C ++中的短路`运算符<<`输出 [英] Short circuit `operator<<` output in C++

问题描述

  if（debug）{
 Output< < f1：<< f1（）<< \\\
; 
} 
  

现在我想做的是写一个流类 Debug 其中我可以这样写

  Debug< f1：<< f1（）<< \\\
; 
  

如果设置了一些全局标志，那么这将产生输出，否则。

现在：通过 Debug 可以很容易地返回一个流到 / dev / null 这将吞噬输出。问题是， f1（）仍然被评估（和'rendered'成一个文本表示，甚至可能更昂贵），这可能是相当糟糕的性能。 p>

现在我的问题：是否有任何技巧允许跳过

的'评估'

 f1：< f1（）<< \\\

  

完全如果调试决定不应该进行输出？类似于C ++为 f（）&&& g（）其中 g（） c $ c> false （我认真考虑编写一个使用&& 作为输出运算符的流类，没有为过载的操作符和&&& ）执行循环。

解决方案

你可以做的是定义这个宏：

  #define Debug_Stream \ 
 if else output 
  

这将使得：

  Debug_Stream<< f1：<< f1（）<< \\\
; 
  

变成等价的：

  if（debug）{
输出<< f1：<< f1（）<< \\\
; 
} 
  

但实际上（加上可读性的空格）

  if（！debug）; 
 else 
输出<< f1：<< f1（）<< \\\
; 
  

I have some code that is sprinkled with constructs like this

if(debug) {
    Output << "f1: " << f1() << "\n";
}

Now what I want to do is write a stream class Debug where I could write it like this

Debug << "f1: " << f1() << "\n";

If some global flag is set then this would generate output, otherwise not.

Now: this can be quite easily done by making Debug return a stream that goes to /dev/null which would swallow the output. The problem is that f1() still gets evaluated (and 'rendered' into a textual representation which might be even more expensive) which might be quite bad for the performance.

Now my question: is there any trick that allows the skipping of the 'evaluation' of

"f1: " << f1() << "\n"

completely if Debug decides that no output should be done? Similar to the short circuiting that C++ does for f() && g() where g() is not evaluated if f()is false (I seriously considered writing a stream class that uses && as the output operator but from what I read short-circuiting is not done for overloaded operator&&)

解决方案

What you can do is define this macro:

#define Debug_Stream \
if(!debug); else Output

This would make this:

Debug_Stream << "f1: " << f1() << "\n";

become equivalent to this:

if(debug) {
    Output << "f1: " << f1() << "\n";
}

But literally (plus whitespace for readability)

if(!debug);
else
    Output << "f1: " << f1() << "\n";

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