How to tell CMake to download external projects before compiling my sources

Question

I have my own code which depends on third-party libraries, such as yaml-cpp for example. I want to download these third-party libraries before compiling my own sources. However, I don't manage to do it with CMake.

I managed to download yaml-cpp like this:

ExternalProject_Add(yaml-cpp
                URL https://yaml-cpp.googlecode.com/files/yaml-cpp-0.5.1.tar.gz
                PREFIX ${CMAKE_CURRENT_BINARY_DIR}/yaml-cpp
                CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR>

But when I type

cmake ..
make

my sources are compiled before. Actually, yaml-cpp is downloaded at the very last step.

Do you know how to tell CMake to download third-party libraries at the very first building step?

Thanks!

Answer 1

ExternalProject creates a target. You can make your first target, e.g. your library, dependent from that target. Then it gets downloaded before you start building your library.