external-project-add

How best to use ExternalProject_Add (in my opinion)

Overview

For the longest time I just didn't understand how to effectively use ExternalProject_Add (me not understanding things is a common theme in this repo...).

I tried dabbling with it a little and spent some time searching for idiomatic examples but couldn't find anything particularly helpful. After I discovered FetchContent (example here) I didn't really understand the use of ExternalProject_Add outside of esoteric circumstances (dealing with different platforms/toolchain files/compilers etc.)... until now 🙂

The realization I had was that ExternalProject_Add has no business being in the same CMakeLists.txt file as your project! It should be off in its own CMakeLists.txt file to automate the process of downloading/building/installing libraries you depend on.

All ExternalProject_Add does is automate the manual steps of say, going to GitHub, cloning a project, configuring/building and installing it. You can just have ExternalProject_Add do all of this for you, and then simply use find_package in your CMakeLists.txt (optionally paired with -DMAKE_INSTALL_PREFIX if the libraries were installed to a custom location).

By doing this you don't have to mess around with any of the properties in the external dependency (e.g. ExternalProject_Get_Property to manually extract include paths etc...). You just do find_package(<your-dependency>) like in the other examples (making sure to link against it in target_link_libraries) and you're away!

Instructions

• First navigate to the external/ folder and run configure_and_build.bat/sh (this will download, configure, build and install the external dependency to the build/ folder in external/).
• Next, cd up to external-project-add (this folder) and run configure.bat/sh and then build.bat/sh (please do take a look inside to see what's going on - the only really important thing is how we set CMAKE_PREFIX_PATH to know to look in the external/build/ folder).
• Run build/<debug/release>/external-project-add-example.