C++学习笔记-Variant

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#include <iostream>
#include <variant>
#include <string>

int main()
{
    std::cout << "=== Basic Variant Usage ===" << std::endl;
    
    std::variant<std::string, int> data;
    data = "Jerry";
    std::cout << "String value: " << std::get<std::string>(data) << std::endl;
    
    data = 2;
    std::cout << "Int value: " << std::get<int>(data) << std::endl;

    /* 检查判断类型是否匹配 */
    if (data.index() != 1) // 索引从0开始，1表示第二个int
        std::cout << "data not int" << std::endl;

    // 第二种方式：std::get_if
    auto value = std::get_if<int>(&data);
    if (!value)
        std::cout << "data not int" << std::endl;
    else
        std::cout << "data is int: " << *value << std::endl;

    // Variant 是一个类型安全的Union
    std::cout << "sizeof(int): " << sizeof(int) << std::endl;
    std::cout << "sizeof(std::string): " << sizeof(std::string) << std::endl;
    std::cout << "sizeof(variant): " << sizeof(data) << std::endl;
    
    return 0;
}

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#include <iostream>
#include <variant>
#include <string>
#include <vector>

void TypeCheckingDemo()
{
    std::cout << "=== Type Checking Demo ===" << std::endl;
    
    std::variant<int, double, std::string> var;
    
    // 设置不同类型的值
    std::vector<std::variant<int, double, std::string>> values{
        42,
        3.14,
        std::string("Hello"),
        100,
        2.718
    };
    
    for (size_t i = 0; i < values.size(); ++i)
    {
        auto& v = values[i];
        std::cout << "Value " << i << ": ";
        
        // 方法1：使用index()检查类型
        switch (v.index())
        {
            case 0:
                std::cout << "int = " << std::get<int>(v);
                break;
            case 1:
                std::cout << "double = " << std::get<double>(v);
                break;
            case 2:
                std::cout << "string = " << std::get<std::string>(v);
                break;
        }
        std::cout << std::endl;
    }
    
    std::cout << "\nUsing std::holds_alternative:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        auto& v = values[i];
        std::cout << "Value " << i << ": ";
        
        // 方法2：使用std::holds_alternative
        if (std::holds_alternative<int>(v))
        {
            std::cout << "int = " << std::get<int>(v);
        }
        else if (std::holds_alternative<double>(v))
        {
            std::cout << "double = " << std::get<double>(v);
        }
        else if (std::holds_alternative<std::string>(v))
        {
            std::cout << "string = " << std::get<std::string>(v);
        }
        std::cout << std::endl;
    }
    
    std::cout << "\nUsing std::get_if:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        auto& v = values[i];
        std::cout << "Value " << i << ": ";
        
        // 方法3：使用std::get_if（安全访问）
        if (auto* intPtr = std::get_if<int>(&v))
        {
            std::cout << "int = " << *intPtr;
        }
        else if (auto* doublePtr = std::get_if<double>(&v))
        {
            std::cout << "double = " << *doublePtr;
        }
        else if (auto* stringPtr = std::get_if<std::string>(&v))
        {
            std::cout << "string = " << *stringPtr;
        }
        std::cout << std::endl;
    }
}

int main()
{
    TypeCheckingDemo();
    return 0;
}

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#include <iostream>
#include <variant>
#include <string>
#include <vector>

// 访问者函数对象
struct PrintVisitor
{
    void operator()(int value) const
    {
        std::cout << "Integer: " << value;
    }
    
    void operator()(double value) const
    {
        std::cout << "Double: " << value;
    }
    
    void operator()(const std::string& value) const
    {
        std::cout << "String: " << value;
    }
};

// 泛型访问者
struct GenericVisitor
{
    template<typename T>
    void operator()(const T& value) const
    {
        std::cout << "Generic: " << value << " (type: " << typeid(T).name() << ")";
    }
};

// 返回值的访问者
struct SizeVisitor
{
    size_t operator()(int value) const
    {
        return sizeof(int);
    }
    
    size_t operator()(double value) const
    {
        return sizeof(double);
    }
    
    size_t operator()(const std::string& value) const
    {
        return value.length();
    }
};

void VisitorPatternDemo()
{
    std::cout << "=== Visitor Pattern Demo ===" << std::endl;
    
    std::vector<std::variant<int, double, std::string>> values{
        42,
        3.14159,
        std::string("Hello World"),
        -100,
        2.718
    };
    
    std::cout << "Using custom visitor:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        std::cout << "Value " << i << ": ";
        std::visit(PrintVisitor{}, values[i]);
        std::cout << std::endl;
    }
    
