Biến, phạm vi và thời gian tồn tại

#include <iostream>
#include <string>

// Static storage duration — lives for the entire program
int global_count = 0;                    // Initialized before main()
static int file_local = 42;              // Same, but only visible in this file

void demonstrate_storage() {
    // Automatic storage duration — created on entry, destroyed on exit
    int local = 10;                      // Born here, dies at closing brace

    // Static storage duration (local scope)
    static int call_count = 0;           // Initialized ONCE, persists across calls
    ++call_count;
    std::cout << "Call #" << call_count << '\n';

    // Thread-local storage duration (C++11)
    // thread_local int per_thread = 0;  // One copy per thread, lives until thread ends
}

int main() {
    demonstrate_storage();  // Call #1
    demonstrate_storage();  // Call #2 — call_count is still alive!
    demonstrate_storage();  // Call #3

    // Dynamic storage duration — you control the lifetime
    int* heap_int = new int{99};         // Born here
    std::cout << *heap_int << '\n';
    delete heap_int;                     // Dies here — you must not forget!
    heap_int = nullptr;                  // Good practice: nullify after delete

    // In modern C++, prefer smart pointers over raw new/delete
    // auto ptr = std::make_unique<int>(99);  // Covered in later modules
}

#include <iostream>

struct Point {
    int x;
    int y;
};

static int global_int;          // Zero-initialized → 0
static Point global_point;      // Zero-initialized → {0, 0}

int main() {
    int a;                      // Default-initialized → INDETERMINATE (UB to read!)
    int b{};                    // Value-initialized → 0
    int c = int();              // Value-initialized → 0
    int d = {};                 // Value-initialized → 0

    Point p1;                   // Default-initialized → members are INDETERMINATE
    Point p2{};                 // Value-initialized → {0, 0}
    Point p3{1, 2};             // Aggregate-initialized → {1, 2}

    // Array initialization
    int arr1[5];                // Default-initialized → INDETERMINATE values!
    int arr2[5]{};              // Value-initialized → {0, 0, 0, 0, 0}
    int arr3[5]{1, 2};          // Partially initialized → {1, 2, 0, 0, 0}

    std::cout << "global_int: " << global_int << '\n';  // Safe: 0
    std::cout << "b: " << b << '\n';                    // Safe: 0
    // std::cout << a;  // DANGER: UB — reading uninitialized variable!

    std::cout << "p2: (" << p2.x << ", " << p2.y << ")\n";  // Safe: (0, 0)
    std::cout << "p3: (" << p3.x << ", " << p3.y << ")\n";  // Safe: (1, 2)
}

#include <string>
#include <iostream>

// BAD: global depends on another global — order is unspecified across files
// std::string bad_prefix = "[LOG] ";  // What if this is used before construction?

// GOOD: Construct on First Use idiom
std::string& log_prefix() {
    static std::string prefix = "[LOG] ";  // Constructed on first call, thread-safe
    return prefix;
}

std::string& app_name() {
    static std::string name = "MyApp";
    return name;
}

void log(const std::string& message) {
    // Safe: log_prefix() and app_name() are guaranteed to be initialized
    std::cout << log_prefix() << app_name() << ": " << message << '\n';
}

int main() {
    log("Starting up");
    log("Ready");
}