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Re: Problem with PFE approach [Was: Faster compilation speed]

On Saturday, August 17, 2002, at 08:04  PM, Daniel Berlin wrote:

On Sat, 17 Aug 2002, Timothy J. Wood wrote:

So, another problem with PFE that I've noticed after working with it
for a while...

If you put all your commonly used headers in a PFE, then changing any
of these headers causes the PFE header to considered changed. And,
since this header is imported into every single file in your project,
you end up in a situation where changing any header causes the entire
project to be rebuilt.
Um, this header should *not* be explicitly included in the files.
It's *prefix* header.
I'm not saying that I'm #including it in my sources. What I'm saying is that the IDE knows that all my files depend upon it (they all end up including it due to it being the prefix header, regardless of whether it is listed or not). This means that they may have depedencies on the its contents and must be rebuilt if it or any header it includes changes.

The way I think about this is that the prefix header mess is just a hack to avoid having a #include at the top of each file. There should be nothing else special about the header -- it is just assumed that there is a #include at the top of your file.

The only thing that would need to be rebuilt in this case is the prefix header.
Everything else that would normally not be rebuilt will not be rebuilt.
Nope... everything needs to be rebuilt. The problem is that the prefix header might satisfy some symbol or macro that a source file needs (assume that the source file doesn't explicitly include headers it needs). If the only approach to PCH is having a prefix header, then this is the way things are going to end up working -- people will not #include the correct set of headers (they will try, but they will invariably fail).

Thus, the only (correct) choice the IDE has is to treat the PFE as a included file. To do otherwise will cause the user to potentially get unexpected results (and we're back to the usability issue again).


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