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深入理解C++23的Deducing this特性(下):高级应用与实战技巧

2024/12/25 21:11:18 来源:https://blog.csdn.net/weixin_61470881/article/details/144636117  浏览:    关键词:深入理解C++23的Deducing this特性(下):高级应用与实战技巧

1. 高级应用场景

1.1 设计模式实现优化

1.1.1 Builder模式

传统实现:

class Builder {std::string str_;int num_;
public:Builder& setString(std::string s) & {str_ = std::move(s);return *this;}Builder&& setString(std::string s) && {str_ = std::move(s);return std::move(*this);}Builder& setNumber(int n) & {num_ = n;return *this;}Builder&& setNumber(int n) && {num_ = n;return std::move(*this);}
};

使用Deducing this的实现:

class Builder {std::string str_;int num_;
public:template<typename Self>decltype(auto) setString(this Self&& self, std::string s) {self.str_ = std::move(s);return std::forward<Self>(self);}template<typename Self>decltype(auto) setNumber(this Self&& self, int n) {self.num_ = n;return std::forward<Self>(self);}
};
1.1.2 访问者模式
// 基础组件
struct Component {template<typename Self, typename Visitor>decltype(auto) accept(this Self&& self, Visitor&& visitor) {return std::forward<Visitor>(visitor)(std::forward<Self>(self));}
};// 具体组件
struct ConcreteComponent : Component {int data = 42;
};// 访问者
struct Visitor {void operator()(const Component& c) {std::cout << "访问基础组件" << std::endl;}void operator()(const ConcreteComponent& c) {std::cout << "访问具体组件,数据:" << c.data << std::endl;}
};

1.2 与协程的结合使用

class AsyncOperation {
public:template<typename Self>auto start(this Self&& self) -> std::future<void> {co_await std::async(std::launch::async, [self = std::forward<Self>(self)]() {// 异步操作});}template<typename Self>auto with_timeout(this Self&& self, std::chrono::milliseconds timeout) -> std::future<bool> {auto future = self.start();if (future.wait_for(timeout) == std::future_status::timeout) {co_return false;}co_return true;}
};

1.3 CRTP进阶应用

// 基础混入类
template<typename Derived>
class Comparable {
public:template<typename Self, typename Other>bool less_than(this Self&& self, Other&& other) const {return static_cast<const Derived&>(self).compare(other) < 0;}template<typename Self, typename Other>bool greater_than(this Self&& self, Other&& other) const {return static_cast<const Derived&>(self).compare(other) > 0;}template<typename Self, typename Other>bool equals(this Self&& self, Other&& other) const {return static_cast<const Derived&>(self).compare(other) == 0;}
};// 使用示例
class String : public Comparable<String> {std::string data_;
public:explicit String(std::string s) : data_(std::move(s)) {}int compare(const String& other) const {return data_.compare(other.data_);}
};

2. 模板元编程技巧

2.1 类型特征与概念

// 检查类型是否支持特定操作
template<typename T>
concept HasToString = requires(T t) {{ t.toString() } -> std::convertible_to<std::string>;
};class StringConverter {
public:template<typename Self>requires HasToString<std::remove_reference_t<Self>>std::string convert(this Self&& self) {return self.toString();}// 回退版本template<typename Self>std::string convert(this Self&& self) {return "Conversion not supported";}
};

2.2 完美转发与值类别保持

class PerfectForwarder {
public:template<typename Self, typename... Args>decltype(auto) forward_to_impl(this Self&& self, Args&&... args) {if constexpr (std::is_lvalue_reference_v<Self>) {return self.implementation(std::forward<Args>(args)...);} else {return std::move(self).implementation(std::forward<Args>(args)...);}}private:template<typename... Args>auto implementation(Args&&... args) & {std::cout << "左值版本" << std::endl;return process(std::forward<Args>(args)...);}template<typename... Args>auto implementation(Args&&... args) && {std::cout << "右值版本" << std::endl;return process(std::forward<Args>(args)...);}template<typename... Args>auto process(Args&&... args) {// 实际处理逻辑return sizeof...(args);}
};

