Why must a launch policy be specified when using std::async with Callable with two or more argument?

Question

This is a question on the usage of std::async in Visual Studio 11 Beta. Given the following code:

#include <future>
#include <functional>

int main()
{
	std::function<int ()> f0 = []()
	{
		return 22;
	};

	std::async(f0);	// Fine.


	std::function<int (int)> f1 = [](int)
	{
		return 22;
	};

	std::async(f1, 11);	// Fine.


	std::function<int (int, int)> f2 = [](int, int)
	{
		return 22;
	};

	std::async(f2, 11, 22);	// Fails compilation.
	
	std::async(std::launch::any, f2, 11, 22);	// Fine.

	return 0;
}

The line that fails compilation fails with an error message that seems to imply that f2 is interpreted as a Callable that takes a single argument. The line right after it doesn't suffer from the problem. The only difference between the two is that a launch policy is defined. The two lines should have no functional differences.

The line that fails does work under Visual Studio 2010 with just::thread's implementation of <future>.

Is this normal?

Answer 1

Hi,

It seems that this behavior is an issue in Visual Studio 11. In addition, I also notice that you have submit a feedback in Connect.:http://connect.microsoft.com/VisualStudio/feedback/details/729760/std-async-fails-compilation-for-callable-with-two-or-more-arguments

Best Regards,

Rob

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Rob Pan [MSFT]
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