Question

I’ve been assigned to completely run a project using CMake.

Basically, the project has over 20 modules, and for each module i created a CMake file
such as:

# Module: CFS
file(
    GLOB_RECURSE
    files
    *.c
    *.cpp
)

include_directories("${PROJECT_SOURCE_DIR}/include/PEM/cfs")
include_directories("${PROJECT_SOURCE_DIR}/include/PEM/kernel2")
SET(LIBRARY_OUTPUT_PATH ${PROJECT_BINARY_DIR}/lib)
add_library(cfs ${files})

kernel2 is another module and has its own CMakeFile.

Now the problem is that a third module: m3 requires headers from cfs (which also require headers from kernel2)

So i basically go with:

# Module: m3
file( ... )
include_directories("${PROJECT_SOURCE_DIR}/include/PEM/cfs")
add_library(m3 ${files})
target_link_library(m3 cfs)

Unfortunately this is not enough, kernel2 included files won’t be found when i compile unless I add:

include_directories("${PROJECT_SOURCE_DIR}/include/PEM/kernel2")

Am I doing it wrong? Perhaps I should also add include files using add_library directive?

Answer 1

If you have #include directives in cfs’s headers, then you should use

include_directories("${PROJECT_SOURCE_DIR}/include/PEM/kernel2")

It’s not the problem of CMake, but how C/C++ compiler works.

For example, you have following header in cfs:

#include "kernel2/someclass.h"

class SomeCfsClass
{
private:
    SomeKernelClass kernelObject;
}

Now if you wish to instantiate SomeCfsClass in your m3 module, the compiler should know it’s size. But knowing it’s size is not possible without knowning SomeKernelClass definition from kernel2/someclass.h.

This situation can be resolved by storing not the object, but pointer to it inside SomeCfsClass:

class SomeKernelClass; // forward declare SomeKernelClass

class SomeCfsClass
{
private:
    SomeKernelClass * kernelObject;
}

But of course, there are cases, when it’s simply impossible to avoid including.

As an alternative, i can suggest to use relative paths in #include directives, but this solution is somewhat hackish.