    std::cout << "\nUsing generic visitor:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        std::cout << "Value " << i << ": ";
        std::visit(GenericVisitor{}, values[i]);
        std::cout << std::endl;
    }
    
    std::cout << "\nUsing lambda visitor:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        std::cout << "Value " << i << ": ";
        std::visit([](const auto& value) {
            std::cout << "Lambda: " << value;
        }, values[i]);
        std::cout << std::endl;
    }
    
    std::cout << "\nUsing size visitor:" << std::endl;
    for (size_t i = 0; i < values.size(); ++i)
    {
        auto size = std::visit(SizeVisitor{}, values[i]);
        std::cout << "Value " << i << " size: " << size << std::endl;
    }
}

int main()
{
    VisitorPatternDemo();
    return 0;
}

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#include <iostream>
#include <variant>
#include <string>
#include <vector>
#include <memory>

// 表达式求值器
struct Number
{
    double value;
    Number(double v) : value(v) {}
};

struct BinaryOp
{
    char op;
    std::unique_ptr<struct Expression> left;
    std::unique_ptr<struct Expression> right;
    
    BinaryOp(char o, std::unique_ptr<Expression> l, std::unique_ptr<Expression> r)
        : op(o), left(std::move(l)), right(std::move(r)) {}
};

struct Expression
{
    std::variant<Number, BinaryOp> data;
    
    Expression(Number n) : data(std::move(n)) {}
    Expression(BinaryOp op) : data(std::move(op)) {}
};

// 求值访问者
struct EvaluateVisitor
{
    double operator()(const Number& num) const
    {
        return num.value;
    }
    
    double operator()(const BinaryOp& op) const
    {
        double leftVal = std::visit(*this, op.left->data);
        double rightVal = std::visit(*this, op.right->data);
        
        switch (op.op)
        {
            case '+': return leftVal + rightVal;
            case '-': return leftVal - rightVal;
            case '*': return leftVal * rightVal;
            case '/': return rightVal != 0 ? leftVal / rightVal : 0;
            default: return 0;
        }
    }
};

// 打印访问者
struct PrintVisitor
{
    void operator()(const Number& num) const
    {
        std::cout << num.value;
    }
    
    void operator()(const BinaryOp& op) const
    {
        std::cout << "(";
        std::visit(*this, op.left->data);
        std::cout << " " << op.op << " ";
        std::visit(*this, op.right->data);
        std::cout << ")";
    }
};

// 错误处理
enum class ErrorType
{
    None,
    FileNotFound,
    PermissionDenied,
    InvalidFormat,
    NetworkError
};

template<typename T>
using Result = std::variant<T, ErrorType>;

Result<std::string> ReadFile(const std::string& filename)
{
    if (filename.empty())
        return ErrorType::InvalidFormat;
    
    if (filename == "missing.txt")
        return ErrorType::FileNotFound;
    
    if (filename == "protected.txt")
        return ErrorType::PermissionDenied;
    
    return std::string("Content of " + filename);
}

Result<int> ParseNumber(const std::string& str)
{
    try
    {
        return std::stoi(str);
    }
    catch (...)
    {
        return ErrorType::InvalidFormat;
    }
}

void PracticalExamplesDemo()
{
    std::cout << "=== Practical Examples Demo ===" << std::endl;
    
    // 表达式求值
    std::cout << "Expression evaluation:" << std::endl;
    
    // 创建表达式: (3 + 4) * 2
    auto expr = Expression(BinaryOp('*',
        std::make_unique<Expression>(BinaryOp('+',
            std::make_unique<Expression>(Number(3)),
            std::make_unique<Expression>(Number(4))
        )),
        std::make_unique<Expression>(Number(2))
    ));
    
    std::cout << "Expression: ";
    std::visit(PrintVisitor{}, expr.data);
    std::cout << " = " << std::visit(EvaluateVisitor{}, expr.data) << std::endl;
    
    // 错误处理
    std::cout << "\nError handling with variant:" << std::endl;
    
    std::vector<std::string> files{"data.txt", "missing.txt", "protected.txt", ""};
    
    for (const auto& file : files)
    {
        auto result = ReadFile(file);
        
        std::visit([&file](const auto& value) {
            using T = std::decay_t<decltype(value)>;
            if constexpr (std::is_same_v<T, std::string>)
            {
                std::cout << "Success reading " << file << ": " << value << std::endl;
            }
            else if constexpr (std::is_same_v<T, ErrorType>)
            {
                std::cout << "Error reading " << file << ": ";
                switch (value)
                {
                    case ErrorType::FileNotFound:
                        std::cout << "File not found";
                        break;
                    case ErrorType::PermissionDenied:
                        std::cout << "Permission denied";
                        break;
                    case ErrorType::InvalidFormat:
                        std::cout << "Invalid format";
                        break;
                    default:
                        std::cout << "Unknown error";
                }
                std::cout << std::endl;
            }
        }, result);
    }
    
