最烦人的解析

Question

我从here获得了代码。

class Timer {
 public:
  Timer();
};

class TimeKeeper {
 public:
  TimeKeeper(const Timer& t);

  int get_time()
  {
      return 1;
  }
};

int main() {
  TimeKeeper time_keeper(Timer());
  return time_keeper.get_time();
}

从它的外观来看，它应该由于以下行而出现编译错误：

TimeKeeper time_keeper(Timer());

但只有在存在return time_keeper.get_time(); 时才会发生。

为什么这条线很重要，编译器会发现 time_keeper(Timer() ) 构造的歧义。

Answer 1

这是因为TimeKeeper time_keeper(Timer()); 被解释为函数声明而不是变量定义。这本身并不是一个错误，但是当您尝试访问 time_keeper 的 get_time() 成员（它是一个函数，而不是 TimeKeeper 实例）时，您的编译器会失败。

这是编译器查看代码的方式：

int main() {
  // time_keeper gets interpreted as a function declaration with a function argument.
  // This is definitely *not* what we expect, but from the compiler POV it's okay.
  TimeKeeper time_keeper(Timer (*unnamed_fn_arg)());

  // Compiler complains: time_keeper is function, how on earth do you expect me to call
  // one of its members? It doesn't have member functions!
  return time_keeper.get_time();
}