Calling a bound lambda through a std::function variable

Question

I am unsucessfully trying to compile the following code in Visual Studio 2010. For what it is worth, the code compiles fine in GCC 4.5.1 with the -std=c++0x flag. Can someone help me figure out why this is not working?

#include <functional>
#include <iostream>
#include <string>

int main()
{
  int value = 42;
  auto myBoundLambda = [&](const std::string& header) -> void
  {
    std::cout << header << value << std::endl;
  };

  std::function<void()> myFunction = std::bind(myBoundLambda, "Answer to life is: ");
  myFunction();

  return 0;
}

Error 1 error C2903: 'result' : symbol is neither a class template nor a function template c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 2 error C2039: 'result' : is not a member of '`anonymous-namespace'::<lambda0>' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 3 error C2143: syntax error : missing ';' before '<' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 4 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 5 error C2039: 'type' : is not a member of '`global namespace'' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 6 error C2238: unexpected token(s) preceding ';' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 28 1 CPP0X Lambda Test
Error 7 error C2039: '_Type' : is not a member of 'std::tr1::_Result_type1<__formal,_Fty,_Arg0>' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 40 1 CPP0X Lambda Test
Error 8 error C2146: syntax error : missing ';' before identifier '_Type' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 40 1 CPP0X Lambda Test
Error 9 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 40 1 CPP0X Lambda Test
Error 10 error C2602: 'std::tr1::_Result_of1<_Fty,_Farg0>::_Type' is not a member of a base class of 'std::tr1::_Result_of1<_Fty,_Farg0>' c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 40 1 CPP0X Lambda Test
Error 11 error C2868: 'std::tr1::_Result_of1<_Fty,_Farg0>::_Type' : illegal syntax for using-declaration; expected qualified-name c:\program files (x86)\microsoft visual studio 10.0\vc\include\xxresult 40 1 CPP0X Lambda Test

Answer 1

As far as I can tell, this is a deficiency in VC++ 2010's standard library implementation, rather than a bug in VC++ 2010's lambda implementation. By design, C++0x lambdas do not implement the TR1 result_of protocol, because C++0x's result_of is decltype-based rather than using nested result<>/result_type types. The problem is that VC++ 2010's result_of appears to still rely on the TR1 protocol rather than use decltype. Presumably this will be addressed in a service-pack or in VC11, but in the meantime if you want to formally submit a bug report (because this is indeed a bug), submit it on MS Connect and post a link back here so we can vote it up.

Answer 2

If this is presenting a problem for you in real code, a workaround is to avoid use of std::result_of, which in practical terms means also avoiding use of std::bind -- use a lambda instead:

#include <functional>
#include <iostream>
#include <string>

int main()
{
	int value = 42;
	auto myBoundLambda = [&](const std::string& header) -> void
	{
		std::cout << header << value << std::endl;
	};

	std::function<void()> myFunction = [=]() { myBoundLambda("Answer to life is: "); };
	myFunction();

	return 0;
}

Answer 3

Thank you for the help. Here is a link to the feedback posted to MS Connect:

https://connect.microsoft.com/VisualStudio/feedback/details/618807