异常安全保证

#include <fstream>
#include <string>
#include <stdexcept>
#include <iostream>

class DatabaseConnection {
public:
    explicit DatabaseConnection(const std::string& conn_str)
        : conn_str_(conn_str), connected_(false) {
        // Simulate connection
        if (conn_str.empty()) {
            throw std::invalid_argument("empty connection string");
        }
        connected_ = true;
        std::cout << "Connected to: " << conn_str_ << '\n';
    }

    ~DatabaseConnection() {
        if (connected_) {
            // Cleanup never throws
            std::cout << "Disconnected from: " << conn_str_ << '\n';
            connected_ = false;
        }
    }

    void execute(const std::string& query) {
        if (!connected_) throw std::runtime_error("not connected");
        // Even if this function throws, the destructor
        // will still close the connection during unwinding
        std::cout << "Executing: " << query << '\n';
    }

    // Non-copyable, movable
    DatabaseConnection(const DatabaseConnection&) = delete;
    DatabaseConnection& operator=(const DatabaseConnection&) = delete;
    DatabaseConnection(DatabaseConnection&& other) noexcept
        : conn_str_(std::move(other.conn_str_)),
          connected_(other.connected_) {
        other.connected_ = false;
    }

private:
    std::string conn_str_;
    bool connected_;
};

void process_data() {
    DatabaseConnection db("postgres://localhost/mydb");
    db.execute("SELECT * FROM users");
    // If execute() throws, ~DatabaseConnection() still runs
    // — the connection is always properly closed
}

#include <algorithm>
#include <utility>
#include <cstddef>

class Matrix {
public:
    Matrix(std::size_t rows, std::size_t cols)
        : rows_(rows), cols_(cols),
          data_(new double[rows * cols]{}) {}

    // Copy constructor — may throw (allocates memory)
    Matrix(const Matrix& other)
        : rows_(other.rows_), cols_(other.cols_),
          data_(new double[other.rows_ * other.cols_]) {
        std::copy(other.data_,
                  other.data_ + rows_ * cols_,
                  data_);
    }

    // Move constructor — never throws
    Matrix(Matrix&& other) noexcept
        : rows_(other.rows_), cols_(other.cols_),
          data_(other.data_) {
        other.data_ = nullptr;
        other.rows_ = other.cols_ = 0;
    }

    ~Matrix() { delete[] data_; }

    // No-throw swap — the key ingredient
    friend void swap(Matrix& a, Matrix& b) noexcept {
        using std::swap;
        swap(a.rows_, b.rows_);
        swap(a.cols_, b.cols_);
        swap(a.data_, b.data_);
    }

    // Copy-and-swap assignment: STRONG guarantee
    // Takes parameter by value (invokes copy/move ctor)
    Matrix& operator=(Matrix other) noexcept {
        swap(*this, other);
        return *this;
        // 'other' is destroyed here, freeing old data
    }

private:
    std::size_t rows_, cols_;
    double* data_;
};

#include <vector>
#include <string>
#include <algorithm>
#include <stdexcept>

class UserRegistry {
public:
    // NO-THROW: simple getter, can never fail
    std::size_t size() const noexcept {
        return users_.size();
    }

    // STRONG GUARANTEE: either the user is added or nothing changes
    void add_user(const std::string& name) {
        if (name.empty()) {
            throw std::invalid_argument("name cannot be empty");
        }
        // If push_back throws (e.g., bad_alloc during reallocation),
        // vector guarantees the original data is intact.
        users_.push_back(name);
    }

    // BASIC GUARANTEE: if an exception occurs partway through,
    // some users may have been processed and some may not.
    // But the object is still in a valid state (no leaks, no corruption).
    void normalize_all() {
        for (auto& user : users_) {
            // std::transform could theoretically throw
            // (e.g., if user string allocation fails during tolower)
            std::transform(user.begin(), user.end(), user.begin(),
                          [](unsigned char c) { return std::tolower(c); });
        }
    }

private:
    std::vector<std::string> users_;
};