cpp-expert skill

---
name: cpp-expert
version: 1.0.0
description: Expert-level C++ development with modern C++20/23, STL, memory management, and performance
category: languages
tags: [cpp, c++, c++20, c++23, stl, templates, performance, memory]
allowed-tools:
  - Read
  - Write
  - Edit
  - Bash(g++:*, clang++:*, cmake:*, make:*)
---

# C++ Expert

Expert guidance for modern C++ development including C++20/23 features, STL, templates, memory management, and high-performance programming.

## Core Concepts

### Modern C++ Features (C++20/23)
- Concepts and constraints
- Ranges and views
- Coroutines
- Modules
- Three-way comparison (spaceship operator)
- std::format
- std::span
- Designated initializers
- consteval and constinit

### Memory Management
- RAII (Resource Acquisition Is Initialization)
- Smart pointers (unique_ptr, shared_ptr, weak_ptr)
- Move semantics and perfect forwarding
- Memory allocation strategies
- Custom allocators
- Memory pools

### Performance
- Zero-cost abstractions
- Inline optimization
- Template metaprogramming
- Compile-time computation (constexpr)
- Cache-friendly data structures
- SIMD operations

## Modern C++ Syntax

### Concepts (C++20)
```cpp
#include <concepts>
#include <iostream>
#include <vector>

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

template<typename T>
concept Printable = requires(T t, std::ostream& os) {
    { os << t } -> std::same_as<std::ostream&>;
};

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

template<Printable T>
void print(const T& value) {
    std::cout << value << '\n';
}

// Concept with multiple requirements
template<typename T>
concept Container = requires(T container) {
    typename T::value_type;
    { container.begin() } -> std::same_as<typename T::iterator>;
    { container.end() } -> std::same_as<typename T::iterator>;
    { container.size() } -> std::convertible_to<std::size_t>;
};

template<Container C>
void process(const C& container) {
    for (const auto& item : container) {
        std::cout << item << ' ';
    }
}
```

### Ranges and Views (C++20)
```cpp
#include <ranges>
#include <vector>
#include <algorithm>

// Ranges
std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

// Filter and transform with views
auto even_squares = numbers
    | std::views::filter([](int n) { return n % 2 == 0; })
    | std::views::transform([](int n) { return n * n; });

for (int value : even_squares) {
    std::cout << value << ' '; // 4 16 36 64 100
}

// Take first N elements
auto first_three = numbers | std::views::take(3);

// Drop first N elements
auto skip_two = numbers | std::views::drop(2);

// Reverse
auto reversed = numbers | std::views::reverse;

// Join multiple ranges
std::vector<std::vector<int>> nested = {{1, 2}, {3, 4}, {5, 6}};
auto flattened = nested | std::views::join;

// Split string
std::string text = "one,two,three";
auto words = text | std::views::split(',');

// Lazy evaluation - nothing computed yet
auto lazy = numbers
    | std::views::filter([](int n) { return n > 5; })
    | std::views::transform([](int n) { return n * 2; })
    | std::views::take(3);

// Computation happens here
std::vector<int> result(lazy.begin(), lazy.end()); // {12, 14, 16}
```

### Coroutines (C++20)
```cpp
#include <coroutine>
#include <iostream>
#include <stdexcept>

// Generator coroutine
template<typename T>
class Generator {
public:
    struct promise_type {
        T current_value;

        auto get_return_object() {
            return Generator{std::coroutine_handle<promise_type>::from_promise(*this)};
        }

        auto initial_suspend() { return std::suspend_always{}; }
        auto final_suspend() noexcept { return std::suspend_always{}; }

        auto yield_value(T value) {
            current_value = value;
            return std::suspend_always{};
        }

        void return_void() {}
        void unhandled_exception() { std::terminate(); }
    };

    explicit Generator(std::coroutine_handle<promise_type> h) : handle(h) {}
    ~Generator() { if (handle) handle.destroy(); }

    bool next() {
        handle.resume();
        return !handle.done();
    }

    T value() const {
        return handle.promise().current_value;
    }

private:
    std::coroutine_handle<promise_type> handle;
};

// Use generator
Generator<int> fibonacci() {
    int a = 0, b = 1;
    while (true) {
        co_yield a;
        auto next = a + b;
        a = b;
        b = next;
    }
}

int main() {
    auto fib = fibonacci();
    for (int i = 0; i < 10; ++i) {
        fib.next();
        std::cout << fib.value() << ' '; // 0 1 1 2 3 5 8 13 21 34
    }
}
```

