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To represent aspects of target configuration such as
debug and release variants, or single- and multi-threaded
builds portably, Boost.Build uses features with
associated values. For
example, the debug-symbols feature can have a value of on or
off. A property is just a (feature,
value) pair. When a user initiates a build, Boost.Build
automatically translates the requested properties into appropriate
command-line flags for invoking toolset components like compilers
and linkers.
There are many built-in features that can be combined to
produce arbitrary build configurations. The following command
builds the project's release variant with inlining
disabled and debug symbols enabled:
b2 release inlining=off debug-symbols=on
Properties on the command-line are specified with the syntax:
feature-name=feature-value
The release and debug that we have seen
in b2 invocations are just a shorthand way to specify
values of the variant feature. For example, the
command above could also have been written this way:
b2 variant=release inlining=off debug-symbols=on
variant is so commonly-used that it has been given
special status as an implicit feature—
Boost.Build will deduce its identity just from the name of one of its
values.
A complete description of features can be found in the section called “Features and properties”.
The set of properties specified on the command line constitutes
a build request—a description of
the desired properties for building the requested targets (or,
if no targets were explicitly requested, the project in the
current directory). The actual
properties used for building targets are typically a
combination of the build request and properties derived from
the project's Jamroot (and its other
Jamfiles, as described in the section called “Project Hierarchies”). For example, the
locations of #included header files are normally
not specified on the command-line, but described in
Jamfiles as target
requirements and automatically combined with the
build request for those targets. Multithread-enabled
compilation is another example of a typical target
requirement. The Jamfile fragment below
illustrates how these requirements might be specified.
exe hello
: hello.cpp
: <include>boost <threading>multi
;
When hello is built, the two requirements specified
above will always be present. If the build request given on the
b2 command-line explictly contradicts a target's
requirements, the target requirements usually override (or, in the case
of “free”” features like
<include>,
[1]
augment) the build request.
The value of the <include> feature is
relative to the location of Jamroot where it is
used.
If we want the same requirements for our other target,
hello2, we could simply duplicate them. However,
as projects grow, that approach leads to a great deal of repeated
boilerplate in Jamfiles.
Fortunately, there's a better way. Each project can specify a set of
attributes, including requirements:
project
: requirements <include>/home/ghost/Work/boost <threading>multi
;
exe hello : hello.cpp ;
exe hello2 : hello.cpp ;
The effect would be as if we specified the same requirement for both
hello and hello2.