C++ Spickzettel

#include <iostream>

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

int age = 30;
double pi = 3.14159;
char grade = 'A';
bool is_dev = true;
std::string name = "Alice";
auto x = 42;          // int
auto y = 3.14;        // double
const double TAX = 0.08;

#include <iostream>
#include <string>

int main() {
    std::string name;
    int age;
    std::cout << "Name: ";
    std::getline(std::cin, name);
    std::cout << "Age: ";
    std::cin >> age;
    std::cout << "Hi " << name << ", " << age << "\n";
}

int x = 10;
int &ref = x;   // alias for x
ref = 20;
std::cout << x;   // 20

void increment(int &n) { n++; }
int a = 5;
increment(a);   // a is now 6

double d = 3.99;
int i = (int)d;               // C-style: 3
int j = static_cast<int>(d);  // C++: 3

int n = 65;
char c = static_cast<char>(n);  // 'A'

// string <-> number
int num = std::stoi("42");
std::string s = std::to_string(3.14);

#include <string>
std::string s = "Hello";
std::cout << s.length();      // 5
std::cout << s[0];            // H
s += ", World!";
std::cout << s;               // Hello, World!
std::cout << s.substr(0, 3);  // Hel
std::cout << s.find("World"); // 7

std::string a = "apple", b = "banana";
if (a == b) { /* equal */ }
if (a < b) { /* apple comes before banana */ }
if (a.find("pp") != std::string::npos) {
    std::cout << "found\n";
}
size_t pos = a.rfind("p");  // last occurrence

#include <sstream>
// Build a string
std::ostringstream oss;
oss << "Name=" << "Alice" << "&Age=" << 30;
std::string result = oss.str();

// Parse tokens
std::istringstream iss("10 20 30");
int a, b, c;
iss >> a >> b >> c;   // a=10, b=20, c=30

std::string raw = R"(C:\Users\name\file.txt)";
// No need to escape backslashes

std::string json = R"({
  "name": "Alice",
  "age": 30
})";

char cstr[] = "Hello";   // null-terminated, size 6
std::string cppstr = "Hello";

#include <cstring>
std::cout << strlen(cstr);      // 5
std::cout << cppstr.length();   // 5

std::string from_c = cstr;          // C -> C++
const char* to_c = cppstr.c_str();  // C++ -> C

#include <cstdint>
int32_t a = 100;
int64_t big = 9'000'000'000LL;
uint8_t byte = 255;
size_t sz = sizeof(a);   // 4

float f = 1.5f;          // 4 bytes
double d = 3.14159265;   // 8 bytes (preferred)

#include <limits>
std::cout << std::numeric_limits<int>::max();    // 2147483647
std::cout << std::numeric_limits<int>::min();    // -2147483648
std::cout << std::numeric_limits<double>::infinity();
std::cout << std::numeric_limits<double>::epsilon();

#include <cmath>
double x = 2.5;
std::pow(x, 3);     // 15.625
std::sqrt(x);       // 1.581
std::abs(-5);       // 5
std::floor(3.7);    // 3.0
std::ceil(3.2);     // 4.0
std::round(3.5);    // 4.0
std::fmod(10.5, 3); // 1.5

#include <random>
std::random_device rd;
std::mt19937 gen(rd());  // Mersenne Twister engine
std::uniform_int_distribution<int> dist(1, 100);

for (int i = 0; i < 5; i++) {
    std::cout << dist(gen) << " ";
}

int a = 100'000;
int b = a * a;   // overflow! undefined behavior
long long safe = (long long)a * a;  // OK

// Check before multiplying
if (a > 0 && b > INT_MAX / a) {
    // would overflow, handle it
}

int score = 85;
if (score >= 90) {
    std::cout << "A\n";
} else if (score >= 80) {
    std::cout << "B\n";
} else {
    std::cout << "C\n";
}

int day = 3;
switch (day) {
    case 1: std::cout << "Mon"; break;
    case 2: std::cout << "Tue"; break;
    case 3: std::cout << "Wed"; break;
    default: std::cout << "Other";
}

// Classic for
for (int i = 0; i < 5; i++) {
    std::cout << i << " ";
}

// Range-based for (C++11)
std::vector<int> v = {1, 2, 3};
for (int n : v) std::cout << n;
for (const auto &n : v) std::cout << n;  // no copy

int n = 5;
while (n > 0) {
    std::cout << n-- << " ";
}

int x;
do {
    std::cin >> x;
} while (x < 0);  // runs at least once

for (int i = 0; i < 10; i++) {
    if (i == 3) continue;  // skip 3
    if (i == 7) break;     // stop at 7
    std::cout << i << " ";  // 0 1 2 4 5 6
}

// C++ has no labeled break; use a flag
bool found = false;
for (int i = 0; i < n && !found; i++) {
    for (int j = 0; j < m; j++) {
        if (matrix[i][j] == target) { found = true; break; }
    }
}

#include <tuple>
std::tuple<int, int> divide(int a, int b) {
    return {a / b, a % b};
}

auto [q, r] = divide(17, 5);  // q=3, r=2 (C++17)

inline int power(int base, int exp = 2) {
    int r = 1;
    for (int i = 0; i < exp; i++) r *= base;
    return r;
}
// power(3) == 9, power(2, 5) == 32

int add(int a, int b) { return a + b; }
double add(double a, double b) { return a + b; }
std::string add(std::string a, std::string b) { return a + b; }

add(1, 2);          // int version
add(1.5, 2.5);      // double version
add("a", "b");      // string version

auto square = [](int x) { return x * x; };
std::cout << square(5);  // 25

int factor = 3;
auto multiply = [factor](int x) { return x * factor; };

std::vector<int> v = {1, 2, 3};
std::for_each(v.begin(), v.end(), [](int n) {
    std::cout << n << " ";
});

#include <functional>
int add(int a, int b) { return a + b; }

int (*fp)(int, int) = add;             // raw function pointer
std::function<int(int, int)> f = add;  // flexible wrapper
f = [](int a, int b) { return a * b; };

std::cout << f(3, 4);  // 12

#include <vector>
std::vector<int> v = {1, 2, 3};
v.push_back(4);
v.pop_back();
v.size();          // 3
v[0] = 0;
v.at(5);           // throws std::out_of_range
for (int n : v) std::cout << n;

