Creating a library

CMake allows us to create libraries used by the OpenCV build system. Factorizing shared code among multiple applications is a common and useful practice in software development. In big applications, or common code shared in multiple applications, this practice is very useful. In this case, we do not create a binary executable, but instead we create a compiled file that includes all the functions, classes, and so on. We can then share this library file with other applications without sharing our source code.

CMake includes the add_library function to this end:

# Create our hello library 
    add_library(Hello hello.cpp hello.h) 
 
# Create our application that uses our new library 
    add_executable(executable main.cpp) 
 
# Link our executable with the new library 
    target_link_libraries(executable Hello) 

The lines starting with # add comments and are ignored by CMake. The add_library (Hello hello.cpp hello.h) command defines the source files of our library and its name, where Hello is the library name and hello.cpp and hello.h are the source files. We add the header file too to allow IDEs such as Visual Studio to link to the header files. This line is going to generate a shared (.so for Mac OS X, and Unix or .dll for Windows) or static library (.a for Mac OS X, and Unix or .lib for Windows) file, depending on whether we add a SHARED or STATIC word between library name and source files. target_link_libraries(executable Hello) is the function that links our executable to the desired library, in our case, the Hello library.