CODEWITHSUNDEEP
c++functionc++11variadic-templates
Wussup
Wussup

Reputation: 17

why pass reference of a varaible to a varadic template function by universal reference operator (&&) failed?

I was intend to pass a reference of a variable into a member function pointer, which is a parameter of another varadic template function to invoke any kind of member function of a class, but from the printing result, it is not passed by reference, but just by value.

#include <functional>
#include <iostream>
#include <memory>

template <class WorkerType> class Delegate {
public:
  template <typename... Args>
  using WrkFunc = void (WorkerType::*)(Args... args);

  explicit Delegate(WorkerType &wrk) : m_worker(&wrk) {}

  template <typename... Args>
  void workerDo(WrkFunc<Args...> func, Args &&... args) {
    auto fn = std::bind(func, m_worker.get(), std::forward<Args>(args)...);
    fn();
  }

private:
  std::shared_ptr<WorkerType> m_worker;
};

class SomeWorker {
public:
  SomeWorker() = default;

  void doSomething(int &a) {
    a = 1000;
    std::cout << "2# address: " << &a << ", value: " << a << std::endl;
  }
};

int main() {
  SomeWorker wrk;
  Delegate<SomeWorker> del(wrk);
  int a = 0;
  std::cout << "1# address: " << &a << ", value: " << a << std::endl;
  del.workerDo(&SomeWorker::doSomething, a);
  std::cout << "3# address: " << &a << ", value: " << a << std::endl;
  return 0;
}

what I expected result like this:
1# address: 0x7fffc1dc621c, value: 0
2# address: 0x7fffc1dc621c, value: 1000
3# address: 0x7fffc1dc621c, value: 1000

but the actual result is:
1# address: 0x7fffc1dc621c, value: 0
2# address: 0x7fffc1dc61d0, value: 1000
3# address: 0x7fffc1dc621c, value: 0

Upvotes: 0

Views: 64

Answers (1)

yumetodo
yumetodo

Reputation: 1202

At first, your Delegate class's constructor is completely broken. That is a wrong usage of std::shared_ptr. You should fix like below:

// define
explicit Delegate(std::shared_ptr<WorkerType> wrk) : m_worker(std::move(wrk)) {}
//call
Delegate<SomeWorker> del(std::make_shared<SomeWorker>());

To pass a object lvalue reference to std::bind, you must use std::ref.

How to bind function to an object by reference?

When you pass std::ref result to workerDo, WrkFunc will disturb calling.

So, you need to rewrite type check using std::is_invocable_r_v

#include <functional>
#include <iostream>
#include <memory>
#include <type_traits>
template <class WorkerType> class Delegate {
public:

  explicit Delegate(std::shared_ptr<WorkerType> wrk) : m_worker(std::move(wrk)) {}

  template <typename Func, typename ...Args, std::enable_if_t<std::is_invocable_r_v<void, Func, WorkerType, Args...>, std::nullptr_t> = nullptr>
  void workerDo(Func func, Args && ...args) {
    auto fn = std::bind(func, m_worker.get(), std::forward<Args>(args)...);
    fn();
  }

private:
  std::shared_ptr<WorkerType> m_worker;
};

class SomeWorker {
public:
  SomeWorker() = default;

  void doSomething(int &a) {
    a = 1000;
    std::cout << "2# address: " << &a << ", value: " << a << std::endl;
  }
};

int main() {
  Delegate<SomeWorker> del(std::make_shared<SomeWorker>());
  int a = 0;
  std::cout << "1# address: " << &a << ", value: " << a << std::endl;
  del.workerDo(&SomeWorker::doSomething, std::ref(a));
  std::cout << "3# address: " << &a << ", value: " << a << std::endl;
  return 0;
}

https://wandbox.org/permlink/fnBThXw5Uh72JeGQ

is there any method to avoid using std::ref everywhere call the workerDo ?

using wrapper function like below will solve.

template <class T, class U, std::enable_if_t<
    (std::is_lvalue_reference_v<T> ? std::is_lvalue_reference_v<U> : true) &&
    std::is_convertible_v<std::remove_reference_t<U>*, std::remove_reference_t<T>*>,
    std::nullptr_t
> = nullptr>
inline decltype(auto) forward_or_construct_reference_wrapper(U&& u)
{
    if constexpr(std::is_lvalue_reference_v<T>) {
        return std::reference_wrapper{std::forward<T>(u)};
    }
    else {
        return static_cast<T&&>(u);
    }
}

https://wandbox.org/permlink/wli9Dh9vXSjzBWMA

Upvotes: 1

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