c++11 chrono unreferenced local variable

Question

When I use the following code to perform some action for 1 second, I get a C4101 warning from Visual Studio: warning C4101: 'highResClock' : unreferenced local variable. I don't understand why I get this warning when I use highResClock twice after declaring it.

chrono::high_resolution_clock highResClock;
chrono::duration<int, ratio<1, 1> > dur(1);
chrono::time_point<chrono::high_resolution_clock> end = highResClock.now() + dur;

while (highResClock.now() < end)
{
    // do something repeatedly for 1 second
}

Edit: It looks like the warning from Visual Studio is because std::chrono::high_resolution_clock::now() is a static function. The highResClock variable wasn't needed to access now(), even though that is the particular method I chose to use. Visual Studio seems to interpret this as a non-use of the variable. When I use the following I no longer get any warnings:

chrono::duration<int, ratio<1, 1> > dur(1);
chrono::time_point<chrono::high_resolution_clock> end = chrono::high_resolution_clock::now() + dur;

while (chrono::high_resolution_clock::now() < end)
{
    // do nothing
}

Answer 1

You're using a static method on an instance of a class, which causes this warning:

However, this warning will also occur when calling a static member function through an instance of the class:

// C4101b.cpp
// compile with:  /W3
struct S {
   static int func()
   {
      return 1;
   }
};

int main() {
   S si;   // C4101, si is never used
   int y = si.func();
   return y;
}

In this situation, the compiler uses information about si to access the static function, but the instance of the class is not needed to call the static function; hence the warning [emphasis added].

The MSDN article also provides additional information how to get rid of the warning:

To resolve this warning, you could:

• Add a constructor, in which the compiler would use the instance of si in the call to func.

• Remove the static keyword from the definition of func.

• Call the static function explicitly: int y = S::func();.

Since you're using a standard class, you should resort to the latter, e.g. std::chrono::high_resolution_clock::now():

auto end = std::chrono::high_resolution_clock::now() + std::chrono::seconds(1);

while(std::chrono::high_resolution_clock::now() < end)
{
    // do nothing
}

That being said, you shouldn't use a busy-loop to wait, there are other ways to do this (e.g. condition variables, or std::this_thread::sleep_*).