#include <map>
#include <unordered_map>
std::map<std::string, int> ordered;   // sorted by key (red-black tree)
ordered["Alice"] = 30;

std::unordered_map<std::string, int> hashed;  // hash table
hashed["Bob"] = 25;

for (auto &[k, v] : ordered) {  // C++17 structured binding
    std::cout << k << ":" << v;
}

#include <set>
std::set<int> s = {3, 1, 4, 1, 5};
// s contains: 1, 3, 4, 5 (sorted, unique)
s.insert(2);
s.erase(1);
if (s.count(4)) std::cout << "found";
auto it = s.lower_bound(3);  // first >= 3

#include <array>
#include <deque>
std::array<int, 3> arr = {1, 2, 3};  // fixed size, stack-allocated
arr.size();  // 3

std::deque<int> dq = {1, 2, 3};
dq.push_front(0);
dq.push_back(4);
// dq: 0, 1, 2, 3, 4

#include <tuple>
std::tuple<int, std::string, double> t = {1, "Alice", 3.14};
auto [id, name, val] = t;  // C++17 structured binding

std::pair<int, int> p = {1, 2};
std::cout << p.first << p.second;

auto [a, b] = std::make_pair(10, 20);

int x = 10;
int *ptr = &x;        // address of x
std::cout << *ptr;    // 10 (dereference)
*ptr = 20;
std::cout << x;       // 20

int arr[] = {1, 2, 3};
int *p = arr;
std::cout << *(p + 1);  // 2

int x = 10;
int &ref = x;    // must init, cannot reseat
int *ptr = &x;   // can be null, can reassign

ref = 20;        // x = 20
*ptr = 30;       // x = 30

// References cannot be null, safer for parameters
void foo(const std::string &s);  // preferred
void bar(std::string *s);        // s might be null

#include <memory>
auto p = std::make_unique<int>(42);
std::cout << *p;  // 42
// auto p2 = p;   // ERROR: cannot copy
auto p2 = std::move(p);  // transfer ownership
// p is now nullptr

#include <memory>
auto a = std::make_shared<int>(42);
auto b = a;  // both point to same object
std::cout << a.use_count();  // 2

std::weak_ptr<int> w = a;  // observer, no ownership
if (auto locked = w.lock()) {
    std::cout << *locked;  // 42
}

// Manual new/delete (avoid in modern C++)
int *p = new int(42);
delete p;

// RAII: resource acquisition is initialization
class Buffer {
    int *data;
public:
    Buffer(size_t n) : data(new int[n]) {}
    ~Buffer() { delete[] data; }  // auto cleanup
};

// Modern: use containers/smart pointers instead
std::vector<int> buf(100);  // no manual delete needed

class Person {
    std::string name;
    int age;
public:
    Person(std::string n, int a) : name(std::move(n)), age(a) {}
    std::string getName() const { return name; }
    int getAge() const { return age; }
};

Person p("Alice", 30);
std::cout << p.getName();

class Account {
private:
    double balance;
protected:
    std::string owner;
public:
    Account(double b) : balance(b) {}
    double getBalance() const { return balance; }
    void deposit(double amt) { if (amt > 0) balance += amt; }
};

class Animal {
public:
    virtual void speak() { std::cout << "..."; }
    virtual ~Animal() = default;  // virtual destructor!
};

class Dog : public Animal {
public:
    void speak() override { std::cout << "Woof"; }
};

Animal *a = new Dog();
a->speak();  // Woof (polymorphism)
delete a;

class Shape {
public:
    virtual double area() const = 0;  // pure virtual
    virtual ~Shape() = default;
};

class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() const override {
        return 3.14159 * r * r;
    }
};

class Vec2 {
    double x, y;
public:
    Vec2(double x, double y) : x(x), y(y) {}
    Vec2 operator+(const Vec2 &o) const {
        return {x + o.x, y + o.y};
    }
    friend std::ostream &operator<<(std::ostream &os, const Vec2 &v) {
        return os << "(" << v.x << "," << v.y << ")";
    }
};

Vec2 a(1, 2), b(3, 4);
std::cout << a + b;  // (4,6)

template <typename T>
T max_val(T a, T b) {
    return (a > b) ? a : b;
}

std::cout << max_val(3, 7);           // int: 7
std::cout << max_val(3.14, 2.71);     // double: 3.14
std::cout << max_val<std::string>("a", "b");  // explicit

template <typename T>
class Stack {
    std::vector<T> data;
public:
    void push(T v) { data.push_back(v); }
    T pop() { T v = data.back(); data.pop_back(); return v; }
    bool empty() const { return data.empty(); }
};

Stack<int> si;
si.push(1); si.push(2);
Stack<std::string> ss;
ss.push("hi");

template <typename T>
T identity(T x) { return x; }

// Full specialization for bool
template <>
bool identity<bool>(bool x) {
    std::cout << "bool!";
    return x;
}

identity(42);     // generic
identity(true);   // specialized

template <typename T>
void print(T v) { std::cout << v << "\n"; }

template <typename T, typename... Args>
void print(T first, Args... rest) {
    std::cout << first << ", ";
    print(rest...);  // recurse
}

print(1, "hello", 3.14);  // 1, hello, 3.14

#include <concepts>
template <std::integral T>
T add(T a, T b) { return a + b; }

template <typename T>
requires std::floating_point<T>
T half(T x) { return x / 2; }

