Std :: thread Warning Visual Studio 2012

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 get a warning that says warning C4930: 'scoped_thread t (std :: thread (__cdecl *) (local_functor))': the prototyped function was not called (was the variable defined?)

Yes, that was the definition of the variable that was intended. How am I supposed to do this?

Answer 1

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

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

In addition, the scoped_thread constructor lacks & :

 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 in another pair of brackets: scoped_thread t((std::thread(local_functor(some_local_state))));

Answer 2

You stumbled upon the most unpleasant parsing .

You can solve this problem with uniform initialization syntax:

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