Re: CMake and Boost

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-06-11 18:51:10

Robert Ramey wrote:
> 1) The first lines show
> 
> mkdir __build
> cd __build
> cmake ..
> cmake --build .
> 
> Does this presume that one starts from the library directory?
> Does this syntax work for out of source tree builds?  That is, would the
> following work?
> 
> mkdir /temp/__build
> cd /temp/__build
> cmake ..
> cmake --build .

You can put your binary directory wherever you like, but then you
need to pass the correct source directory at the `cmake ..` point,
because `..` is no longer the source directory.

> 2) BOOST_INCLUDE_LIBRARY
> 
> I would like my CMakeLists.txt file to explicitly list the direct antecedent
> libraries (dependencies - bad word usage).  So I'd expect to see something like
> 
> add_directory(../filesystem)
> add_directory(../regex)

This can only work if your library is the root project. If it's not, you can't
just add_subdirectory your dependencies, because the root project may
use more than one Boost library. If each of these tries to add_subdirectory
other Boost libraries, there will be duplication and things are not going to
work.

In fact things are not going to work even in your case (if all libraries did what
you're trying to do) because `filesystem` may perform `add_subdirectory(regex)`
and then your own `add_subdirectory(regex)` will fail.

As a general rule, subprojects should never use add_subdirectory for their
dependencies.

The intended use of BOOST_INCLUDE_LIBRARIES is from the Boost root:

mkdir __build__ && cd __build__
cmake -DBOOST_INCLUDE_LIBRARIES=serialization ..

This tells the superproject to perform the necessary add_subdirectory calls
for Serialization and its dependencies.