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【C++】类型擦除 + 工厂模式，告别 if-else

news 来源：原创 2024/9/21 8:40:55

文章目录

一、使用std::variant和std::visit
二、使用虚函数
参考

需求点：假设一个第三方库有提供了很多Msg类，这些Msg类都提供了固定的一个成员函数，但是却没有虚基函数。如何在自己的项目代码中更好的使用这些Msg类内？

一、使用std::variant和std::visit

#include <iostream>
#include <memory>  // for std::unique_ptr and std::make_unique
#include <string>
#include <utility>  // for std::move
#include <variant>
#include <vector>// 外部库 Start
struct MoveMsg {int x;int y;void speak() { std::cout << "Move " << x << ", " << y << '\n'; }
};struct JumpMsg {int height;void speak() { std::cout << "Jump " << height << '\n'; }
};struct SleepMsg {int time;void speak() { std::cout << "Sleep " << time << '\n'; }
};struct ExitMsg {void speak() { std::cout << "Exit" << '\n'; }
};
// 外部库 Endint main() {using Msg = std::variant<MoveMsg, JumpMsg, SleepMsg, ExitMsg>;std::vector<Msg> msgs;msgs.push_back(MoveMsg{1, 2});msgs.push_back(JumpMsg{1});for (auto&& msg : msgs) {std::visit([](auto& msg) { msg.speak(); }, msg);}return 0;
}

二、使用虚函数

C++20语法

#include <iostream>
#include <memory>  // for std::unique_ptr and std::make_unique
#include <string>
#include <utility>  // for std::move
#include <variant>
#include <vector>// 外部库 Start
struct MoveMsg {int x;int y;void speak() { std::cout << "Move " << x << ", " << y << '\n'; }
};struct JumpMsg {int height;void speak() { std::cout << "Jump " << height << '\n'; }void happy() { std::cout << "happy " << height << '\n'; }
};struct SleepMsg {int time;void speak() { std::cout << "Sleep " << time << '\n'; }
};struct ExitMsg {void speak() { std::cout << "Exit" << '\n'; }
};
// 外部库 Endstruct MsgBase {virtual void speak() = 0;virtual void happy() = 0;virtual std::shared_ptr<MsgBase> clone() const = 0;virtual ~MsgBase() = default;
};//内部代码使用MsgBase去包装库的speak()和happy()函数
template <class Msg>
struct MsgImpl : MsgBase {Msg msg;template <class ...Ts>MsgImpl(Ts &&...ts) : msg{std::forward<Ts>(ts)...} {}void speak() override {msg.speak();}void happy() override {if constexpr (requires {msg.happy();} ){msg.happy();}else{std::cout<< "no happy\n";}}std::shared_ptr<MsgBase> clone() const override {return std::make_shared<MsgImpl<Msg>>(msg);}
};template <class Msg, class ...Ts>
std::shared_ptr<MsgBase> makeMsg(Ts &&...ts) {return std::make_shared<MsgImpl<Msg>>(std::forward<Ts>(ts)...);
}int main() {std::vector<std::shared_ptr<MsgBase>> msgs;msgs.push_back(makeMsg<MoveMsg>(1, 2));msgs.push_back(makeMsg<JumpMsg>(1));for (auto&& msg : msgs) {msg->speak(); msg->happy(); }return 0;
}

