Compile different code depending on library dependencies installed on machine - c++

Question

I have c++ code which is built using cmake. I have written a class which depends on a library called Gurobi. One of Gurobi's competitors is CPLEX, another library which allows for similar optimization tasks. I have to rewrite my classes in CPLEX for a research project (because our funding comes from an agency which only has access to CPLEX). Therefore, I want to change my cmake file so that it looks for Gurobi and CPLEX and compiles my code using the GurobiClass.cpp file if it finds Gurobi, and uses the CPLEX.cpp file if it finds cplex.
I'm basically asking how to ignore code in the compilation step, so that a user does not have to have both libraries to compile my code.
I'm sure this question has been asked already, but I'm not sure what the correct name for this is / what to search, so I'm asking it again, but feel free to point me to online resources.
Thanks for any help you can provide!

Answer 1

Considering you haven't posted any code, there are a number of options. If the libraries use CMake themselves, and come with a FindGurobi.cmake or FindCPLEX.cmake, you can use CMake's find_package() to locate them. You can determine which files to include (and libraries to link) to your target using if-statements. A simple example would be something like this:
find_package(Gurobi)
find_package(CPLEX)
if(Gurobi_FOUND)
# Compile with Gurobi classes.
add_library(MyLibrary
TopLevelClass.cpp
GurobiClass1.cpp
GurobiClass2.cpp
...
)
target_include_directories(MyLibrary PUBLIC ${Gurobi_INCLUDE_DIR})
target_link_libraries(MyLibrary PUBLIC ${Gurobi_LIBRARIES})
elseif(CPLEX_FOUND)
# Compile with CPLEX classes.
add_library(MyLibrary
TopLevelClass.cpp
CPLEXClass1.cpp
CPLEXClass2.cpp
...
)
target_include_directories(MyLibrary PUBLIC ${CPLEX_INCLUDE_DIR})
target_link_libraries(MyLibrary PUBLIC ${CPLEX_LIBRARIES})
else()
message(FATAL_ERROR "Neither Gurobi nor CPLEX library was found.")
endif()
If you are hard-coding the paths to these libraries somehow, you could also do something like this (to check if the library files exist):
set(Gurobi_INCLUDE_DIR /path/to/local/gurobi/include)
set(Gurobi_LIBRARY /path/to/local/gurobi/lib/gurobi.lib)
set(CPLEX_INCLUDE_DIR /path/to/local/CPLEX/include)
set(CPLEX_LIBRARY /path/to/local/CPLEX/lib/cplex.lib)
if(EXISTS ${Gurobi_LIBRARY})
# Compile with Gurobi classes...
elseif(EXISTS ${CPLEX_LIBRARY})
# Compile with CPLEX classes...
else()
message(FATAL_ERROR "Neither Gurobi nor CPLEX library was found.")
endif()