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How it works

In this example, the warning flags -Wall, -Wextra, and -Wpedantic will be added to the compile options for the geometry target; both the compute-areas and geometry targets will use the -fPIC flag. Compile options can be added with three levels of visibility: INTERFACE, PUBLIC, and PRIVATE.

The visibility levels have the following meaning:

With the PRIVATE attribute, compile options will only be applied to the given target and not to other targets consuming it. In our examples, compiler options set on the geometry target will not be inherited by the compute-areas, even though compute-areas will link against the geometry library.
With the INTERFACE attribute, compile options on a given target will only be applied to targets consuming it.
With the PUBLIC attribute, compile options will be applied to the given target and all other targets consuming it.

The visibility levels of target properties are at the core of a modern usage of CMake and we will revisit this topic often and extensively throughout the book. Adding compile options in this way does not pollute the CMAKE_<LANG>_FLAGS_<CONFIG> global CMake variables and gives you granular control over what options are used on which targets.

How can we verify whether the flags are correctly used as we intended to? Or in other words, how can you discover which compile flags are actually used by a CMake project? One approach is the following and it uses CMake to pass additional arguments, in this case the environment variable VERBOSE=1, to the native build tool:

$ mkdir -p build
$ cd build
$ cmake ..
$ cmake --build . -- VERBOSE=1

... lots of output ...

[ 14%] Building CXX object CMakeFiles/geometry.dir/geometry_circle.cpp.o
/usr/bin/c++ -fPIC -Wall -Wextra -Wpedantic -o CMakeFiles/geometry.dir/geometry_circle.cpp.o -c /home/bast/tmp/cmake-cookbook/chapter-01/recipe-08/cxx-example/geometry_circle.cpp
[ 28%] Building CXX object CMakeFiles/geometry.dir/geometry_polygon.cpp.o
/usr/bin/c++ -fPIC -Wall -Wextra -Wpedantic -o CMakeFiles/geometry.dir/geometry_polygon.cpp.o -c /home/bast/tmp/cmake-cookbook/chapter-01/recipe-08/cxx-example/geometry_polygon.cpp
[ 42%] Building CXX object CMakeFiles/geometry.dir/geometry_rhombus.cpp.o
/usr/bin/c++ -fPIC -Wall -Wextra -Wpedantic -o CMakeFiles/geometry.dir/geometry_rhombus.cpp.o -c /home/bast/tmp/cmake-cookbook/chapter-01/recipe-08/cxx-example/geometry_rhombus.cpp
[ 57%] Building CXX object CMakeFiles/geometry.dir/geometry_square.cpp.o
/usr/bin/c++ -fPIC -Wall -Wextra -Wpedantic -o CMakeFiles/geometry.dir/geometry_square.cpp.o -c /home/bast/tmp/cmake-cookbook/chapter-01/recipe-08/cxx-example/geometry_square.cpp

... more output ...

[ 85%] Building CXX object CMakeFiles/compute-areas.dir/compute-areas.cpp.o
/usr/bin/c++ -fPIC -o CMakeFiles/compute-areas.dir/compute-areas.cpp.o -c /home/bast/tmp/cmake-cookbook/chapter-01/recipe-08/cxx-example/compute-areas.cpp

... more output ...

The preceding output confirms that the compile flags were correctly set according to our instructions.

The second approach to controlling compiler flags involves no modifications to CMakeLists.txt. If one wants to modify compiler options for the geometry and compute-areas targets in this project, it is as easy as invoking CMake with an additional argument:

$ cmake -D CMAKE_CXX_FLAGS="-fno-exceptions -fno-rtti" ..

As you might have guessed, this command will compile the project, deactivating exceptions and runtime type identification (RTTI).

The two approaches can also be coupled. One can use a basic set of flags globally, while keeping control of what happens on a per target basis. We can use CMakeLists.txt and running this command:

$ cmake -D CMAKE_CXX_FLAGS="-fno-exceptions -fno-rtti" ..

This will configure the geometry target with -fno-exceptions -fno-rtti -fPIC -Wall -Wextra -Wpedantic, while configuring compute-areas with -fno-exceptions -fno-rtti -fPIC.

In the rest of the book, we will generally set compiler flags on a per target basis and this is the practice that we recommend for your projects. Using target_compile_options() not only allows for a fine-grained control over compilation options, but it also integrates better with more advanced features of CMake.