c++怎么实现一个类型擦除(Type Erasure)的函数包装器_C++泛型编程与类型擦除技巧

#include 
#include 

class Function {
  struct CallableBase {
    virtual ~CallableBase() = default;
    virtual int call() = 0;
    virtual std::unique_ptr clone() const = 0;
  };

  template 
  struct CallableModel : CallableBase {
    F func;
    explicit CallableModel(F f) : func(std::move(f)) {}

    int call() override { return func(); }

    std::unique_ptr clone() const override {
      return std::make_unique(func);
    }
  };

  std::unique_ptr callable;

public:
  Function() = default;

  template 
  Function(F f) : callable(std::make_unique>(std::move(f))) {}

  Function(const Function& other)
    : callable(other.callable ? other.callable->clone() : nullptr) {}

  Function& operator=(const Function& other) {
    if (this != &other) {
      callable = other.callable ? other.callable->clone() : nullptr;
    }
    return *this;
  }

  Function(Function&&) = default;
  Function& operator=(Function&&) = default;

  int operator()() const {
    if (!callable) throw std::bad_function_call();
    return callable->call();
  }

  explicit operator bool() const { return !!callable; }
};

int foo() { return 42; }

int main() {
  Function f1 = []{ return 100; };
  Function f2 = foo;

  std::cout << f1() << "\n"; // 输出 100
  std::cout << f2() << "\n"; // 输出 42

  Function f3 = f1;
  std::cout << f3() << "\n"; // 输出 100
}

template 
class Function;

template 
class Function {
  struct CallableBase {
    virtual ~CallableBase() = default;
    virtual R call(Args... args) = 0;
    virtual std::unique_ptr clone() const = 0;
  };

  template 
  struct CallableModel : CallableBase {
    F func;
    explicit CallableModel(F f) : func(std::move(f)) {}

    R call(Args... args) override { return func(std::forward(args)...); }

    std::unique_ptr clone() const override {
      return std::make_unique(func);
    }
  };

  std::unique_ptr callable;

public:
  Function() = default;

  template 
  Function(F f) : callable(std::make_unique>(std::move(f))) {}

  Function(const Function& other)
    : callable(other.callable ? other.callable->clone() : nullptr) {}

  Function& operator=(const Function& other) {
    if (this != &other) {
      callable = other.callable ? other.callable->clone() : nullptr;
    }
    return *this;
  }

  Function(Function&&) = default;
  Function& operator=(Function&&) = default;

  R operator()(Args... args) const {
    if (!callable) throw std::bad_function_call();
    return callable->call(std::forward(args)...);
  }

  explicit operator bool() const { return !!callable; }
};