// 1. Preprocessor directive — runs before compilation
#include <iostream>

// 2. Namespace — avoids name collisions
using namespace std;

// 3. Entry point — every C++ program starts here
int main() {
    // 4. Statement — must end with semicolon
    cout << "Hello, C++!" << endl;

    // 5. Return value — 0 = success, non-zero = error
    return 0;
}
// Output: Hello, C++!

// Include standard library headers
#include <iostream>    // Standard I/O (cout, cin)
#include <string>      // std::string
#include <vector>      // std::vector
#include <cmath>       // sin, cos, sqrt, pow

// Include your own headers (use quotes, not angle brackets)
#include "myheader.h"

// Define a constant (no type, no semicolon)
#define PI 3.14159265

// Conditional compilation
#ifdef DEBUG
    cout << "Debug mode is ON\n";
#endif

// Version 1: no command-line arguments
int main() {
    return 0;
}

// Version 2: with command-line arguments
int main(int argc, char* argv[]) {
    // argc = number of arguments (including program name)
    // argv = array of argument strings

    for (int i = 0; i < argc; i++) {
        cout << "Arg " << i << ": " << argv[i] << "\n";
    }
    return 0;
}
// ./program hello world
// Output:
// Arg 0: ./program
// Arg 1: hello
// Arg 2: world

#include <iostream>

// Option 1: Always qualify (most explicit, safest in large projects)
std::cout << "Hello\n";

// Option 2: Bring in everything from std (convenient for small programs)
using namespace std;
cout << "Hello\n";

// Option 3: Bring in only specific names (recommended balance)
using std::cout;
using std::endl;
cout << "Hello" << endl;

// Defining your own namespace
namespace myapp {
    int version = 1;
    void greet() { std::cout << "MyApp v" << version << "\n"; }
}

// Usage
myapp::greet();   // MyApp v1

int x = 10;           // Declaration statement
x = x + 5;           // Assignment statement
cout << x << "\n";   // Expression statement
return 0;             // Return statement

// Multiple statements on one line (legal but bad style)
int a = 1; int b = 2; int c = a + b;

// A block is a group of statements in curly braces
{
    int local = 42;   // local only exists inside these braces
    cout << local;
}
// cout << local;  // ERROR: 'local' not in scope here

// Single-line comment — everything after // is ignored

/*
   Multi-line comment
   Useful for longer explanations
   or temporarily disabling code
*/

/**
 * Doxygen-style comment for documentation generation
 * @param x  The first number
 * @param y  The second number
 * @return   Their sum
 */
int add(int x, int y) {
    return x + y;
}

#include <iostream>
using namespace std;

int main() {
    // cout = character output stream
    cout << "Hello";         // Print without newline
    cout << " World\n";      // \n = newline character
    cout << "Line 2" << endl; // endl = newline + flush buffer

    // Printing multiple values with << (chaining)
    int age = 25;
    double pi = 3.14159;
    cout << "Age: " << age << ", Pi: " << pi << "\n";

    // Escape sequences
    cout << "Tab:\tEnd\n";
    cout << "Quote: \"Hello\"\n";
    cout << "Backslash: \\\n";

    return 0;
}
// Output:
// Hello World
// Line 2
// Age: 25, Pi: 3.14159
// Tab:    End
// Quote: "Hello"
// Backslash: \

// Missing semicolon
int x = 10   // ERROR: expected ';' before '}'

// Wrong bracket type
cout << "Hello";          // Correct: angle brackets for std lib
#include "iostream"       // Wrong: should use <iostream>

// Assignment vs equality
if (x = 5) { }   // WARNING: this assigns 5 to x, always true
if (x == 5) { }  // Correct: tests if x equals 5

// Case sensitivity
Int x = 10;   // ERROR: 'Int' is not a type; 'int' is
COUT << "hi"; // ERROR: should be 'cout'

#include <iostream>
#include <string>

// Global constant (available throughout the file)
const std::string APP_NAME = "C++ Demo";
const int VERSION = 1;

// Forward declaration — tells the compiler the function exists
void printBanner();

int main() {
    printBanner();

    // Local variables
    std::string userName;
    int userAge;

    // Prompt user
    std::cout << "Enter your name: ";
    std::cin >> userName;

    std::cout << "Enter your age: ";
    std::cin >> userAge;

    // Output with expression
    std::cout << "Hello, " << userName << "!\n";
    std::cout << "In 10 years you will be " << (userAge + 10) << "\n";

    return 0;
}

// Function definition (below main — allowed because of forward declaration)
void printBanner() {
    std::cout << "======================\n";
    std::cout << "  " << APP_NAME << " v" << VERSION << "\n";
    std::cout << "======================\n";
}
// Output:
// ======================
//   C++ Demo v1
// ======================
// Enter your name: Alice
// Enter your age: 30
// Hello, Alice!
// In 10 years you will be 40