c++11 chrono 未引用的局部变量

Question

当我使用以下代码执行某些操作 1 秒时，我从 Visual Studio 收到 C4101 警告:warning C4101: 'highResClock' : unreferenced local variable。我不明白为什么在声明后两次使用 highResClock 时会收到此警告。

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
}

编辑: Visual Studio 发出的警告似乎是因为 std::chrono::high_resolution_clock::now() 是一个静态函数。访问 now() 不需要 highResClock 变量，即使这是我选择使用的特定方法。 Visual Studio 似乎将此解释为未使用变量。当我使用以下内容时，我不再收到任何警告:

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

您在一个类的实例上使用静态方法，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].

MSDN 文章还提供了如何消除警告的附加信息:

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();.

由于您使用的是标准类，因此您应该求助于后者，例如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
}

也就是说，您不应该使用忙循环来等待，还有其他方法可以做到这一点(例如，条件变量，或 std::this_thread::sleep_*)。