TTG uses CMake metabuild system. Each dependency can be either:
- found via
find_package(thus it must be discoverable by e.g. adding its installation prefix toCMAKE_PREFIX_PATH), or - downloaded and built from source as a CMake subproject of the TTG repo.
Due to the fragility/sensitivity of ABI of C++ code to seemingly trivial toolchain/platform details (and even the API of preprocessor-infected C/C++ code) it is stronly recommended to let TTG download and build all prerequisites from source! Pre-building dependencies should be left to the package maintainers.
Managing subprojects in TTG
Dependency source code deployment is managed by the FetchContent module. It pulls the source from the origin (git repo, tarball URL, etc.) to the <BUILDDIR>/_deps/<SUBPROJECT>-src/ directory and sets up the code for building in the <BUILD_DIR>/_deps/<SUBPROJECT>-build directory. Although these directories are located in the build tree, they are made part of the source tree by add_subdirectory. Thus in effect this mechanism is equivalent to manually copying the source tree of the dependency into the TTG source tree. The only difference from building the dependency as a part of TTG and standalone is the CMake state: when configured as a subproject the dependency CMake code will see the cache and non-cache variables and targets defined by TTG itself and the prior dependency subprojects. Thus to be usable as a subproject dependency CMake code needs to be designed to avoid variable and target name clashes with the host project.
Keeping the subprojects in sync
In order to keep TTG in sync with each subproject, one needs to update the following files:
cmake/modules/FindOrFetch*.cmake: these files decide what is the source of the git repository for each projectcmake/modules/ExternalDependenciesVersions.cmake: define, for each subproject the {branch, tag, revision hash} to use.
The build system should update the subproject source when these files are changed. However, for clean rebuild you should, in addition to wiping out CMakeCache.txt, remove the build/_deps directory.
