记录函数的返回值

问题描述

我想记录一个函数的返回值。问题是，函数可能有很多出口点，我不想在每个人之前添加一个日志调用。

我想到有一个内部对象它负责进行 Log 调用。但仍然，我必须在每个返回语句之前通知它的返回值。

创建一个宏，在从函数返回之前调用记录器。像：

  #define RETURN（ret）Logger.log（__ FUNCTION__，ret）; return ret; 
  

但我想避免这样做。

感谢

解决方案

我不认为你可以做得更好，更容易。在这种情况下，我认为对源的影响最小的解决方案是使用预处理器，但是你不应该这样做，因为它已经内置了代价。Fx：

  if（done）
 RETURN（something）; 
  

可扩展为：

  if（done）
 Logger.log（function_name，something）;返回东西
  

这意味着 something 如果 done 为true，则返回。

为了使扩展适合单个语句，它通常包装在 do {...} while（0），这将使该示例日志， done 为true。

但是仍然有一个惊喜，因为宏参数扩展了两次，考虑你写 RETURN（something ++）; 然后它会扩展到 Logger.log（__ FUNCTION__，something ++）;返回一些++; 这意味着不幸的副作用。这是C中的一个真正的问题，但不是在C ++。这里的模板很方便：

 模板< typename T& 
 T const& log_and_return（char const * func，const T& value）
 {
 Logger.log（func，value）; 
返回值; 
} 
 
 #define RETURN（value）return log_and_return（__ func__，value）
  

注意，它在标准中被称为 __ func __ （不 __ FUNCTION __ ）。

I would like to log the return value of a function. The problem is that the function might have many exit points and I don't want to add a log call before every one of them.

I thought about having an inner object that's responsible on making the Log call. But still, I would have to notify it about the return value before each return statement.

I also thought about creating a macro that calls the logger before returning from the function. Something like:

#define RETURN(ret) Logger.log(__FUNCTION__, ret); return ret;

But I want to avoid doing that.

Any other thoughts on how I can achieve that nicely and easily?

Thanks

解决方案

I don't think you can do that more nicely and easily. In this case I think the solution with least impact on the source is to use the preprocessor, but you shouldn't do it the way you do because it has surprices built in. Fx:

if( done )
    RETURN(something);

expands to:

if( done )
    Logger.log("function_name", something); return something;

which means that something is sent to the log if done is true, then something is returned anyway.

To make the expansion fit into a single statement it's normally wrapped in a do { ... } while(0) which would make that example log and return only if done is true.

But there's still a surprise since the macro argument is expanded twice, consider the case where you write RETURN(something++); then it will expand to Logger.log(__FUNCTION__, something++); return something++; which means unfortunate side effects. This was a real problem in C, but not in C++. Here templates are handy:

 template<typename T>
 T const& log_and_return(char const* func, const T& value)
 {
      Logger.log(func, value);
      return value;
 }

 #define RETURN(value) return log_and_return(__func__, value)

Note that it is called __func__ in the standard (an not __FUNCTION__).

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