// Abbreviated form
auto square(std::integral auto x) { return x * x; }

#include <algorithm>
std::vector<int> v = {3, 1, 4, 1, 5, 9};
std::sort(v.begin(), v.end());          // 1 1 3 4 5 9
std::sort(v.begin(), v.end(), std::greater<>());  // descending

auto it = std::find(v.begin(), v.end(), 4);
if (it != v.end()) std::cout << "found";

std::vector<int> v = {1, 2, 3, 4};
std::vector<int> squared(v.size());
std::transform(v.begin(), v.end(), squared.begin(),
               [](int x) { return x * x; });
// squared: 1 4 9 16

std::for_each(v.begin(), v.end(), [](int &x) { x *= 2; });
// v: 2 4 6 8

#include <numeric>
std::vector<int> v = {1, 2, 3, 4, 5};
int sum = std::accumulate(v.begin(), v.end(), 0);  // 15
int product = std::accumulate(v.begin(), v.end(), 1,
                              std::multiplies<>());  // 120

int cnt = std::count(v.begin(), v.end(), 3);  // 1

std::vector<int> v = {1, 2, 3, 4, 5};
std::vector<int> dest;
std::copy(v.begin(), v.end(), std::back_inserter(dest));

auto end = std::remove(v.begin(), v.end(), 3);  // erase-remove idiom
v.erase(end, v.end());

std::vector<int> u = {1, 1, 2, 2, 3};
u.erase(std::unique(u.begin(), u.end()), u.end());  // 1 2 3

int a = 3, b = 7;
std::cout << std::max(a, b);   // 7
std::cout << std::min(a, b);   // 3

auto [mn, mx] = std::minmax({3, 1, 4, 1, 5});  // mn=1, mx=5

int score = 105;
int clamped = std::clamp(score, 0, 100);  // 100

#include <stdexcept>
try {
    int x = 10, y = 0;
    if (y == 0) throw std::runtime_error("division by zero");
    std::cout << x / y;
} catch (const std::exception &e) {
    std::cerr << "Error: " << e.what();
}

#include <stdexcept>
throw std::runtime_error("runtime error");
throw std::logic_error("logic error");
throw std::out_of_range("index out of range");
throw std::invalid_argument("bad argument");
throw std::bad_alloc();  // thrown by new on allocation failure

// std::exception is the base; .what() returns the message

class FileError : public std::runtime_error {
public:
    FileError(const std::string &msg, const std::string &path)
        : std::runtime_error(msg + ": " + path), path_(path) {}
    const std::string &path() const { return path_; }
private:
    std::string path_;
};

try { throw FileError("not found", "data.txt"); }
catch (const FileError &e) {
    std::cerr << e.what() << " at " << e.path();
}

void safe_op() noexcept {
    // guaranteed not to throw
}

void risky() {
    std::vector<int> v(1000);  // RAII: cleanup on exception
    throw std::runtime_error("oops");
    // v's destructor still runs during stack unwinding
}

#include <cassert>
double sqrt_safe(double x) {
    assert(x >= 0 && "sqrt of negative");
    return std::sqrt(x);
}
// In release builds (NDEBUG defined), assert is removed

#include <fstream>
#include <string>
std::ifstream file("input.txt");
if (!file) { std::cerr << "cannot open"; return 1; }

std::string line;
while (std::getline(file, line)) {
    std::cout << line << "\n";
}

#include <fstream>
std::ofstream out("output.txt");
if (!out) return 1;

out << "Line 1\n";
out << "Value: " << 42 << "\n";

out.close();  // optional, destructor closes
// Append mode: std::ofstream("f.txt", std::ios::app);

#include <sstream>
// Build a string from mixed types
std::ostringstream oss;
oss << "Name=" << "Alice" << "&Age=" << 30;
std::string url = oss.str();

// Parse tokens from a string
std::istringstream iss("10 20 30");
int a, b, c;
iss >> a >> b >> c;

#include <fstream>
struct Record { int id; double value; };

// Write
std::ofstream out("data.bin", std::ios::binary);
Record r{1, 3.14};
out.write(reinterpret_cast<char*>(&r), sizeof(r));

// Read
std::ifstream in("data.bin", std::ios::binary);
Record r2;
in.read(reinterpret_cast<char*>(&r2), sizeof(r2));

#include <format>  // C++20
std::string s = std::format("Hello, {}! You are {}.", "Alice", 30);
std::cout << std::format("{:.2f}", 3.14159);  // 3.14
std::cout << std::format("{:>10}", "right");   // padded

// Pre-C++20: use iomanip
#include <iomanip>
std::cout << std::fixed << std::setprecision(2) << 3.14159;

#include <memory>
std::unique_ptr<int> p1 = std::make_unique<int>(42);
// std::unique_ptr<int> p2 = p1;  // ERROR: cannot copy
std::unique_ptr<int> p2 = std::move(p1);  // OK: transfer ownership
// p1 is now nullptr

// Custom deleter
auto deleter = [](FILE* f) { if (f) fclose(f); };
std::unique_ptr<FILE, decltype(deleter)> fp(fopen("f.txt", "r"), deleter);

#include <memory>
auto p1 = std::make_shared<int>(42);
auto p2 = p1;  // OK: both share ownership
std::cout << *p1 << " " << p1.use_count();  // 42 2

// Control block holds refcount + deleter + allocator
// Refcount is atomic (thread-safe), but object access is NOT
std::shared_ptr<int> p3{new int{10}};  // uses non-array new

// Thread-safe: refcount operations are atomic
// NOT thread-safe: accessing the pointed-to object

#include <memory>
struct Node {
    std::shared_ptr<Node> next;
    std::weak_ptr<Node> prev;  // weak to avoid cycle
};

auto a = std::make_shared<Node>();
auto b = std::make_shared<Node>();
a->next = b;
b->prev = a;  // weak_ptr from shared_ptr

// To use a weak_ptr, lock() into a shared_ptr
if (auto locked = b->prev.lock()) {
    std::cout << "prev exists";
} else {
    std::cout << "prev expired";
}

#include <memory>
// C++17: shared_ptr supports arrays
std::shared_ptr<int[]> arr1(new int[10]);
arr1[0] = 42;

// unique_ptr with arrays (partial specialization exists)
std::unique_ptr<int[]> arr2 = std::make_unique<int[]>(10);
arr2[5] = 100;

// Prefer std::array or std::vector over raw arrays
#include <vector>
std::vector<int> v(10);  // better choice

#include <memory>
class Widget : public std::enable_shared_from_this<Widget> {
public:
    std::shared_ptr<Widget> getPtr() {
        return shared_from_this();  // safe
        // return shared_ptr<Widget>(this);  // BAD: double delete
    }
};

auto w = std::make_shared<Widget>();
auto w2 = w->getPtr();  // shares ownership, refcount = 2

int x = 10;          // x is an lvalue
int& lref = x;       // lvalue reference
int&& rref = 20;     // rvalue reference (binds to temporary)
// int& bad = 20;    // ERROR: can't bind lref to rvalue
const int& cref = 20;  // OK: const lref binds to rvalue

int y = x + 5;       // (x + 5) is an rvalue (prvalue)
std::string s1 = "hi";
std::string&& rr = std::move(s1);  // rr is named rvalue

class Buffer {
    int* data;
    size_t size;
public:
    // Move constructor: steal resources
    Buffer(Buffer&& other) noexcept
        : data(other.data), size(other.size) {
        other.data = nullptr;
        other.size = 0;
    }
    // Move assignment
    Buffer& operator=(Buffer&& other) noexcept {
        if (this != &other) {
            delete[] data;
            data = other.data; size = other.size;
            other.data = nullptr; other.size = 0;
        }
        return *this;
    }
};