3. 实际项目中的应用

3.1 智能指针包装器

template<typename T>
class SmartWrapper {std::shared_ptr<T> ptr_;public:explicit SmartWrapper(T* ptr) : ptr_(ptr) {}template<typename Self>auto get_pointer(this Self&& self) {if constexpr (std::is_const_v<std::remove_reference_t<Self>>) {return static_cast<const T*>(self.ptr_.get());} else {return self.ptr_.get();}}template<typename Self, typename F>auto with_lock(this Self&& self, F&& func) {std::lock_guard<std::mutex> lock(self.mutex_);return std::forward<F>(func)(self.get_pointer());}private:std::mutex mutex_;
};

3.2 线程安全容器

template<typename T>
class ThreadSafeContainer {std::vector<T> data_;mutable std::shared_mutex mutex_;public:template<typename Self>auto read(this Self&& self) const {std::shared_lock lock(self.mutex_);return self.data_;}template<typename Self, typename U>void write(this Self&& self, U&& value) {std::unique_lock lock(self.mutex_);self.data_.push_back(std::forward<U>(value));}template<typename Self, typename Pred>auto find_if(this Self&& self, Pred&& pred) const {std::shared_lock lock(self.mutex_);return std::find_if(self.data_.begin(), self.data_.end(),std::forward<Pred>(pred));}
};

4. 性能优化技巧

4.1 移动语义优化

class ResourceManager {std::vector<std::unique_ptr<Resource>> resources_;public:template<typename Self>auto acquire_resource(this Self&& self) {if constexpr (std::is_rvalue_reference_v<Self&&>) {// 对于右值对象,可以直接移动资源return std::move(self.resources_.back());} else {// 对于左值对象,需要克隆资源return std::make_unique<Resource>(*self.resources_.back());}}
};

4.2 内存分配优化

class MemoryOptimizer {std::vector<char> buffer_;public:template<typename Self, typename T>T* allocate(this Self&& self, std::size_t count = 1) {std::size_t required = count * sizeof(T);if (self.buffer_.size() < required) {if constexpr (std::is_rvalue_reference_v<Self&&>) {// 右值对象可以重用bufferself.buffer_.resize(required);} else {// 左值对象需要重新分配self.buffer_ = std::vector<char>(required);}}return reinterpret_cast<T*>(self.buffer_.data());}
};

5. 调试与测试策略

5.1 类型检查与断言

class DebugHelper {
public:template<typename Self>void debug_info(this Self&& self) {// 输出类型信息std::cout << "对象类型: " << typeid(Self).name() << std::endl;// 检查const限定符if constexpr (std::is_const_v<std::remove_reference_t<Self>>) {std::cout << "const对象" << std::endl;}// 检查值类别if constexpr (std::is_rvalue_reference_v<Self&&>) {std::cout << "右值对象" << std::endl;} else {std::cout << "左值对象" << std::endl;}// 检查是否可移动if constexpr (std::is_move_constructible_v<std::remove_reference_t<Self>>) {std::cout << "可移动对象" << std::endl;}}
};

5.2 单元测试策略

class TestSubject {
public:template<typename Self>bool test_method(this Self&& self) {// 使用static_assert进行编译期检查static_assert(std::is_same_v<std::remove_cvref_t<Self>,TestSubject>, "类型不匹配");// 运行时检查assert(self.validate());return true;}private:bool validate() const { return true; }
};// 测试用例
void run_tests() {TestSubject subject;const TestSubject const_subject;// 测试左值assert(subject.test_method());// 测试const左值assert(const_subject.test_method());// 测试右值assert(TestSubject{}.test_method());
}

参考资料

[1] P0847R7: Deducing this
[2] C++23 Standard
[3] CppCon 2021: “The C++23 this parameter” - Gašper Ažman
[4] C++ Reference: Member Functions
[5] Modern C++ Design Patterns
[6] Effective Modern C++

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