memory-safety-patterns

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# Memory Safety Patterns

Cross-language patterns for memory-safe programming including RAII, ownership, smart pointers, and resource management.

## When to Use This Skill

- Writing memory-safe systems code
- Managing resources (files, sockets, memory)
- Preventing use-after-free and leaks
- Implementing RAII patterns
- Choosing between languages for safety
- Debugging memory issues

## Core Concepts

### 1. Memory Bug Categories

| Bug Type | Description | Prevention |
|----------|-------------|------------|
| **Use-after-free** | Access freed memory | Ownership, RAII |
| **Double-free** | Free same memory twice | Smart pointers |
| **Memory leak** | Never free memory | RAII, GC |
| **Buffer overflow** | Write past buffer end | Bounds checking |
| **Dangling pointer** | Pointer to freed memory | Lifetime tracking |
| **Data race** | Concurrent unsynchronized access | Ownership, Sync |

### 2. Safety Spectrum

```
Manual (C) → Smart Pointers (C++) → Ownership (Rust) → GC (Go, Java)
Less safe                                              More safe
More control                                           Less control
```

## Patterns by Language

### Pattern 1: RAII in C++

```cpp
// RAII: Resource Acquisition Is Initialization
// Resource lifetime tied to object lifetime

#include <memory>
#include <fstream>
#include <mutex>

// File handle with RAII
class FileHandle {
public:
    explicit FileHandle(const std::string& path)
        : file_(path) {
        if (!file_.is_open()) {
            throw std::runtime_error("Failed to open file");
        }
    }

    // Destructor automatically closes file
    ~FileHandle() = default; // fstream closes in its destructor

    // Delete copy (prevent double-close)
    FileHandle(const FileHandle&) = delete;
    FileHandle& operator=(const FileHandle&) = delete;

    // Allow move
    FileHandle(FileHandle&&) = default;
    FileHandle& operator=(FileHandle&&) = default;

    void write(const std::string& data) {
        file_ << data;
    }

private:
    std::fstream file_;
};

// Lock guard (RAII for mutexes)
class Database {
public:
    void update(const std::string& key, const std::string& value) {
        std::lock_guard<std::mutex> lock(mutex_); // Released on scope exit
        data_[key] = value;
    }

    std::string get(const std::string& key) {
        std::shared_lock<std::shared_mutex> lock(shared_mutex_);
        return data_[key];
    }

private:
    std::mutex mutex_;
    std::shared_mutex shared_mutex_;
    std::map<std::string, std::string> data_;
};

// Transaction with rollback (RAII)
template<typename T>
class Transaction {
public:
    explicit Transaction(T& target)
        : target_(target), backup_(target), committed_(false) {}

    ~Transaction() {
        if (!committed_) {
            target_ = backup_; // Rollback
        }
    }

    void commit() { committed_ = true; }

    T& get() { return target_; }

private:
    T& target_;
    T backup_;
    bool committed_;
};
```

### Pattern 2: Smart Pointers in C++

```cpp
#include <memory>

// unique_ptr: Single ownership
class Engine {
public:
    void start() { /* ... */ }
};

class Car {
public:
    Car() : engine_(std::make_unique<Engine>()) {}

    void start() {
        engine_->start();
    }

    // Transfer ownership
    std::unique_ptr<Engine> extractEngine() {
        return std::move(engine_);
    }

private:
    std::unique_ptr<Engine> engine_;
};

// shared_ptr: Shared ownership
class Node {
public:
    std::string data;
    std::shared_ptr<Node> next;

    // Use weak_ptr to break cycles
    std::weak_ptr<Node> parent;
};

void sharedPtrExample() {
    auto node1 = std::make_shared<Node>();
    auto node2 = std::make_shared<Node>();

    node1->next = node2;
    node2->parent = node1; // Weak reference prevents cycle

    // Access weak_ptr
    if (auto parent = node2->parent.lock()) {
        // parent is valid shared_ptr
    }
}

// Custom deleter for resources
class Socket {
public:
    static void close(int* fd) {
        if (fd && *fd >= 0) {
            ::close(*fd);

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