# C++ Quick Reference

*Classes, templates, STL, smart pointers, modern C++ essentials*

> Source: C++ Reference (cppreference.com) · MIT

## Basics

### Hello World

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

### Compile & Run

```
g++ -std=c++20 -Wall -o app main.cpp
./app
clang++ -std=c++20 -o app main.cpp
```

### Variables & Constants

```
int x = 42;
auto y = 3.14;             // type deduction
const int MAX = 100;
constexpr int SIZE = 256;  // compile-time constant
```

### Namespaces

```
namespace math {
    double pi = 3.14159;
}
using namespace std;       // use sparingly
using std::cout;           // prefer selective
```

## Classes

### Class Definition

```
class Rectangle {
    double w_, h_;
public:
    Rectangle(double w, double h) : w_(w), h_(h) {}
    double area() const { return w_ * h_; }  };
```

### Inheritance

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

### Access Specifiers

| Command | Description |
|---------|-------------|
| `public` | Accessible from anywhere |
| `protected` | Accessible in class and derived classes |
| `private` | Accessible only within the class |
| `friend` | Grant access to specific function or class |

### Special Members

| Command | Description |
|---------|-------------|
| `Constructor` | `MyClass(args)` — initialize object |
| `Destructor` | `~MyClass()` — clean up resources |
| `Copy ctor` | `MyClass(const MyClass&)` |
| `Move ctor` | `MyClass(MyClass&&)` — transfer ownership |
| `Copy assign` | `operator=(const MyClass&)` |
| `Move assign` | `operator=(MyClass&&)` |

## Templates

### Function Template

```
template <typename T>
T max_val(T a, T b) {
    return (a > b) ? a : b;
}
auto result = max_val(3, 7);  // deduced as int
```

### Class Template

```
template <typename T>
class Stack {
    std::vector<T> data_;
public:
    void push(const T& v) { data_.push_back(v); }  };
```

### Concepts (C++20)

```
template <typename T>
concept Numeric = std::integral<T> || std::floating_point<T>;
template <Numeric T>
T add(T a, T b) { return a + b; }
```

## STL Containers

### Sequence Containers

| Command | Description |
|---------|-------------|
| `vector<T>` | Dynamic array, fast random access |
| `deque<T>` | Double-ended queue |
| `list<T>` | Doubly-linked list |
| `array<T,N>` | Fixed-size array (compile-time size) |
| `forward_list<T>` | Singly-linked list |

### Associative Containers

| Command | Description |
|---------|-------------|
| `map<K,V>` | Ordered key-value pairs (red-black tree) |
| `set<T>` | Ordered unique elements |
| `unordered_map<K,V>` | Hash map, O(1) average lookup |
| `unordered_set<T>` | Hash set, O(1) average lookup |
| `multimap<K,V>` | Ordered, allows duplicate keys |

### Vector Operations

```
std::vector<int> v = {1, 2, 3};
v.push_back(4);
v.emplace_back(5);         // construct in place
v.size();  v.empty();
v[0];  v.at(0);            // at() has bounds check
```

## Iterators & Algorithms

### Iterator Usage

```
std::vector<int> v = {3, 1, 4, 1, 5};
for (auto it = v.begin(); it != v.end(); ++it) {
    std::cout << *it << " ";
}
for (const auto& val : v) { }  // range-based for
```

### Common Algorithms

| Command | Description |
|---------|-------------|
| `sort(begin, end)` | Sort elements in ascending order |
| `find(begin, end, val)` | Find first occurrence of value |
| `count(begin, end, val)` | Count occurrences of value |
| `transform(b, e, out, fn)` | Apply function to each element |
| `accumulate(b, e, init)` | Reduce elements (sum by default) |
| `reverse(begin, end)` | Reverse element order |
| `unique(begin, end)` | Remove consecutive duplicates |

### Ranges (C++20)

```
namespace rv = std::views;
auto evens = v | rv::filter([](int n){ return n % 2 == 0; })
               | rv::transform([](int n){ return n * n; });
```