Buffer a(100);
Buffer b = std::move(a);  // calls move ctor, a is now empty

#include <utility>
template <typename T, typename Arg>
auto make_unique(Arg&& arg) {
    return std::unique_ptr<T>(new T(std::forward<Arg>(arg)));
}

template <typename... Args>
void log(Args&&... args) {
    // std::forward preserves value category
    log_impl(std::forward<Args>(args)...);
}

std::string s = "hello";
log(s);              // Arg = std::string&  (lvalue)
log(std::string());  // Arg = std::string&& (rvalue)

// Rule of Five: if you define any of these, define all 5
class Resource {
    int* data;
public:
    Resource(size_t n) : data(new int[n]) {}
    ~Resource() { delete[] data; }
    Resource(const Resource& o);             // copy ctor
    Resource& operator=(const Resource& o);  // copy assign
    Resource(Resource&& o) noexcept;         // move ctor
    Resource& operator=(Resource&& o) noexcept;  // move assign
};

// Rule of Zero: prefer to use RAII types
class Better {
    std::vector<int> data;  // handles everything
public:
    Better() = default;  // compiler-generated funcs are correct
};

std::vector<int> makeVec() {
    std::vector<int> v;
    v.push_back(1); v.push_back(2);
    return v;  // NRVO: no copy, no move!
}

std::string makeStr() {
    return std::string("hello");  // RVO
}

// C++17 guarantees RVO (mandatory copy elision) for prvalues
std::vector<int> v = makeVec();  // constructed in place

// Don't std::move return values—it disables RVO!
std::vector<int> bad() {
    std::vector<int> v;
    return std::move(v);  // WORSE: forces move, blocks NRVO
}

#include <thread>
#include <iostream>

void worker(int id) {
    std::cout << "Thread " << id << "\n";
}

int main() {
    std::thread t1(worker, 1);
    std::thread t2([]{ std::cout << "Lambda thread\n"; });

    t1.join();   // wait for t1 to finish
    t2.join();   // must join or detach before destruction
    // t1.detach();  // runs independently (risky)

    // Hardware concurrency hint
    unsigned n = std::thread::hardware_concurrency();
}

#include <mutex>
#include <vector>

std::mutex mtx;
std::vector<int> shared;

void safePush(int val) {
    // RAII lock: unlocks even if exception thrown
    std::lock_guard<std::mutex> lock(mtx);
    shared.push_back(val);
}  // lock released here

// std::scoped_lock (C++17) locks multiple mutexes deadlock-free
std::mutex m1, m2;
void transfer() {
    std::scoped_lock lock(m1, m2);  // atomic
}

// std::unique_lock: lockable/unlockable, movable
std::unique_lock<std::mutex> ul(mtx);
ul.unlock();
ul.lock();

#include <condition_variable>
#include <queue>
#include <thread>

std::mutex mtx;
std::condition_variable cv;
std::queue<int> tasks;

void producer() {
    {
        std::lock_guard<std::mutex> lock(mtx);
        tasks.push(42);
    }
    cv.notify_one();  // wake one waiting consumer
}

void consumer() {
    std::unique_lock<std::mutex> lock(mtx);
    cv.wait(lock, []{ return !tasks.empty(); });  // predicate prevents spurious wakeup
    int task = tasks.front();
    tasks.pop();
}

#include <future>
#include <iostream>

int slowComputation() {
    std::this_thread::sleep_for(std::chrono::seconds(1));
    return 42;
}

// async policy: may run in new thread or deferred
auto fut = std::async(std::launch::async, slowComputation);
// do other work...
int result = fut.get();  // blocks until ready

// std::launch::async: definitely new thread
// std::launch::deferred: lazy, runs on get()
// std::launch::async | deferred: implementation chooses

// std::promise for manual control
std::promise<int> p;
std::future<int> f = p.get_future();
std::thread([&p]{ p.set_value(10); }).detach();
f.get();  // 10

#include <atomic>
#include <iostream>

std::atomic<int> counter{0};
std::atomic<bool> ready{false};

void worker() {
    while (!ready.load(std::memory_order_acquire));
    counter.fetch_add(1, std::memory_order_relaxed);
}

// Compare-and-swap (CAS) loop
std::atomic<int> val{0};
int expected = 0;
bool success = val.compare_exchange_weak(
    expected, 1,
    std::memory_order_acq_rel);

// Atomic is lock-free for most primitive types
static_assert(std::atomic<int>::is_always_lock_free);

// std::atomic_flag: guaranteed lock-free, for spinlocks
std::atomic_flag spin = ATOMIC_FLAG_INIT;
while (spin.test_and_set(std::memory_order_acquire));  // acquire
spin.clear(std::memory_order_release);  // release

// Primary template
template <typename T>
struct TypeName {
    static const char* get() { return "unknown"; }
};

// Full specialization for int
template <>
struct TypeName<int> {
    static const char* get() { return "int"; }
};

// Full specialization for const char*
template <>
struct TypeName<const char*> {
    static const char* get() { return "string"; }
};

std::cout << TypeName<int>::get();           // "int"
std::cout << TypeName<double>::get();        // "unknown"
std::cout << TypeName<const char*>::get();   // "string"

#include <type_traits>

// SFINAE: Substitution Failure Is Not An Error
// Only enable this overload if T is integral
template <typename T,
          typename = std::enable_if_t<std::is_integral_v<T>>>
void process(T x) { std::cout << "integral\n"; }

// Only enable if T is floating point
template <typename T,
          std::enable_if_t<std::is_floating_point_v<T>, int> = 0>
void process(T x) { std::cout << "floating\n"; }

process(10);    // integral
process(3.14);  // floating

// void_t trick (C++17) for detecting member existence
template <typename T, typename = void>
struct has_size : std::false_type {};
template <typename T>
struct has_size<T, std::void_t<decltype(std::declval<T>().size())>>
    : std::true_type {};

