Re: [boost] How to specify Boost headers location properly

Subject: Re: [boost] How to specify Boost headers location properly
From: Vladimir Prus (ghost_at_[hidden])
Date: 2010-03-11 01:16:01

Mateusz Loskot wrote:
> Steven Watanabe wrote:
>> Mateusz Loskot wrote:
>>> For 1st example for the case 1),
>>> in trunk/libs/accumulators/example/Jamfile.v2, Boost headers are
>>> located by relative path and mysterious $(BOOST_ROOT)
>>>
>>> exe example
>>>     :
>>>         main.cpp
>>>     :
>>>         <include>../../..
>>>         <include>$(BOOST_ROOT)
>>>     ;
>>>
>>>   
>> <include>$(BOOST_ROOT) can be replaced with <dependency>/boost//headers
> 
> Yet another option I've just learned, but...
> 
> What is the difference between using headers-only library by specifying
> this requirement using <source> and <dependency>?
> 
> The manual says:
> 
> "(...)a dependency on the target (...) (so it will be brought up-to-date
> whenever the target being declared is). The dependency is not used in
> any other way."
> 
> What does bringing up-to-date mean for headers-only?
> Also, the "not used in any other way" suggests to me that it does not
> implicitly update <include> directories.

Actually, for /boost//headers there's no difference between <source>
and <dependency>. Every single metatarget in Boost.Build, when built,
produces two things:

- actual targets (corresponding to files)
- set of properties that must be propagated to dependents. These
are known as usage requirements.

Now, there are three predefined build properties:

1. <source> tries to consume the produced targets, and adds usage requirements.
2. <dependency> sets a dependency on the produced targets, and also adds
usage requirements.
3. <use> only sets usage requirements

Now, because the 'headers' metatarget never produces any real file, only
usage requirements, all three are exactly the same.

- Volodya