最烦躁的解析 [英] Most vexing parse

问题描述

我从这里获得了代码。

  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，它应该得到编译错误，

  TimeKeeper time_keeper（Timer 
  

但是它只会发生在 return time_keeper.get_time（）; time_keeper（Timer（））发现歧义 code>

<这是由于 TimeKeeper time_keeper（Timer（））的原因，这是因为 ）; 被解释为一个函数声明，而不是一个变量定义。这本身不是一个错误，但是当您尝试访问time_keeper（这是一个函数，而不是TimeKeeper实例）的 get_time（）成员时，失败。

这是编译器如何查看代码：

  int main（）{
 // time_keeper被解释为一个带有函数参数的函数声明。 
 //这绝对是*不是*我们期望的，但是从编译器POV它没关系。 
 TimeKeeper time_keeper（Timer（* unnamed_fn_arg）（））; 
 
 //编译器抱怨：time_keeper是函数，你怎么期望我调用
 //它的成员？它没有成员函数！ 
 return time_keeper.get_time（）; 
} 
  

I got the code from here.

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();
}

From the looks of it, it should get compile error due to the line:

TimeKeeper time_keeper(Timer());

But it only happens if return time_keeper.get_time(); is present.

Why would this line even matter, the compiler would spot ambiguity on time_keeper(Timer() ) construction.

解决方案

This is due to the fact that TimeKeeper time_keeper(Timer()); is interpreted as a function declaration and not as a variable definition. This, by itself, is not an error, but when you try to access the get_time() member of time_keeper (which is a function, not a TimeKeeper instance), your compiler fails.

This is how your compiler view the code:

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();
}

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