std::thread Visual Studio 2012 Warning

Question

I am trying to understand how to use the new std::thread using Visual Studio 2012. I am trying to compile the following code.

#include <iostream>
#include <thread>

class scoped_thread
{
    std::thread t_;

public:

    explicit scoped_thread(std::thread & t): t_(std::move(t))
    {
        if(!t_.joinable())throw std::logic_error("No thread");
    }

    ~scoped_thread()
    {
        t_.join();
    }

private:

    scoped_thread(scoped_thread const &);
    scoped_thread & operator=(scoped_thread const &);
};

struct local_functor
{
    int& i_;

    local_functor(int & i):i_(i){}

    void operator()()
    {
        while(i_ < 1e5)i_++;        
    }
};

// can potentially throw exceptions
void callAnotherFunc()
{
    std::cout << "this function can throw an exception" << std::endl;   
    // try (un)commenting the line below and see the behaviour
    throw std::out_of_range("WTF2");
}


int main()
{
    int some_local_state =  0;

    try
    {   
        scoped_thread t(std::thread(local_functor(some_local_state)));
        callAnotherFunc();

        std::cout << "Proper exit of function" << std::endl;
    }
    catch(const std::exception & e)
    {
        std::cout << e.what() << " exception occurred!" << std::endl;
    }
    catch(...)
    {
        std::cout << "Unhandled exception!" << std::endl;
    }

    return 0;
}

I am getting a warning that says warning C4930: 'scoped_thread t(std::thread (__cdecl *)(local_functor))': prototyped function not called (was a variable definition intended?)

Yes, it was a variable definition that was intended. How am I supposed to do that?

Answer 1

The warning tells you, that the first line in your try block is parsed as a function declaration. That happes sometimes if you use C++03 initialization style. Use uniform initialization instead:

scoped_thread t{std::thread{local_functor{some_local_state}}};

In addition, you have a missing & in the scoped_thread constructor:

explicit scoped_thread(std::thread && t): t_(std::move(t))
//                                 ^-- use r-value ref

PS: If your compiler does not support uniform initialization, wrap the initializer into another pair of brackets: scoped_thread t((std::thread(local_functor(some_local_state))));

Answer 2

You stumbled upon the most vexing parse.

You could solve it by using the uniform initialization syntax:

scoped_thread t{std::thread(local_functor(some_local_state))};