    // 数字解析
    std::cout << "\nNumber parsing:" << std::endl;
    std::vector<std::string> numbers{"123", "abc", "456", "12.34"};
    
    for (const auto& numStr : numbers)
    {
        auto result = ParseNumber(numStr);
        
        if (std::holds_alternative<int>(result))
        {
            std::cout << "Parsed '" << numStr << "': " << std::get<int>(result) << std::endl;
        }
        else
        {
            std::cout << "Failed to parse '" << numStr << "'" << std::endl;
        }
    }
}

int main()
{
    PracticalExamplesDemo();
    return 0;
}

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#include <iostream>
#include <variant>
#include <string>

// 传统union
union TraditionalUnion
{
    int intValue;
    double doubleValue;
    // char* stringValue; // 不能包含非平凡类型
};

// 带标记的union
enum class UnionType { INT, DOUBLE, STRING };

struct TaggedUnion
{
    UnionType type;
    union
    {
        int intValue;
        double doubleValue;
        char stringValue[32]; // 固定大小
    };
    
    TaggedUnion(int value) : type(UnionType::INT), intValue(value) {}
    TaggedUnion(double value) : type(UnionType::DOUBLE), doubleValue(value) {}
    TaggedUnion(const char* value) : type(UnionType::STRING)
    {
        strncpy(stringValue, value, 31);
        stringValue[31] = '\0';
    }
    
    void Print() const
    {
        switch (type)
        {
            case UnionType::INT:
                std::cout << "int: " << intValue;
                break;
            case UnionType::DOUBLE:
                std::cout << "double: " << doubleValue;
                break;
            case UnionType::STRING:
                std::cout << "string: " << stringValue;
                break;
        }
    }
};

void ComparisonDemo()
{
    std::cout << "=== Union vs Variant Comparison ===" << std::endl;
    
    // 传统union的问题
    std::cout << "Traditional union problems:" << std::endl;
    TraditionalUnion tu;
    tu.intValue = 42;
    std::cout << "Set int: " << tu.intValue << std::endl;
    
    tu.doubleValue = 3.14;
    std::cout << "Set double: " << tu.doubleValue << std::endl;
    std::cout << "Int value after setting double: " << tu.intValue << " (corrupted)" << std::endl;
    
    // 带标记的union
    std::cout << "\nTagged union:" << std::endl;
    TaggedUnion tagged1(42);
    TaggedUnion tagged2(3.14);
    TaggedUnion tagged3("Hello");
    
    tagged1.Print(); std::cout << std::endl;
    tagged2.Print(); std::cout << std::endl;
    tagged3.Print(); std::cout << std::endl;
    
    // std::variant的优势
    std::cout << "\nstd::variant advantages:" << std::endl;
    std::variant<int, double, std::string> var1 = 42;
    std::variant<int, double, std::string> var2 = 3.14;
    std::variant<int, double, std::string> var3 = std::string("Hello World");
    
    auto printVariant = [](const auto& var) {
        std::visit([](const auto& value) {
            std::cout << "variant: " << value << " (type: " << typeid(value).name() << ")";
        }, var);
        std::cout << std::endl;
    };
    
    printVariant(var1);
    printVariant(var2);
    printVariant(var3);
    
    // 大小比较
    std::cout << "\nSize comparison:" << std::endl;
    std::cout << "TraditionalUnion: " << sizeof(TraditionalUnion) << " bytes" << std::endl;
    std::cout << "TaggedUnion: " << sizeof(TaggedUnion) << " bytes" << std::endl;
    std::cout << "std::variant: " << sizeof(var1) << " bytes" << std::endl;
    
    std::cout << "\nVariant advantages:" << std::endl;
    std::cout << "- Type safety" << std::endl;
    std::cout << "- Exception safety" << std::endl;
    std::cout << "- Supports complex types (std::string, etc.)" << std::endl;
    std::cout << "- Visitor pattern support" << std::endl;
    std::cout << "- Runtime type information" << std::endl;
    std::cout << "- Automatic memory management" << std::endl;
}

int main()
{
    ComparisonDemo();
    return 0;
}

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#include <iostream>
#include <variant>
#include <string>
#include <vector>
#include <optional>

// 递归variant（需要前向声明）
struct JsonValue;
using JsonArray = std::vector<JsonValue>;
using JsonObject = std::map<std::string, JsonValue>;

struct JsonValue
{
    std::variant<
        std::nullptr_t,
        bool,
        int,
        double,
        std::string,
        JsonArray,
        JsonObject
    > data;
    