### Smart Pointers
```cpp
#include <memory>
#include <vector>

class Resource {
    int* data;
public:
    Resource(int size) : data(new int[size]) {
        std::cout << "Resource acquired\n";
    }

    ~Resource() {
        delete[] data;
        std::cout << "Resource released\n";
    }
};

// unique_ptr - exclusive ownership
std::unique_ptr<Resource> create_resource() {
    return std::make_unique<Resource>(100);
}

auto resource = create_resource();
// resource.reset(); // Manually release
// auto copy = resource; // Error: cannot copy unique_ptr
auto moved = std::move(resource); // Transfer ownership

// shared_ptr - shared ownership
std::shared_ptr<Resource> shared = std::make_shared<Resource>(100);
{
    std::shared_ptr<Resource> shared2 = shared; // Reference count = 2
    std::cout << "Use count: " << shared.use_count() << '\n'; // 2
} // shared2 destroyed, ref count = 1
// shared destroyed, Resource released

// weak_ptr - non-owning reference
std::weak_ptr<Resource> weak = shared;
if (auto locked = weak.lock()) {
    // Use resource
}

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

### Move Semantics
```cpp
#include <utility>
#include <vector>

class Buffer {
    int* data;
    size_t size;

public:
    // Constructor
    Buffer(size_t s) : size(s), data(new int[s]) {
        std::cout << "Constructor\n";
    }

    // Destructor
    ~Buffer() {
        delete[] data;
        std::cout << "Destructor\n";
    }

    // Copy constructor
    Buffer(const Buffer& other) : size(other.size), data(new int[other.size]) {
        std::copy(other.data, other.data + size, data);
        std::cout << "Copy constructor\n";
    }

    // Move constructor
    Buffer(Buffer&& other) noexcept : size(other.size), data(other.data) {
        other.data = nullptr;
        other.size = 0;
        std::cout << "Move constructor\n";
    }

    // Copy assignment
    Buffer& operator=(const Buffer& other) {
        if (this != &other) {
            delete[] data;
            size = other.size;
            data = new int[size];
            std::copy(other.data, other.data + size, data);
            std::cout << "Copy assignment\n";
        }
        return *this;
    }

    // Move assignment
    Buffer& operator=(Buffer&& other) noexcept {
        if (this != &other) {
            delete[] data;
            data = other.data;
            size = other.size;
            other.data = nullptr;
            other.size = 0;
            std::cout << "Move assignment\n";
        }
        return *this;
    }
};

// Perfect forwarding
template<typename T, typename... Args>
std::unique_ptr<T> make_unique_custom(Args&&... args) {
    return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
}
```

### Templates and Metaprogramming
```cpp
#include <type_traits>
#include <iostream>

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

// Class template
template<typename T, size_t N>
class Array {
    T data[N];

public:
    constexpr size_t size() const { return N; }

    T& operator[](size_t index) { return data[index]; }
    const T& operator[](size_t index) const { return data[index]; }
};

// Variadic templates
template<typename... Args>
void print(Args... args) {
    ((std::cout << args << ' '), ...); // Fold expression (C++17)
    std::cout << '\n';
}