## Smart Pointers

### unique_ptr

```
auto p = std::make_unique<int>(42);
std::cout << *p << std::endl;
// auto-deleted when out of scope
// cannot be copied, only moved
```

### shared_ptr

```
auto sp = std::make_shared<std::string>("hello");
auto sp2 = sp;               // reference count: 2
std::cout << sp.use_count(); // 2
```

### Comparison

| Command | Description |
|---------|-------------|
| `unique_ptr<T>` | Exclusive ownership, zero overhead |
| `shared_ptr<T>` | Shared ownership via reference counting |
| `weak_ptr<T>` | Non-owning observer of `shared_ptr` |
| `make_unique<T>()` | Preferred way to create `unique_ptr` |
| `make_shared<T>()` | Preferred way to create `shared_ptr` |

## Lambdas

### Lambda Syntax

```
auto add = [](int a, int b) { return a + b; };
int sum = add(3, 4);         // 7
```

### Capture Modes

| Command | Description |
|---------|-------------|
| `[x]` | Capture `x` by value (copy) |
| `[&x]` | Capture `x` by reference |
| `[=]` | Capture all used variables by value |
| `[&]` | Capture all used variables by reference |
| `[=, &x]` | All by value, `x` by reference |
| `[this]` | Capture enclosing object pointer |

### Lambda with STL

```
std::vector<int> v = {5, 2, 8, 1};
std::sort(v.begin(), v.end(),
    [](int a, int b) { return a > b; });  // descending
auto it = std::find_if(v.begin(), v.end(),
    [](int n) { return n > 3; });
```

## Strings & I/O

### std::string

```
std::string s = "hello";
s += " world";              // concatenation
s.substr(0, 5);             // "hello"
s.find("world");            // 6 (position)
s.length();  s.empty();
```

### String Conversions

| Command | Description |
|---------|-------------|
| `std::to_string(42)` | Number to string |
| `std::stoi(s)` | String to `int` |
| `std::stod(s)` | String to `double` |
| `std::stol(s)` | String to `long` |

### I/O Streams

```
std::cout << "output" << std::endl;
std::cin >> variable;
std::getline(std::cin, line);
```

### File I/O

```
std::ofstream out("file.txt");
out << "hello" << std::endl;
std::ifstream in("file.txt");
std::string line;
while (std::getline(in, line)) { }
```

## Error Handling

### Exceptions

```
try {
    throw std::runtime_error("something failed");
} catch (const std::exception& e) {
    std::cerr << e.what() << std::endl;
} catch (...) { /* unknown error */ }
```

### Standard Exceptions

| Command | Description |
|---------|-------------|
| `std::exception` | Base class for all standard exceptions |
| `std::runtime_error` | Runtime error with message |
| `std::logic_error` | Logic error (pre-condition violation) |
| `std::out_of_range` | Index or iterator out of range |
| `std::invalid_argument` | Invalid function argument |
| `std::bad_alloc` | Memory allocation failure |

### noexcept

```
void safe_func() noexcept {
    // guaranteed not to throw
}
bool can_throw = noexcept(safe_func());  // true
```

## Modern C++ (17/20)

### Structured Bindings (C++17)

```
std::map<std::string, int> m = {{"a", 1}, {"b", 2}};
for (auto& [key, value] : m) {
    std::cout << key << ": " << value << "\n";
}
```

### std::optional (C++17)

```
std::optional<int> find(int id) {
    if (id > 0) return id * 10;
    return std::nullopt;
}
auto val = find(3);             // has_value() == true
```

### std::variant & std::any (C++17)

```
std::variant<int, std::string> v = "hello";
std::cout << std::get<std::string>(v);
std::any a = 42;
int n = std::any_cast<int>(a);
```

### Key Modern Features

| Command | Description |
|---------|-------------|
| `auto` | Type deduction for variables and return types |
| `constexpr` | Compile-time evaluation |
| `if constexpr` | Compile-time conditional (C++17) |
| `std::span<T>` | Non-owning view over contiguous data (C++20) |
| `std::format()` | Type-safe formatting (C++20) |
| `co_await` | Coroutine support (C++20) |