text: Q:
  
  C++ Thread Error - compile-time error with class argument
  
  I found the page C++ thread error: no type named type, with a similar error message to the one I am reporting below.  The answer on that page doesn't cover this case, as best I can tell.  Admittedly, there must be something simple that I am simply clueless about here.
  I have been trying use threads in a C++ program I am working on.  I had an initial version working with boost::thread without any problems.  This morning, I tried to rewrite my code to use std::thread instead of boost::thread.  That's when I suddenly ran into compile-time errors that I do not understand.  I have reduced the problem to the following piece of code.
  The upshot?  As soon as I try to pass a reference to one of my own user-defined classes as a function argument, the program fails to compile.  
  #include <iostream>
  #include <thread>
  
  class TestClass { } ;
  
  void testfunc1 ( void )           { std::cout << "Hello World TF1" << std::endl ; }
  void testfunc2 ( double val )     { std::cout << "Hello World TF2" << std::endl ; }
  void testfunc3 ( TestClass & tc ) { std::cout << "Hello World TF3" << std::endl ; }
  
  int main ( int argc, char *argv[] )
  {
      std::thread t1 ( testfunc1 ) ;
  
      double tv ;
      std::thread t2 ( testfunc2, tv ) ;
  
      TestClass tc ;
      std::thread t3 ( testfunc3, tc ) ;    //  compiler generates error here
  
      return 0 ;
  }
  
  The code will compile as long as I comment out that last line of code.  When it is present, however, I get the following compile-time error.
  $ g++ -std=c++11 test.cpp
  In file included from /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/thread:39:0,
                   from test.cpp:3:
  /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/functional: In instantiation of ‘struct std::_Bind_simple<void (*(TestClass))(TestClass&)>’:
  /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/thread:142:59:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = void (&)(TestClass&); _Args = {TestClass&}]’
  test.cpp:19:33:   required from here
  /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/functional:1505:61: error: no type named ‘type’ in ‘class std::result_of<void (*(TestClass))(TestClass&)>’
         typedef typename result_of<_Callable(_Args...)>::type result_type;
                                                               ^
  /usr/lib/gcc/x86_64-pc-cygwin/5.4.0/include/c++/functional:1526:9: error: no type named ‘type’ in ‘class std::result_of<void (*(TestClass))(TestClass&)>’
           _M_invoke(_Index_tuple<_Indices...>)
  
  There is clearly some kind of type-related issue, but I can't decipher this error message.  Any idea what the issue is?  (I happen to be using Cygwin on a Windows 10 machine, but I don't think that's relevant to the problem described.)
  
  A:
  
  That's because std::thread can't store C++ references, it's similar to how std::vector<T&> can't exist. So, in order to pass references around, there's a reference wrapper in the standard library. It's basically a pointer under the hood, which mimics some of the language-wise reference's behavior. The std::ref and std::cref (for const references) functions are used to create an object of std::reference_wrapper (they're convenient functions, as they have template type deduction and shorter names).
  The only thing you have to add in your code is the std::ref function call, like so:
  TestClass tc;
  std::thread t3(testfunc3, std::ref(tc));
  
  