#include <type_traits>
template <typename T>
auto getValue(T t) {
    if constexpr (std::is_pointer_v<T>) {
        return *t;  // only compiled if T is a pointer
    } else if constexpr (std::is_integral_v<T>) {
        return t * 2;
    } else {
        return t;
    }
}

int x = 5;
int* p = &x;
getValue(x);   // 10
getValue(p);   // 5
getValue(3.14);  // 3.14

// Compile-time recursion with termination
template <int N>
constexpr int factorial() {
    if constexpr (N <= 1) return 1;
    else return N * factorial<N - 1>();
}

#include <iostream>

// Parameter pack
template <typename... Args>
void print(Args... args) {
    // C++17 fold expression
    ((std::cout << args << " "), ...);
    std::cout << "\n";
}

print(1, "hello", 3.14, 'x');  // 1 hello 3.14 x

// Sum with fold
template <typename... T>
auto sum(T... args) {
    return (args + ...);  // binary fold: ((a+b)+c)+d
}

// Sum with initial value
template <typename... T>
auto sumFrom0(T... args) {
    return (0 + ... + args);  // left fold with init
}

// Base case recursion (pre-C++17)
template <typename T>
void printOne(T t) { std::cout << t; }

#include <concepts>

// Define a concept
template <typename T>
concept Numeric = std::integral<T> || std::floating_point<T>;

// Use as constraint
template <Numeric T>
T add(T a, T b) { return a + b; }

// Requires clause
template <typename T>
requires requires(T t) { t.size(); }
auto getSize(const T& t) { return t.size(); }

// Concepts in abbreviated function templates
void process(std::integral auto x) { /* ... */ }
void process(std::ranges::range auto& r) { /* ... */ }

// Standard concepts: integral, floating_point, equality_comparable,
// totally_ordered, copyable, movable, default_initializable, etc.

int x = 10, y = 20;
std::vector<int> v;

auto a = [] { /* no capture */ };
auto b = [x] { return x; };           // capture x by value
auto c = [&x] { x = 100; };           // capture x by reference
auto d = [=] { return x + y; };       // capture all by value
auto e = [&] { x = 1; y = 2; };       // capture all by reference
auto f = [x, &y] { return x + y; };   // mixed
auto g = [=, &x] { x = 1; return y; }; // default value, x by ref
auto h = [this] { return member; };   // capture this (C++17: *this)
auto i = [x = x + 5] { return x; };   // init capture (C++14)

// Best practice: capture only what you need, explicitly

// auto parameters (C++14)
auto add = [](auto a, auto b) { return a + b; };
add(1, 2);        // int
add(1.5, 2.5);    // double
add(std::string("a"), std::string("b"));

// Template lambdas (C++20)
auto getSize = []<typename T>(const std::vector<T>& v) {
    return v.size();
};

// Perfect forwarding in lambdas (C++20)
auto wrapper = []<typename T>(T&& arg) {
    return process(std::forward<T>(arg));
};

// Recursive lambda with std::function
#include <functional>
std::function<int(int)> fib = [&](int n) {
    return n < 2 ? n : fib(n - 1) + fib(n - 2);
};

#include <functional>
int counter = 0;

// Without mutable, value-captured vars are const
auto inc = [counter]() mutable {
    return ++counter;  // modifies the captured copy
};
inc(); inc();
std::cout << counter;  // still 0 (original unchanged)

// Stateful counter with std::function
auto makeCounter(int start) {
    return [count = start]() mutable { return ++count; };
}
auto c = makeCounter(10);
c(); c();  // 11, 12

// Lambda as callback storage
std::vector<std::function<void()>> callbacks;
callbacks.push_back([&]{ std::cout << "click"; });

#include <algorithm>
#include <vector>

std::vector<int> v = {5, 2, 8, 1, 9, 3};

// Sort descending
std::sort(v.begin(), v.end(), [](int a, int b) {
    return a > b;
});

// Find first even
auto it = std::find_if(v.begin(), v.end(),
    [](int x) { return x % 2 == 0; });

// Remove odds (erase-remove idiom)
v.erase(std::remove_if(v.begin(), v.end(),
    [](int x) { return x % 2 != 0; }), v.end());

// Transform
std::transform(v.begin(), v.end(), v.begin(),
    [](int x) { return x * x; });

// C++20 ranges: cleaner
auto evens = v | std::views::filter([](int x){ return x%2==0; })
               | std::views::transform([](int x){ return x*2; });

#include <functional>
// Function returning a function
auto makeMultiplier(int factor) {
    return [factor](int x) { return x * factor; };
}
auto doubleIt = makeMultiplier(2);
auto tripleIt = makeMultiplier(3);
doubleIt(5);  // 10
tripleIt(5);  // 15

// Function taking a function
template <typename F>
void applyTwice(int x, F f) {
    std::cout << f(f(x));
}
applyTwice(5, [](int x){ return x + 3; });  // 11

// Composition
auto compose = [](auto f, auto g) {
    return [f, g](auto x) { return f(g(x)); };
};
auto addOne = [](int x){ return x + 1; };
auto square = [](int x){ return x * x; };
auto sqThenAdd = compose(addOne, square);
sqThenAdd(3);  // 10

namespace math {
    double pi = 3.14159;
    double square(double x) { return x * x; }

    namespace geometry {  // nested
        double circleArea(double r) { return pi * square(r); }
    }
}

// Usage
double a = math::square(5);
double area = math::geometry::circleArea(2.0);

// Namespace alias
namespace geo = math::geometry;
geo::circleArea(1.0);

// Anonymous namespace: internal linkage (like 'static')
namespace {
    int internalVar = 42;  // only visible in this file
}

#include <iostream>
#include <vector>

// using declaration: brings one name
using std::cout;
using std::vector;
cout << "hello";
vector<int> v;

// using directive: brings entire namespace
using namespace std;  // brings ALL of std
// AVOID in headers! Pollutes global namespace.