C++14写法1

#include <iostream>
#include <memory>  // for std::unique_ptr and std::make_unique
#include <string>
#include <utility>  // for std::move
#include <variant>
#include <vector>// 外部库 Start
struct MoveMsg {int x;int y;void speak() { std::cout << "Move " << x << ", " << y << '\n'; }
};struct JumpMsg {int height;void speak() { std::cout << "Jump " << height << '\n'; }void happy() { std::cout << "happy " << height << '\n'; }
};struct SleepMsg {int time;void speak() { std::cout << "Sleep " << time << '\n'; }
};struct ExitMsg {void speak() { std::cout << "Exit" << '\n'; }
};
// 外部库 Endstruct MsgBase {virtual void speak() = 0;virtual void happy() = 0;virtual std::shared_ptr<MsgBase> clone() const = 0;virtual ~MsgBase() = default;
};// 特质，用于检测 happy 方法是否存在
template<typename T>
struct has_happy {
private:typedef char YesType[1];typedef char NoType[2];template <typename C> static YesType& test(decltype(&C::happy));template <typename C> static NoType& test(...);public:enum { value = sizeof(test<T>(0)) == sizeof(YesType) };
};// 函数重载用于调用 happy 或者输出 "no happy"
template <typename T>
typename std::enable_if<has_happy<T>::value>::type call_happy(T& t) {t.happy();
}template <typename T>
typename std::enable_if<!has_happy<T>::value>::type call_happy(T&) {std::cout << "no happy\n";
}// MsgImpl 用于包装库的消息类型
template <class Msg>
struct MsgImpl : MsgBase {Msg msg;template <class ...Ts>MsgImpl(Ts &&...ts) : msg{std::forward<Ts>(ts)...} {}void speak() override {msg.speak();}void happy() override {call_happy(msg);}std::shared_ptr<MsgBase> clone() const override {return std::make_shared<MsgImpl<Msg>>(msg);}
};template <class Msg, class ...Ts>
std::shared_ptr<MsgBase> makeMsg(Ts &&...ts) {return std::make_shared<MsgImpl<Msg>>(std::forward<Ts>(ts)...);
}int main() {std::vector<std::shared_ptr<MsgBase>> msgs;msgs.push_back(makeMsg<MoveMsg>(1, 2));msgs.push_back(makeMsg<JumpMsg>(1));for (auto&& msg : msgs) {msg->speak(); msg->happy(); }return 0;
}

has_happy 模板：

has_happy 模板用于检测 Msg 类中是否有 happy 方法。
SFINAE 技术用于检查特定方法是否存在。
test 函数用于确定 happy 方法的存在性，如果存在，返回 sizeof(char)，否则返回 sizeof(int)。

call_happy 函数：

根据 std::integral_constant 的值，选择调用 happy 方法或输出 “no happy”。
std::true_type 和 std::false_type 是 std::integral_constant 的特例化，分别表示 true 和 false。

C++14写法2：

#include <iostream>
#include <memory>
#include <string>
#include <utility>
#include <vector>
#include <type_traits>// 外部库 Start
struct MoveMsg {int x;int y;void speak() { std::cout << "Move " << x << ", " << y << '\n'; }
};struct JumpMsg {int height;void speak() { std::cout << "Jump " << height << '\n'; }void happy() { std::cout << "happy " << height << '\n'; }
};struct SleepMsg {int time;void speak() { std::cout << "Sleep " << time << '\n'; }
};struct ExitMsg {void speak() { std::cout << "Exit" << '\n'; }
};
// 外部库 Endstruct MsgBase {virtual void speak() = 0;virtual void happy() = 0;virtual std::shared_ptr<MsgBase> clone() const = 0;virtual ~MsgBase() = default;
};// 检测 happy 方法存在性的模板
template<typename T>
struct has_happy {// 利用 SFINAE 技术检测 happy 方法template<typename U>static auto test(int) -> decltype(std::declval<U>().happy(), std::true_type{});template<typename>static std::false_type test(...);static constexpr bool value = decltype(test<T>(0))::value;
};// MsgImpl 包装库的消息类型
template <class Msg>
struct MsgImpl : MsgBase {Msg msg;template <class ...Ts>MsgImpl(Ts&&... ts) : msg{std::forward<Ts>(ts)...} {}void speak() override {msg.speak();}void happy() override {call_happy(msg, std::integral_constant<bool, has_happy<Msg>::value>{});}std::shared_ptr<MsgBase> clone() const override {return std::make_shared<MsgImpl<Msg>>(msg);}private:// 当 Msg 有 happy 方法时调用template<typename T>void call_happy(T& t, std::true_type) {t.happy();}// 当 Msg 没有 happy 方法时调用template<typename T>void call_happy(T&, std::false_type) {std::cout << "no happy\n";}
};template <class Msg, class ...Ts>
std::shared_ptr<MsgBase> makeMsg(Ts&&... ts) {return std::make_shared<MsgImpl<Msg>>(std::forward<Ts>(ts)...);
}int main() {std::vector<std::shared_ptr<MsgBase>> msgs;msgs.push_back(makeMsg<MoveMsg>(1, 2));msgs.push_back(makeMsg<JumpMsg>(1));for (auto&& msg : msgs) {msg->speak(); msg->happy(); }return 0;
}