# Can you combine FetchContent and ExternalProject?

**URL:** <https://discourse.cmake.org/t/can-you-combine-fetchcontent-and-externalproject/228>\
**Category:** Code\
**Created:** [November 18, 2019, 8:15pm UTC](https://discourse.cmake.org/t/can-you-combine-fetchcontent-and-externalproject/228 "2019-11-18T20:15:42Z")\
**Posts on this page:** 1\
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**Author:** ![craig.scott](https://discourse.cmake.org/user_avatar/discourse.cmake.org/craig.scott/32/20_2.png) [@craig.scott](https://discourse.cmake.org/u/craig.scott)\
**Post date:** [November 19, 2019, 11:08am UTC](https://discourse.cmake.org/t/can-you-combine-fetchcontent-and-externalproject/228/2 "2019-11-19T11:08:05Z")

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Ordinarily, I advise choosing one or the other rather than mixing `FetchContent` and `ExternalProject`. More specifically, if you can use `FetchContent` to not just download the dependencies but also to add them directly to your build, that is typically simpler than `ExternalProject` (read up on using [`FetchContent_MakeAvailable()`](https://cmake.org/cmake/help/latest/module/FetchContent.html#command:fetchcontent_makeavailable) specifically). It relies on the dependency projects not doing things like assuming they are the top level project or using target names that other projects also define, but if you don’t hit either of those problems, then it’s probably the simpler route.

In your situation, it may still make sense to mix the two approaches. You want the sources to be downloaded at configure time, which FetchContent can achieve for you (you can’t use `FetchContent_MakeAvailable()` then though). If you genuinely do need to build the dependencies as external builds, then `ExternalProject` is the right tool for that. You can tell `ExternalProject` to use the source locations that `FetchContent` will have already populated for you at configure time. In the call to `ExternalProject_Add()`, omit any download or update details and instead provide `SOURCE_DIR`, with a value being provided by `FetchContent`. For example (untested, but hopefully you get the idea):

```cmake
FetchContent_Declare(someproj
    GIT_REPOSITORY https://github.com/someGroup/someproj
    ...
)
FetchContent_GetProperties(someproj)
if(NOT someproj_POPULATED)
  FetchContent_Populate(someproj)
endif()

ExternalProject_Add(someproj
    SOURCE_DIR ${someproj_SOURCE_DIR} # Provided by FetchContent
    ...
)

```

Just to be clear, your approach relies on the head node being identically configured to the compute nodes (same paths, exact same tool versions, etc.).

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