// Namespace-scoped using (safe)
namespace mylib {
    using std::string;  // only affects mylib
    using std::vector;
    string s;
}

// C++20: using enum
enum class Color { Red, Green, Blue };
void print() {
    using enum Color;
    auto c = Red;  // no Color:: needed here
}

namespace mylib {
    struct Widget {
        int value;
    };
    // Operator overload found via ADL
    std::ostream& operator<<(std::ostream& os, const Widget& w) {
        return os << "Widget(" << w.value << ")";
    }

    void helper(const Widget&) {}
}

int main() {
    mylib::Widget w{42};
    // ADL: finds operator<< in mylib without qualification
    std::cout << w;  // works! no mylib:: needed

    // ADL applies to free functions too
    helper(w);  // wait—needs mylib::helper unless ADL applies
    mylib::helper(w);  // explicit
}

// swap is the classic ADL use case
namespace ns { struct X {};
    void swap(X&, X&) {}  // customized swap
}
void f(ns::X& a, ns::X& b) {
    using std::swap;
    swap(a, b);  // ADL picks ns::swap if available
}

// Inline namespace members are part of the enclosing namespace
namespace mylib {
    inline namespace v2 {
        struct Widget { int x, y, z; };  // newer version
        void process(Widget) {}
    }

    namespace v1 {
        struct Widget { int x, y; };  // older version
    }
}

// v2::Widget is accessible as mylib::Widget (inline)
mylib::Widget w;  // actually mylib::v2::Widget
mylib::process(w);

// Explicitly use v1 if needed
mylib::v1::Widget oldW;

// ABI versioning: change which namespace is inline
// to change default version without breaking old code

// math.cppm (module interface unit)
export module math;

export double pi = 3.14159;

export double square(double x) {
    return x * x;
}

// Internal (not exported)
double internalHelper(double x) {
    return x * 2;
}

// main.cpp
import math;
import std;  // standard library module (C++23)

int main() {
    return square(pi);  // 9.87
    // internalHelper(3);  // ERROR: not exported
}

// Module partitions (sub-modules)
export module math:geometry;
export double circleArea(double r);

// Traditional include guard (header.h)
#ifndef MY_HEADER_H
#define MY_HEADER_H

// declarations here
class Widget { /* ... */ };

#endif  // MY_HEADER_H

// Alternative: #pragma once (non-standard but widely supported)
#pragma once

class Widget { /* ... */ };

// #pragma once pros: simpler, no risk of macro name collision
// Include guards pros: standard, works everywhere
// Both prevent multiple inclusion in a single TU

// Basic macro
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define SQUARE(x) ((x) * (x))

int m = MAX(3, 5);      // ((3) > (5) ? (3) : (5))
int s = SQUARE(4);      // ((4) * (4))

// ALWAYS parenthesize arguments!
#define BAD(x) x * x
BAD(1 + 2);  // 1 + 2 * 1 + 2 = 5, not 9!

// Multi-line macro with do-while
#define LOG(msg) do { \
    std::cerr << __FILE__ << ":" << __LINE__ \
              << " " << msg << "\n"; \
} while (0)

// Variadic macro (C++11)
#define PRINT(...) printf(__VA_ARGS__)
PRINT("x=%d, y=%d\n", x, y);

// # (stringification) turns a macro argument into a string literal
#define STR(x) #x
#define XSTR(x) STR(x)
const char* s1 = STR(hello world);  // "hello world"
const char* s2 = STR(42);           // "42"

// Two-level macro to expand macros before stringifying
#define VERSION 100
const char* v1 = STR(VERSION);   // "VERSION"
const char* v2 = XSTR(VERSION);  // "100"

// ## (token pasting) concatenates tokens
#define CONCAT(a, b) a ## b
#define MAKE_VAR(n) var_ ## n
int MAKE_VAR(1) = 10;  // int var_1 = 10;
int CONCAT(foo, bar) = 5;  // int foobar = 5;

// Useful for generating unique names
#define UNIQUE(prefix) prefix ## __LINE__
int UNIQUE(tmp_) = 0;  // int tmp_42 = 0; (if on line 42)

#define DEBUG 1
#define PLATFORM "windows"

#if DEBUG
    std::cout << "Debug mode\n";
#endif

#ifdef DEBUG
    logDebug("entered function");
#endif

#ifndef NDEBUG
    assert(ptr != nullptr);
#endif

#if defined(_WIN32) || defined(_WIN64)
    #include <windows.h>
#elif defined(__linux__)
    #include <unistd.h>
#elif defined(__APPLE__)
    #include <TargetConditionals.h>
#endif

// Check C++ standard
#if __cplusplus >= 202002L
    #define CPP20_OR_LATER
#elif __cplusplus >= 201703L
    #define CPP17_OR_LATER
#endif

// #error and #warning
#if !defined(VERSION)
    #error "VERSION must be defined"
#endif

// Standard predefined macros
std::cout << __FILE__;       // source file path
std::cout << __LINE__;       // current line number
std::cout << __DATE__;       // compilation date "Mmm dd yyyy"
std::cout << __TIME__;       // compilation time "hh:mm:ss"
std::cout << __cplusplus;    // C++ standard version
std::cout << __func__;       // current function name (not macro)

// Compiler-specific
#ifdef __GNUC__
    std::cout << __GNUC__ << "." << __GNUC_MINOR__;
#endif
#ifdef _MSC_VER
    std::cout << "MSVC " << _MSC_VER;
#endif

// __VA_OPT__ (C++20): expands to its arg if variadic has args
#define LOG(fmt, ...) \
    printf(fmt __VA_OPT__(,) __VA_ARGS__)
LOG("plain");              // printf("plain")
LOG("x=%d", x);            // printf("x=%d", x)

// __has_include (C++17): check if header exists
#if __has_include(<optional>)
    #include <optional>
#endif

class Logger {
public:
    static Logger& instance() {
        // Meyers' singleton: thread-safe in C++11+
        static Logger inst;
        return inst;
    }
    void log(const std::string& msg) {
        std::cout << "[LOG] " << msg << "\n";
    }
    Logger(const Logger&) = delete;
    Logger& operator=(const Logger&) = delete;
private:
    Logger() = default;
};

// Usage
Logger::instance().log("started");

// Avoid: lazy init with new + manual sync (error-prone)
// The static local is initialized once, thread-safely

