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Subject: Re: [Boost-build] feature, properties, variants, and all the rest
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2017-07-29 15:24:25


On 07/29/2017 08:53 AM, Stefan Seefeld via Boost-build wrote:
> I was actually thinking of something rather common when building C/C++,
> in particular with autotools:
> As a first step, a few config checks are run, to detect certain features
> of the build environment. These result in the definition of macros. So I
> was thinking of a chain of two such config checks A and B, where A would
> check for the presence of a header (say), and B would then attempt to
> build a (test) binary with it. If both succeed, the actual executable is
> built (let's call that target C). Now let's assume the header check A
> also sets a include search path, which is used both to compile target B
> and C.
> How would that chain of dependencies be expressed with b2 ? What
> features and properties are used, and how, to pass the header search
> path to B and C ?

This particular scenario would look something like:

obj A : a.cpp : : <include>/a/path ;
obj B : b.cpp : [ check-target-builds A : <use>A ] ;
exe C : C.cpp : [ check-target-builds B : <use>B ]

Now, when building C, it will go something like this:
- Building C
  - Building B from check-target-builds
    - Building A from check-target builds
      - A builds successfully
    - Applying property <use>A
    - Common properties for B: <use>A
    - Building dependencies of B
      - Building A - reuse existing target
    - Apply usage-requirements from A
    - Properties for B: <include>/a/path
    - Usage requirements for B: <include>/a/path (Usage
      requirements propagate up from A)
  - B builds successfully
  - Applying property <use>B
  - Build B and get usage-requirements
  - Build C.o with <include>/a/path

If either A or B fails to build, then the corresponding
<use>A or <use>B will not be applied and the usage-requirement
<include>/a/path will stop propagating upwards.

In Christ,
Steven Watanabe

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