Map include path to different directory - c++

Question

I am looking for a way to tell CMake to make an include directory appear under another name to the compiler.
Let's say I have a project that needs code from a foreign library. This code resides in a directory foreignLib-1.5.0_build123456 in my project's root directory. From time to time I will want to update this library and in this process change the library directory name.
I want to reference this directory as foreignLib in my C++ source files. I want to be able to write
#include "foreignLib/include/lib.h"
and CMake should tell my compiler to translate this to
#include "foreignLib-1.5.0_build123456/include/lib.h"
Now I am wondering:
Does a feature like this exist in CMake?
If yes, how do I use it?
If yes, which compilers are supported?

Answer 1

I'd probably just copy the entire include directory into my build tree and make that available.
A decent way of doing that is to use file(GLOB_RECURSE ...) to gather a list of files in the includes folder, then use configure_file(<input> <output> COPYONLY) to copy them to the build tree.
By using configure_file, the files in the build tree are only replaced as required. This means that re-running CMake won't automatically make these files appear out-of-date to the build tool, hence avoiding an unnecessary recompilation.
set(ForeignLibName foreignLib-1.5.0_build123456)
set(ForeignLibRoot ${CMAKE_SOURCE_DIR}/${ForeignLibName})
file(GLOB_RECURSE IncludeFiles RELATIVE ${ForeignLibRoot} ${ForeignLibRoot}/*)
foreach(IncludeFile ${IncludeFiles})
configure_file(${ForeignLibRoot}/${IncludeFile}
${CMAKE_BINARY_DIR}/${ForeignLibName}/foreignLib/${IncludeFile}
COPYONLY)
endforeach()
include_directories(${CMAKE_BINARY_DIR}/${ForeignLibName})
This should allow you to do:
#include "foreignLib/include/lib.h"
When you update the foreign library, you'd have to ensure that you also updated the CMakeLists.txt so that CMake re-runs the next time you go to build. Updating the CMakeLists.txt should only involve changing the single line set(ForeignLibName ...).

Answer 2

You could have a look at Cmake's configure_file: Rename your source file into yourfile.cpp.in and do
#include ${DIR_TO_LIB}/include/xy.h
In your CMakeLists, you set the variable DIR_TO_LIB somehow (by parsing command line or something) and issue
configure_file(yourfile.cpp.in, yourfile.cpp)
This will yield yourfile.cpp with the correct path set.
This is probably not exactly what you intended, but I think it would work.

Answer 3

As part of the build process generate an include file at a well known location (later on called "boilerplate/foreignLib_lib.h") with the real include path.
your code file:
#include "boilerplate/foreignLib_lib.h"
boilerplate/foreignLib_lib.h:
#include "foreignLib-1.5.0_build123456/include/lib.h"

Answer 4

Use CMake's include_directories statement or add an -I switch to the *_CXX_FLAGS to add the current foreignLib path to your include path.