// SFINAE (Substitution Failure Is Not An Error)
template<typename T>
typename std::enable_if<std::is_integral<T>::value, T>::type
abs(T value) {
    return value < 0 ? -value : value;
}

template<typename T>
typename std::enable_if<std::is_floating_point<T>::value, T>::type
abs(T value) {
    return std::fabs(value);
}

// Type traits
template<typename T>
void check_type() {
    std::cout << std::boolalpha;
    std::cout << "is_integral: " << std::is_integral_v<T> << '\n';
    std::cout << "is_floating_point: " << std::is_floating_point_v<T> << '\n';
    std::cout << "is_pointer: " << std::is_pointer_v<T> << '\n';
}

// Compile-time computation
constexpr int factorial(int n) {
    return n <= 1 ? 1 : n * factorial(n - 1);
}

constexpr int fact10 = factorial(10); // Computed at compile time
```

### std::format (C++20)
```cpp
#include <format>
#include <iostream>

int main() {
    int age = 30;
    std::string name = "Alice";

    // Basic formatting
    std::cout << std::format("Hello, {}!", name) << '\n';

    // Positional arguments
    std::cout << std::format("{1} is {0} years old", age, name) << '\n';

    // Format specifiers
    double pi = 3.14159265359;
    std::cout << std::format("Pi: {:.2f}", pi) << '\n'; // 3.14

    // Width and alignment
    std::cout << std::format("{:<10} {:>10}", "left", "right") << '\n';

    // Numbers
    int num = 42;
    std::cout << std::format("Dec: {0:d}, Hex: {0:x}, Bin: {0:b}", num) << '\n';

    // Padding
    std::cout << std::format("{:0>5}", num) << '\n'; // 00042

    return 0;
}
```

## STL Containers

### Sequential Containers
```cpp
#include <vector>
#include <deque>
#include <list>
#include <array>

// vector - dynamic array
std::vector<int> vec = {1, 2, 3, 4, 5};
vec.push_back(6);
vec.emplace_back(7); // Construct in-place
vec.reserve(100); // Pre-allocate capacity

// deque - double-ended queue
std::deque<int> deq = {1, 2, 3};
deq.push_front(0);
deq.push_back(4);

// list - doubly-linked list
std::list<int> lst = {1, 2, 3};
lst.push_front(0);
lst.push_back(4);
lst.remove(2); // Remove all elements with value 2

// array - fixed-size array
std::array<int, 5> arr = {1, 2, 3, 4, 5};
```

### Associative Containers
```cpp
#include <map>
#include <set>
#include <unordered_map>
#include <unordered_set>

// map - ordered key-value pairs
std::map<std::string, int> ages;
ages["Alice"] = 30;
ages["Bob"] = 25;
ages.insert({"Charlie", 35});

// set - ordered unique elements
std::set<int> numbers = {3, 1, 4, 1, 5, 9};
numbers.insert(2);

// unordered_map - hash table
std::unordered_map<std::string, int> hash_map;
hash_map["key"] = 42;

// unordered_set - hash set
std::unordered_set<int> hash_set = {1, 2, 3};
```

## Algorithms

```cpp
#include <algorithm>
#include <numeric>
#include <vector>

std::vector<int> numbers = {5, 2, 8, 1, 9, 3, 7};

// Sorting
std::sort(numbers.begin(), numbers.end());
std::sort(numbers.begin(), numbers.end(), std::greater<int>());

// Searching
auto it = std::find(numbers.begin(), numbers.end(), 8);
bool found = std::binary_search(numbers.begin(), numbers.end(), 5);

// Transforming
std::vector<int> doubled(numbers.size());
std::transform(numbers.begin(), numbers.end(), doubled.begin(),
    [](int n) { return n * 2; });

// Filtering
std::vector<int> evens;
std::copy_if(numbers.begin(), numbers.end(), std::back_inserter(evens),
    [](int n) { return n % 2 == 0; });

// Accumulate
int sum = std::accumulate(numbers.begin(), numbers.end(), 0);
int product = std::accumulate(numbers.begin(), numbers.end(), 1,
    std::multiplies<int>());

