When you use most of main target rules, Boost.Build automatically figures what commands to run and it what order. As soon as you want to use new file types, or support new tools, one approach is to extend Boost.Build to smoothly support them, as documented in Chapter 6, Extender Manual. However, if there's a single place where the new tool is used, it might be easier to just explicitly specify the commands to run.
Three main target rules can be used for that. The make
rule allows you to construct a single file from any
number of source file, by running a command you specify. The
notfile rule allows you to run an arbitrary
command, without creating any files. And finaly, the
generate rule allows you to describe transformation using
Boost.Build's virtual targets. This is higher-level than file names that
the make rule operates with and allows you
to create more than one target, create differently named targets
depending on properties or use more than one tool.
The make rule is used when you want to
create one file from a number of sources using some specific command.
The notfile is used to unconditionally run
a command.
Suppose you want to create file file.out from file
file.in by running command
in2out. Here is how you would do this in Boost.Build:
make file.out : file.in : @in2out ;
actions in2out
{
in2out $(<) $(>)
}
If you run bjam and file.out
does not exist, Boost.Build will run the in2out
command to create that file. For more details on specifying actions, see
the section called “Boost.Jam Language”.
It could be that you just want to run some command unconditionally, and
that command does not create any specific files. For that you can use
the notfile rule. For example:
notfile echo_something : @echo ;
actions echo
{
echo "something"
}
The only difference from the make rule is
that the name of the target is not considered a name of a file, so
Boost.Build will unconditionally run the action.
The generate rule is used when you want to
express transformations using Boost.Build's virtual targets, as opposed
to just filenames. The generate rule has
the standard main target rule signature, but you are required to specify
the generating-rule property. The value of the
property should be in the form
@, the named rule
should have the following signature:
rule-name
rule generating-rule ( project name : property-set : sources * )
and will be called with an instance of the project-target
class, the name of the main target, an instance of the
property-set class containing build properties, and the
list of instances of the virtual-target class corresponding
to sources. The rule must return a list of virtual-target
instances. The interface of the virtual-target class can be
learned by looking at the build/virtual-target.jam
file. The generate example contained in the
Boost.Build distribution illustrates how the generate
rule can be used.