meson/docs/markdown/Build-targets.md

short-description
Definition of build targets

Sometimes you can't build files from sources but need to utilize an existing object file. A typical case is using an object file provided by a third party. Object files can be specified just like sources.

exe = executable('myexe', 'source.cpp', objects : 'third_party_object.o')

A different case is when you want to use object files built in one target directly in another. A typical case is when you build a shared library and it has an internal class that is not exported in the ABI. This means you can't access it even if you link against the library. Typical workarounds for this include building both a shared and static version of the library or putting the source file in the test executable's source list. Both of these approaches cause the source to be built twice, which is slow.

In Meson you can extract object files from targets and use them as-is on other targets. This is the syntax for it.

lib = shared_library('somelib', 'internalclass.cc', 'file.cc', ...)
eo = lib.extract_objects('internalclass.cc')
executable('classtest', 'classtest.cpp', objects : eo)

Here we take the internal class object and use it directly in the test. The source file is only compiled once.

Note that careless use of this feature may cause strange bugs. As an example trying to use objects of an executable or static library in a shared library will not work because shared library objects require special compiler flags. Getting this right is the user's responsibility. For this reason it is strongly recommended that you only use this feature for generating unit test executables in the manner described above.