// RAII: resource tied to object lifetime
class FileHandle {
    FILE* fp;
public:
    explicit FileHandle(const char* path) : fp(fopen(path, "r")) {
        if (!fp) throw std::runtime_error("open failed");
    }
    ~FileHandle() { if (fp) fclose(fp); }
    FILE* get() { return fp; }
    // disable copy to prevent double-close
    FileHandle(const FileHandle&) = delete;
    FileHandle& operator=(const FileHandle&) = delete;
    // allow move
    FileHandle(FileHandle&& o) noexcept : fp(o.fp) { o.fp = nullptr; }
};

{
    FileHandle f("data.txt");
    // use f.get()
}  // fclose called automatically, even on exception

// widget.h - public interface
class Widget {
public:
    Widget();
    ~Widget();  // must define in .cpp (unique_ptr needs complete type)
    Widget(Widget&&) noexcept;
    Widget& operator=(Widget&&) noexcept;
    void doSomething();
private:
    class Impl;  // forward declaration
    std::unique_ptr<Impl> pimpl;
};

// widget.cpp - implementation
#include "widget.h"
#include <vector>
class Widget::Impl {
public:
    std::vector<int> data;
    void doSomething() { /* ... */ }
};
Widget::Widget() : pimpl(std::make_unique<Impl>()) {}
Widget::~Widget() = default;
Widget::Widget(Widget&&) noexcept = default;
void Widget::doSomething() { pimpl->doSomething(); }

#include <functional>
#include <vector>
#include <string>

class Subject {
    std::vector<std::function<void(const std::string&)>> observers;
public:
    void subscribe(std::function<void(const std::string&)> cb) {
        observers.push_back(cb);
    }
    void notify(const std::string& event) {
        for (auto& cb : observers) cb(event);
    }
};

// Usage
Subject s;
s.subscribe([](const std::string& e){ std::cout << "A: " << e; });
s.subscribe([](const std::string& e){ std::cout << "B: " << e; });
s.notify("hello");  // both callbacks invoked

// For removal, use a token/ID system or observer objects
// instead of std::function

// CRTP: class Derived : public Base<Derived>
template <typename Derived>
struct Comparable {
    bool operator==(const Derived& other) const {
        return static_cast<const Derived*>(this)->equalTo(other);
    }
    bool operator!=(const Derived& other) const {
        return !(*this == other);
    }
};

struct Point : Comparable<Point> {
    int x, y;
    bool equalTo(const Point& o) const { return x == o.x && y == o.y; }
};

Point a{1, 2}, b{1, 2};
a == b;  // true (uses Comparable::operator==)
a != b;  // false

// Static polymorphism (no virtual overhead)
template <typename T>
void draw(const T& shape) {
    static_cast<const T&>(shape).drawImpl();
}

#include <memory>
std::unique_ptr<int> p1 = std::make_unique<int>(42);
// std::unique_ptr<int> p2 = p1;  // Error: cannot copy
std::unique_ptr<int> p3 = std::move(p1);  // OK: transfer ownership
// p1 is now nullptr

auto p1 = std::make_shared<int>(42);
auto p2 = p1;  // OK: shared ownership
std::cout << p1.use_count();  // 2
// Reference counting: deleted when count reaches 0
// Thread-safe for reference count, not for the object

auto shared = std::make_shared<int>(42);
std::weak_ptr<int> weak = shared;  // Does not increase count
if (auto locked = weak.lock()) {  // Try to get shared_ptr
    std::cout << *locked;  // Use the object
} else {
    std::cout << "Object deleted";
}

std::unique_ptr<FILE, decltype(&fclose)> file(fopen("test.txt", "r"), fclose);
// Or with lambda
auto deleter = [](FILE* f) { if (f) fclose(f); };
std::unique_ptr<FILE, decltype(deleter)> file2(fopen("test.txt", "r"), deleter);

class Node : public std::enable_shared_from_this<Node> {
public:
    std::shared_ptr<Node> getPtr() {
        return shared_from_this();  // Safe
        // return std::shared_ptr<Node>(this);  // BUG: double delete
    }
};

class Buffer {
    int* data; size_t size;
public:
    Buffer(Buffer&& other) noexcept : data(other.data), size(other.size) {
        other.data = nullptr; other.size = 0;  // Leave in valid state
    }
    Buffer& operator=(Buffer&& other) noexcept {
        delete[] data;
        data = other.data; size = other.size;
        other.data = nullptr; other.size = 0;
        return *this;
    }
};

std::string s1 = "Hello";
std::string s2 = std::move(s1);  // s1 is now empty
// std::move does not move anything!
// It casts to an rvalue reference, enabling move
// The actual move happens in the constructor/assignment

template<typename T, typename Arg>
auto make(Arg&& arg) {
    return std::make_shared<T>(std::forward<Arg>(arg));
}
// std::forward preserves value category:
// lvalue stays lvalue, rvalue stays rvalue
// Arg&& is a forwarding reference (not rvalue reference)

std::string create() {
    std::string s = "Hello";
    return s;  // NRVO: no copy, no move
}
std::string s = create();  // RVO: no copy
// With -O2, compilers eliminate the copy/move entirely
// Do NOT use std::move on return of local variable!

class Resource {
public:
    Resource();  // Constructor
    ~Resource();  // Destructor
    Resource(const Resource&);  // Copy constructor
    Resource& operator=(const Resource&);  // Copy assignment
    Resource(Resource&&) noexcept;  // Move constructor
    Resource& operator=(Resource&&) noexcept;  // Move assignment
};

template<typename... Args>
void print(Args... args) {
    (std::cout << ... << args) << '\n';  // C++17 fold expression
}
print(1, "hello", 3.14, 'a');  // 1hello3.14a
// Recursive unpacking (pre-C++17)
template<typename T, typename... Rest>
void print(T first, Rest... rest) {
    std::cout << first;
    print(rest...);
}

template<typename T,
         typename = std::enable_if_t<std::is_integral_v<T>>>
void process(T value) { /* integer version */ }
template<typename T,
         typename = std::enable_if_t<std::is_floating_point_v<T>>,
         typename = void>  // Different signature
void process(T value) { /* float version */ }

template<typename T>
auto get_value(T t) {
    if constexpr (std::is_pointer_v<T>)
        return *t;
    else
        return t;
}
// Only the matching branch is compiled
// No SFINAE needed

template<typename T>
concept Number = std::integral<T> || std::floating_point<T>;
template<Number T>
T add(T a, T b) { return a + b; }
// Or: requires clause
template<typename T> requires Number<T>
T multiply(T a, T b) { return a * b; }

