Re: [boost] Proposal for #pragma once support

Subject: Re: [boost] Proposal for #pragma once support
From: Christopher Jefferson (chris_at_[hidden])
Date: 2009-06-09 15:13:50

On 9 Jun 2009, at 20:08, Zachary Turner wrote:

> That being said, compilation of boost itself isn't even the real use  
> case,
> since a user of boost typically just compiles it once and is done.   
> The real
> use case is for people using boost in their own projects, i.e.  
> including
> lots of boost header files at various locations in their program.   
> In this
> case, it has potential to make a huge impact, since developers  
> frequently
> build code tens of times per day.  Even the slightest change to a  
> header
> file and a rebuild of the project to test the change has the  
> potential to
> trigger a rebuild of numerous other files, so there's no reason not  
> to take
> every measure possible to reduce compilation time.

While it would be a bit of work, I'd be very interested in seeing  
convincing numbers to this effect: If you add #pragma once (with  
suitable guards) to boost (or a substantial section of it), then prove  
that the resulting code produces speed improvements, then you would  
convince people!

One thing to watch out for, in case you don't notice in advance: Some  
headers should NOT have #pragma once. Not very many, but such files  
exist. You'll probably want to try to find some way of guessing if a  
file already has a standard include guard (shouldn't be too hard to  
detect), and only #pragma once such files.
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