Cmake could not find boost_python

Question

I am trying to build this simple boost python demo from this link on my MacOS High Sierra.

Following is the hello_ext.cpp :

#include <boost/python.hpp>

char const* greet()
{
  return "hello, world";
}

BOOST_PYTHON_MODULE(hello_ext)
{
  using namespace boost::python;
  def("greet", greet);
}

Following is the CmakeLists.txt :

cmake_minimum_required(VERSION 3.5)

# Find python and Boost - both are required dependencies
find_package(PythonLibs 2.7 REQUIRED)
find_package(Boost COMPONENTS python REQUIRED)

# Without this, any build libraries automatically have names "lib{x}.so"
set(CMAKE_SHARED_MODULE_PREFIX "")

# Add a shared module - modules are intended to be imported at runtime.
# - This is where you add the source files
add_library(hello_ext MODULE hello_ext.cpp)

# Set up the libraries and header search paths for this target
target_link_libraries(hello_ext ${Boost_LIBRARIES} ${PYTHON_LIBRARIES})
target_include_directories(hello_ext PRIVATE ${PYTHON_INCLUDE_DIRS})

I figured I need to install python. Boost 1.69 was already installed and I did brew install boost-python which worked fine. Doing a brew list | grep 'boost' brew list | grep 'boost' lists out boost and boost-python .

But, doing a cmake .. from a build directory complains the following:

Could not find the following Boost libraries:

      boost_python

No Boost libraries were found.  You may need to set BOOST_LIBRARYDIR to 
the directory containing Boost libraries or BOOST_ROOT to the location 
of Boost.

What am I missing here?

Answer 1

From this document :

Note that Boost Python components require a Python version suffix (Boost 1.67 and later), eg python36 or python27 for the versions built against Python 3.6 and 2.7, respectively. This also applies to additional components using Python including mpi_python and numpy. Earlier Boost releases may use distribution-specific suffixes such as 2, 3 or 2.7. These may also be used as suffixes, but note that they are not portable.

The example you found was probably using an older version of Boost. So, you may need to change this line:

find_package(Boost COMPONENTS python27 REQUIRED)

Answer 2

To pass the right python version to find_package(Boost) , I suggest to extract it from the version of python found on the system.

find_package(PythonLibs 3.6 REQUIRED)
# Extract major/minor python version
string(REPLACE "." ";" VERSION_LIST ${PYTHONLIBS_VERSION_STRING})
list(GET VERSION_LIST 0 PYTHONLIBS_VERSION_MAJOR)                                                                                                                             
list(GET VERSION_LIST 1 PYTHONLIBS_VERSION_MINOR)
find_package(Boost COMPONENTS python${PYTHONLIBS_VERSION_MAJOR}${PYTHONLIBS_VERSION_MINOR} REQUIRED)

The 3.6 on the first line is a minimum version, it finds the python38 boost module on my machine due to /usr/lib64/libpython3.8.so .