template<typename T>
class Vector { /* general implementation */ };
template<>
class Vector<bool> {  // Full specialization
    // Bit-packed implementation
};
template<typename T>
class Vector<T*> {  // Partial specialization for pointers
    // Pointer-specific implementation
};

std::vector<int> v = {3, 1, 4, 1, 5, 9};
std::sort(v.begin(), v.end());  // 1,1,3,4,5,9
std::sort(v.begin(), v.end(), std::greater<>());  // Descending
// Custom comparator
std::sort(v.begin(), v.end(), [](int a, int b) {
    return a % 10 < b % 10;  // Sort by last digit
});

std::vector<int> v = {1, 2, 3, 4};
std::vector<int> squared(v.size());
std::transform(v.begin(), v.end(), squared.begin(),
    [](int x) { return x * x; });  // 1,4,9,16
int sum = std::accumulate(v.begin(), v.end(), 0);  // 10
int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<>());

std::vector<int> v = {1, 2, 3, 2, 1};
auto it = std::find(v.begin(), v.end(), 2);  // First 2
size_t cnt = std::count(v.begin(), v.end(), 2);  // 2
auto it2 = std::find_if(v.begin(), v.end(),
    [](int x) { return x > 2; });  // First > 2

std::vector<int> v = {1, 2, 3, 4, 5};
std::vector<int> evens;
std::copy_if(v.begin(), v.end(), std::back_inserter(evens),
    [](int x) { return x % 2 == 0; });  // 2,4
// Remove-erase idiom
v.erase(std::remove(v.begin(), v.end(), 3), v.end());
// C++20: std::erase(v, 3);

auto [min, max] = std::minmax({3, 1, 4, 1, 5});  // C++17
int value = std::clamp(15, 0, 10);  // 10 (clamped to max)
int m = std::min({1, 2, 3});
int m2 = std::max({1, 2, 3}, [](int a, int b) { return a < b; });

#include <thread>
void task(int n) { /* ... */ }
std::thread t1(task, 42);
std::thread t2([]() { /* lambda */ });
t1.join();  // Wait for completion
t2.detach();  // Run independently
// Always join or detach before destruction

std::mutex mtx;
int shared = 0;
void increment() {
    std::lock_guard<std::mutex> lock(mtx);  // RAII
    ++shared;
}  // Auto-unlock
// std::unique_lock for conditional locking
std::unique_lock<std::mutex> ulock(mtx);
ulock.unlock();  // Manual unlock
ulock.lock();    // Re-lock

#include <future>
std::future<int> f = std::async(std::launch::async, []() {
    return 42;
});
int result = f.get();  // Blocks until ready
// std::launch::async: always new thread
// std::launch::deferred: lazy, runs on get()

std::mutex mtx;
std::condition_variable cv;
bool ready = false;
// Waiter
std::unique_lock<std::mutex> lock(mtx);
cv.wait(lock, []() { return ready; });  // Spurious wakeups handled
// Notifier
{
    std::lock_guard<std::mutex> lk(mtx);
    ready = true;
}
cv.notify_one();  // Or notify_all()

#include <atomic>
std::atomic<int> counter{0};
counter++;  // Atomic increment
counter.fetch_add(1, std::memory_order_relaxed);
bool expected = false;
counter.compare_exchange_strong(expected, true);
// Memory orders: relaxed, acquire, release, seq_cst

std::pair p = {1, "hello"};
auto [num, str] = p;  // C++17
std::map<std::string, int> m = {{"a", 1}, {"b", 2}};
for (const auto& [key, value] : m) {
    std::cout << key << ": " << value;
}

#include <optional>
std::optional<int> find(bool found) {
    if (found) return 42;
    return std::nullopt;
}
auto result = find(true);
if (result) std::cout << *result;
// Or: result.value_or(0)

#include <variant>
std::variant<int, std::string> v;
v = 42;
v = "hello";
std::visit([](auto&& arg) {
    using T = std::decay_t<decltype(arg)>;
    if constexpr (std::is_same_v<T, int>)
        std::cout << "int: " << arg;
    else if constexpr (std::is_same_v<T, std::string>)
        std::cout << "string: " << arg;
}, v);

#include <ranges>
namespace rv = std::ranges::views;
auto result = std::vector{1, 2, 3, 4, 5}
    | rv::filter([](int x) { return x % 2 == 0; })
    | rv::transform([](int x) { return x * x; });
// Lazy evaluation: no computation until iterated
for (int x : result) std::cout << x;  // 4, 16

#include <coroutine>
Generator<int> counter() {
    for (int i = 0; ; ++i)
        co_yield i;  // Suspend and yield
}
for (int x : counter()) {
    if (x > 5) break;
    std::cout << x;
}
// co_await: wait for async operation
// co_return: finish coroutine

// BUG: dangling pointer
int* p = new int(42);
delete p;
std::cout << *p;  // Undefined behavior
// FIX: use smart pointers
auto p = std::make_unique<int>(42);
// Or set to nullptr after delete
int* p2 = new int(42);
delete p2; p2 = nullptr;

std::vector<int> v = {1, 2, 3};
// BUG: iterator invalidated
for (auto it = v.begin(); it != v.end(); ++it) {
    if (*it == 2) v.push_back(4);  // May invalidate it
}
// FIX: use index
for (size_t i = 0; i < v.size(); ++i) {
    if (v[i] == 2) v.push_back(4);
}

int x;  // Uninitialized: UB to read
std::cout << x;  // UB
int arr[5];
arr[5] = 0;  // Out of bounds: UB
int* p = nullptr;
*p;  // Null dereference: UB
// Signed overflow: UB (unsigned wraps)
int big = INT_MAX + 1;  // UB

class Base { public: virtual ~Base() {} };
class Derived : public Base { int extra; };
void take(Base b) { /* copies Base part only */ }
Derived d;
take(d);  // Sliced: extra lost
// FIX: pass by reference or pointer
void take(const Base& b) { /* full object */ }

// Static initialization order fiasco
// File1.cpp
int x = computeX();  // May use y
// File2.cpp
int y = computeY();  // May use x
// Order across files is undefined!
// FIX: function-local static
int& getX() {
    static int x = computeX();  // Lazy, thread-safe
    return x;
}