 5` |
          | Comparison | `==`, `!=`, `<`, `>` | `a == b`|
          | Logical | `&&`, `\|\|`, `!` | `a > 0 && b > 0` |
          | Increment | `++`, `--` | `i++` |
          | Scope Resolution | `::` | `std::cout` |

---

## Operator Precedence

- Determines the order of operations in an expression.
          - Just like `*` is evaluated before `+` in math.
          - **Rule of Thumb:** When in doubt, use parentheses `()` to enforce the order you want. It makes your code safer and easier to read.

```cpp
          // Without parentheses, multiplication is first
          int x = 5 + 3 * 2; // result is 11

// With parentheses, addition is first
          int y = (5 + 3) * 2; // result is 16
          ```

# Variable Scope

---

## What is Scope?

- **Visibility:** The region of code where a variable can be accessed by its name.
          - **Lifetime:** The period during which the variable exists in memory.
          
          These are related but distinct concepts!

---

## Types of Scope

- **Local Scope (Block Scope):**
            - Declared inside a function or block `{...}`.
            - Created when the block is entered, destroyed when it's exited.
          - **Global Scope (File Scope):**
            - Declared outside of all functions.
            - Exists for the entire duration of the program.
            - **Warning:** Use sparingly to avoid bugs.

---

## Scope Example

```cpp
          #include <iostream>

int globalVar = 100; // Global scope

void myFunction() {
              int localVar = 10; // Local scope to myFunction
              std::cout << "Local: " << localVar << std::endl;
              std::cout << "Global: " << globalVar << std::endl;
          }

int main() {
              myFunction();
              // ERROR! localVar is not in this scope
              // std::cout << localVar; 
              std::cout << "Global from main: " << globalVar << std::endl;
              return 0;
          }
          ```

# Storage Classes

---

## Concept
          A specifier that controls a variable's **lifetime** and **linkage** (visibility across files).
          
          Main specifiers we care about: `static` and `extern`.

---

### `static` (inside a function)

- **Lifetime:** Becomes the entire program duration.
          - The variable **keeps its value** between function calls.
          - It is initialized only **once**.

```cpp
          cout << "--- Static Variable Example ---" << endl;
            {
                static int count = 0; // persists between block executions
                count++;
                cout << "Called " << count << " times." << endl;
            }
            {
                static int count = 0; // persists across runs of program block
                count++;
                cout << "Called " << count << " times." << endl;
            }
          ```

---

### `extern`

- **Declaration**, not a definition.
          - Tells the compiler: "This global variable is defined in another file, but I want to use it here."
          - Used to **share a single global variable** across multiple `.cpp` files.
          
          ---
          
          **file1.cpp**
          ```cpp
          // Definition of the variable
          int sharedData = 42;
          ```

**file2.cpp**
          ```cpp
          #include <iostream>
          
          // Declaration telling compiler it exists elsewhere
          extern int sharedData;
          
          void useSharedData() {
              std::cout << sharedData; // Accesses 42 from file1.cpp
          }
          ```
          Compile both files by linking them
          ```
          g++ file1.cpp file2.cpp -o myprogram
          ```
          ---

## Summary

- **Local (default):** Temporary, visible only inside a function/block.
          - **Global (default):** Permanent, visible everywhere (potentially dangerous).
          - **`static` (local):** Permanent, but visible only inside a function/block.
          - **`extern`:** A promise that a global variable exists in another file.

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