std::unique_ptr as parameter to function in std::t

Question

This question already has an answer here:

• visual studio implementation of “move semantics” and “rvalue reference” 2 answers

So I'm trying to pass a std::unique_ptr as a parameter to a function that is launched in a separate thread, and I'm getting a strange error at compile time that reads:

1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\functional(1149): error C2280: 'std::unique_ptr<Widget,std::default_delete<_Ty>>::unique_ptr(const std::unique_ptr<_Ty,std::default_delete<_Ty>> &)' : attempting to reference a deleted function

A simplified version of this code, that still reproduces the same issue is:

#include <thread>
#include <memory>
#include <iostream>

class Widget
{
public:
  Widget() : m_data(0)
  {
  }

  void prepareData(int i)
  {
    m_data = i;
  }

  int getData() const
  {
    return m_data;
  }

private:
  int m_data;
};

void processWidget(std::unique_ptr<Widget> widget)
{
  std::cout << widget->getData() << std::endl;
}

int main()
{
  std::unique_ptr<Widget> widget(new Widget());
  widget->prepareData(42);

  std::thread t(processWidget, std::move(widget));
  t.join();

  return 0;
}

My guess is that there is something wrong with the destruction of the Widget object from main(), however I cannot pinpoint the issue. Is it necessary to do something additional to cleanup that variable? By the way, I'm using VS2013.

Answer 1

You are not allowed to make copy of unique_ptr, therefore the copy constructor is disabled. That is pointing the compiler error.

You can fix it with a reference:

void processWidget(std::unique_ptr<Widget>& widget)