c++ - 如何传递一个non-copyable的函数对象给std::function？

Question

#include <memory>
#include <functional>

class A{    //non-copyable
    std::unique_ptr<int> a;
public:
    void operator()(){}    //non-const
};

void func(std::function<void(void)> f)
{}

int main()
{
    A fobj;
    func(fobj);
    return 0;
}

如上，需要传递一个A的函数对象给func，并且fobj不能是const型的。怎样实现呢？

Answer 1

有以下方案：

• 用A的引用。缺點：A被銷毀時引用隨之失效。

• 透過移動語意建構一個shared_ptr。缺點：額外的運行時開銷，需要A可移動構造。

#include <memory>
#include <functional>
#include <utility>

class A{    //non-copyable
    std::unique_ptr<int> a;
public:
    void operator()(){}    //non-const
};

void func(std::function<void(void)> f) {}

int main()
{
  A obj;
  
  // by reference
  func([&obj](){obj();});
  
  // by shared_ptr
  auto ptr = std::make_shared<A>(std::move(obj));
  func([ptr](){(*ptr)();});

  return 0;
}

• 或透過一個在拷貝時移動物件的wrapper來實現（需要A可移動構造）:

#include <memory>
#include <functional>
#include <type_traits>

class A{    //non-copyable
    std::unique_ptr<int> a;
public:
    void operator()(){}    //non-const
};

void func(std::function<void(void)> f) {}

template <class Fn, class Ret, class... Args>
struct FunctionWrapperImpl {
  FunctionWrapperImpl(FunctionWrapperImpl &wrapper)
      : fn_(std::move(wrapper.fn_)) {}
  FunctionWrapperImpl(FunctionWrapperImpl &&wrapper)
      : fn_(std::move(wrapper.fn_)) {}
  FunctionWrapperImpl(Fn &&fn) : fn_(std::move(fn)) {}
  
  template <class T = Ret>
  std::enable_if_t<std::is_same<void, T>::value, Ret>
  operator()(Args... args) {
    fn_(std::forward<Args>(args)...);
  }
  
  template <class T = Ret>
  std::enable_if_t<!std::is_same<void, T>::value, Ret>
  operator()(Args... args) {
    return fn_(std::forward<Args>(args)...);
  }
  
  Fn fn_;
};

template <class Fn>
struct FunctionWrapper {
  FunctionWrapper(FunctionWrapper &&wrapper) : fn_(std::move(wrapper.fn_)) {}
  
  FunctionWrapper(Fn &&fn) : fn_(std::move(fn)) {}
  
  template <class Ret, class... Args>
  operator std::function<Ret(Args...)>() && {
    return std::function<Ret(Args...)>(
        FunctionWrapperImpl<Fn, Ret, Args...>(std::move(fn_)));
  }
  
  Fn fn_;
};

template <class Fn>
auto function_wrapper(Fn &&fn) {
  return FunctionWrapper<Fn>(std::move(fn));
}

int main()
{
  A obj;
  
  // function wrapper
  func(function_wrapper(std::move(obj)));

  return 0;
}