    JsonValue() : data(nullptr) {}
    JsonValue(bool b) : data(b) {}
    JsonValue(int i) : data(i) {}
    JsonValue(double d) : data(d) {}
    JsonValue(const std::string& s) : data(s) {}
    JsonValue(const JsonArray& a) : data(a) {}
    JsonValue(const JsonObject& o) : data(o) {}
};

// JSON打印访问者
struct JsonPrintVisitor
{
    void operator()(std::nullptr_t) const
    {
        std::cout << "null";
    }
    
    void operator()(bool value) const
    {
        std::cout << (value ? "true" : "false");
    }
    
    void operator()(int value) const
    {
        std::cout << value;
    }
    
    void operator()(double value) const
    {
        std::cout << value;
    }
    
    void operator()(const std::string& value) const
    {
        std::cout << "\"" << value << "\"";
    }
    
    void operator()(const JsonArray& array) const
    {
        std::cout << "[";
        for (size_t i = 0; i < array.size(); ++i)
        {
            if (i > 0) std::cout << ", ";
            std::visit(*this, array[i].data);
        }
        std::cout << "]";
    }
    
    void operator()(const JsonObject& object) const
    {
        std::cout << "{";
        bool first = true;
        for (const auto& [key, value] : object)
        {
            if (!first) std::cout << ", ";
            first = false;
            std::cout << "\"" << key << "\": ";
            std::visit(*this, value.data);
        }
        std::cout << "}";
    }
};

// 异常安全的variant操作
template<typename... Types>
class SafeVariant
{
private:
    std::variant<Types...> m_Data;
    
public:
    template<typename T>
    SafeVariant(T&& value) : m_Data(std::forward<T>(value)) {}
    
    template<typename T>
    std::optional<T> Get() const
    {
        if (std::holds_alternative<T>(m_Data))
        {
            return std::get<T>(m_Data);
        }
        return std::nullopt;
    }
    
    template<typename Visitor>
    auto Visit(Visitor&& visitor) const
    {
        return std::visit(std::forward<Visitor>(visitor), m_Data);
    }
    
    size_t Index() const { return m_Data.index(); }
    
    template<typename T>
    bool Is() const { return std::holds_alternative<T>(m_Data); }
};

void AdvancedFeaturesDemo()
{
    std::cout << "=== Advanced Features Demo ===" << std::endl;
    
    // JSON示例
    JsonValue json = JsonObject{
        {"name", JsonValue("John Doe")},
        {"age", JsonValue(30)},
        {"active", JsonValue(true)},
        {"scores", JsonValue(JsonArray{
            JsonValue(95),
            JsonValue(87),
            JsonValue(92)
        })},
        {"address", JsonValue(JsonObject{
            {"street", JsonValue("123 Main St")},
            {"city", JsonValue("Anytown")}
        })}
    };
    
    std::cout << "JSON: ";
    std::visit(JsonPrintVisitor{}, json.data);
    std::cout << std::endl;
    
    // 安全variant示例
    std::cout << "\nSafe variant demo:" << std::endl;
    SafeVariant<int, double, std::string> safeVar(42);
    
    auto intValue = safeVar.Get<int>();
    auto stringValue = safeVar.Get<std::string>();
    
    std::cout << "Int value: " << (intValue ? std::to_string(*intValue) : "none") << std::endl;
    std::cout << "String value: " << (stringValue ? *stringValue : "none") << std::endl;
    
    // 类型检查
    std::cout << "Is int: " << safeVar.Is<int>() << std::endl;
    std::cout << "Is string: " << safeVar.Is<std::string>() << std::endl;
    
    // 访问者模式
    safeVar.Visit([](const auto& value) {
        std::cout << "Safe variant contains: " << value << std::endl;
    });
}

void BestPracticesDemo()
{
    std::cout << "\n=== Best Practices ===" << std::endl;
    
    std::cout << "1. Use std::variant instead of union for type safety" << std::endl;
    std::cout << "2. Prefer std::visit over manual type checking" << std::endl;
    std::cout << "3. Use std::holds_alternative for simple type checks" << std::endl;
    std::cout << "4. Consider std::optional for nullable types" << std::endl;
    std::cout << "5. Use generic lambdas for simple visitors" << std::endl;
    std::cout << "6. Be careful with recursive variants (use pointers/smart pointers)" << std::endl;
    std::cout << "7. Consider performance implications of large variants" << std::endl;
}

int main()
{
    AdvancedFeaturesDemo();
    BestPracticesDemo();
    return 0;
}