Re: [boost] Fwd: Installing CMake configuration files

Subject: Re: [boost] Fwd: Installing CMake configuration files
From: Peter Dimov (pdimov_at_[hidden])
Date: 2018-10-20 18:19:51

Steven Watanabe wrote:

> Before we go there, however, I'd like to discuss whether it's actually the 
> right thing to do.  In particular, I don't want to go down a path that 
> will get in the way of modular installation for headers.
>
> # Ideally it should look something like this:
> link-directory headers : ../include
>   : requirements <location>$(BOOST_ROOT)
>   : usage-requirements <include>$(BOOST_ROOT) ;
> alias boost_any : headers /boost/config//boost_config ;
> boost-install boost_any ;

I'm not sure we want that. A modular header-only library should have 
usage-requirements <include>../include, not <include>$(BOOST-ROOT) (and 
consequently should not link headers when used as a dependency.)

There are two cases: the library using the header-only library is 
"modularity-aware" and has enumerated its header-only dependencies as 
<library>/boost/any, or isn't and hasn't.

If it is and has, it doesn't need the links in boost/.

If it isn't and hasn't, `any` having <include>$(BOOST-ROOT) in its 
usage-requirements doesn't matter as it won't be seen. In this case, the 
dependency on /boost/headers that we inject will both link the headers and 
give the <include>$(BOOST-ROOT).

(I was thinking that /boost/headers can become a regular alias to 
libs/headers/build, whose implicit `stage` will then link the headers. But 
it will work the same either way.)

In case it isn't clear, the above assumes incremental transition to the 
modular layout, where some libraries are modular, others aren't yet. So we 
have four types of libraries:

1. Old-school header-only libraries, without build/Jamfile. These all form 
the metalibrary /boost/headers. When staged, /boost/headers links everything 
into boost/, as before. When installed, it copies all headers that aren't 
otherwise covered below, into the install location.

2. Newfangled header-only libraries, with build/Jamfile. When staged, these 
do nothing. When installed, they install their headers.

3. Old-school buildable libraries. These have a dependency on 
/boost/headers.

4. Newfangled buildable libraries. These enumerate all of their dependencies 
in their Jamfile, including type 2 header-only ones. If they use no type 1 
libraries, they don't need a dependency on /boost/headers, and don't need 
the header links.