// Partition
auto pivot = std::partition(numbers.begin(), numbers.end(),
    [](int n) { return n < 5; });

// Remove
numbers.erase(std::remove(numbers.begin(), numbers.end(), 5), numbers.end());

// Unique (remove consecutive duplicates)
std::sort(numbers.begin(), numbers.end());
numbers.erase(std::unique(numbers.begin(), numbers.end()), numbers.end());
```

## Concurrency

```cpp
#include <thread>
#include <mutex>
#include <future>
#include <atomic>

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

std::thread t1(worker, 1);
std::thread t2(worker, 2);
t1.join();
t2.join();

// Mutex
std::mutex mtx;
int shared_data = 0;

void increment() {
    std::lock_guard<std::mutex> lock(mtx);
    ++shared_data;
}

// Atomic
std::atomic<int> counter{0};
counter++; // Thread-safe
counter.fetch_add(5);

// Future and promise
std::promise<int> prom;
std::future<int> fut = prom.get_future();

std::thread t([&prom]() {
    std::this_thread::sleep_for(std::chrono::seconds(1));
    prom.set_value(42);
});

int result = fut.get(); // Blocks until ready
t.join();

// async
auto future = std::async(std::launch::async, []() {
    return 42;
});

int value = future.get();
```

## Build Systems

### CMake
```cmake
# CMakeLists.txt
cmake_minimum_required(VERSION 3.20)
project(MyApp VERSION 1.0.0 LANGUAGES CXX)

# Set C++ standard
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# Compiler flags
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
    add_compile_options(-Wall -Wextra -Wpedantic -O3)
endif()

# Find packages
find_package(Threads REQUIRED)
find_package(Boost 1.75 REQUIRED COMPONENTS system filesystem)

# Add executable
add_executable(myapp
    src/main.cpp
    src/module.cpp
    include/module.h
)

# Include directories
target_include_directories(myapp PRIVATE include)

# Link libraries
target_link_libraries(myapp PRIVATE
    Threads::Threads
    Boost::system
    Boost::filesystem
)

# Install
install(TARGETS myapp DESTINATION bin)
```

## Best Practices

### RAII
```cpp
// ❌ Bad: Manual resource management
void process_file() {
    FILE* f = fopen("data.txt", "r");
    // ... work with file
    fclose(f); // Easy to forget
}

// ✅ Good: RAII with smart pointers
void process_file() {
    auto file = std::unique_ptr<FILE, decltype(&fclose)>(
        fopen("data.txt", "r"),
        &fclose
    );
    // ... work with file
    // Automatically closed when leaving scope
}
```

### Const Correctness
```cpp
class Data {
    int value;

public:
    // Const method
    int get_value() const { return value; }

    // Non-const method
    void set_value(int v) { value = v; }
};

// Const reference parameter
void process(const Data& data) {
    int v = data.get_value(); // OK
    // data.set_value(42); // Error: cannot modify const object
}
```

### Rule of Zero/Three/Five
- **Rule of Zero**: If you don't manage resources, don't declare special members
- **Rule of Three**: If you declare destructor, copy constructor, or copy assignment, declare all three
- **Rule of Five**: Add move constructor and move assignment

## Anti-Patterns to Avoid

❌ **Raw pointers for ownership**: Use smart pointers
❌ **Manual memory management**: Use RAII
❌ **Using C-style arrays**: Use std::array or std::vector
❌ **Ignoring const correctness**: Mark everything const that can be
❌ **Unnecessary copies**: Use move semantics and references
❌ **Premature optimization**: Profile before optimizing
❌ **Using `new` without `delete`**: Use smart pointers

## Resources

- C++ Reference: https://en.cppreference.com/
- C++ Core Guidelines: https://isocpp.github.io/CppCoreGuidelines/
- Compiler Explorer: https://godbolt.org/
- CPP Reference: https://cplusplus.com/