[CMake] How to link against projects added through FetchContent

[Saad Khattak]

Thank you for the clarifications Craig! The confusion mainly stemmed from
${Catch2_SOURCE_DIR} where 'Catch2' should have been lowercase. I had
originally started with the name 'catch' (which would explain why I
accidentally left it unchanged in the _GetProperties function) but when
that didn't work, I started assuming that perhaps it was conflicting with
the actual library. It was all downhill from there :)

After your suggestions, I was successfully able to get it to work. For
future reference for anyone stumbling on this answer, here's a minimum
example of a working version:
https://github.com/samaursa/cmake_fetch_content

>> I'm a little surprised CMake didn't complain about that, it does for me
when I tested this just now

CMake did not warn me about it. I am on CMake Version 3.11.0-rc3. It would
be great if this warning could be emphasized by CMake as without it, it is
difficult to tell what went wrong since `target_link_libraries` also fails
and I simply get compile errors for missing headers.

On Sun, Mar 11, 2018 at 8:55 PM Craig Scott <craig.scott at crascit.com> wrote:

> Thanks for trying out the new FetchContent module. Comments on your
> question interspersed below.
>
>
> On Mon, Mar 12, 2018 at 11:02 AM, Saad Khattak <saadrustam at gmail.com>
> wrote:
>
>> Hi,
>>
>> I would like to know how to use the FetchContent properly so that I can
>> link against downloaded (CMake enabled) projects. I have looked at the
>> CMake docs, which although are quite thorough, almost always fail to list a
>> complete example which is incredibly crucial to get up and running quickly.
>>
>> With ExternalProject_Add, we use add_dependencies(...) but that doesn't
>> seem to be the case for FetchContent. Since I can immediately call
>> add_subdirectory(...), I assumed that I can simply link to the library. But
>> that doesn't seem to do anything.
>>
>
> This is indeed how it should work. After you've called add_subdirectory(),
> the targets defined by the project being added should be immediately
> available just like any other target your own project would define.
>
>
>
>>
>> Here's my CMakeLists.txt
>> ``````````````````````````````````````````````````````````````
>> cmake_minimum_required(VERSION 3.5)
>>
>
> Your minimum CMake version will be 3.11 if you want to use FetchContent,
> so consider specifying that as the minimum version here.
>
>
>
>
>> project(testProj)
>>
>> include(FetchContent)
>>
>> FetchContent_Declare(
>>   Catch2
>>   GIT_REPOSITORY "https://github.com/catchorg/Catch2"
>>   TEST_COMMAND ""
>>   )
>>
>
> As stated in the docs, TEST_COMMAND will be ignored, so don't set it here.
> FetchContent only downloads, it doesn't build (you're in control of that,
> see further comments below). The ExternalProject_Add() docs also recommend
> you set GIT_TAG to the specific git hash you want to use (apart from being
> more robust, it saves having to contact the remote each time CMake is run
> after it has been cloned). FetchContent delegates all downloading to
> ExternalProject, so the comments apply here as well.
>
>
>
>>
>> FetchContent_GetProperties(catch)
>>
>
> This needs to be the same as the first argument to FetchContent_Declare()
> above, i.e. Catch2
>
>
>> if(NOT Catch2_POPULATED)
>>   FetchContent_Populate(Catch2)
>>   add_subdirectory(${Catch2_SOURCE_DIR} ${Catch2_BINARY_DIR})
>>
>
> The documentation for the FetchContent_Populate() function state that the
> variable names use lowercased names of the content name (i.e.
> catch2_SOURCE_DIR rather than Catch2_SOURCE_DIR), so both of these
> variables used here will be empty. I'm a little surprised CMake didn't
> complain about that, it does for me when I tested this just now. Same for
> the variable name used in the if test (i.e. use catch2_POPULATED rather
> than Catch2_POPULATED).
>
> The reason for the lowercasing is that it was found during the 2 years or
> so when we were dog-fooding this module that while people usually got the
> name right, there would be variations in upper/lowercase which made things
> less reliable.
>
>
>
>> endif()
>>
>> add_executable(testExe main.cpp)
>>
>> target_link_libraries(testExe Catch2)
>>
>
> If the Catch2 project defines a library called Catch2, then this line
> should work fine once you fix the above problems. I think you want Catch
> rather than Catch2 though.
>
>
>
>
>> ``````````````````````````````````````````````````````````````
>>
>> CMake populates Catch2 with Catch2-NOTFOUND.
>>
>> So, my question is, how do I link against projects added through
>> FetchContent?
>>
>
>
>
>
>
> --
> Craig Scott
> Melbourne, Australia
> https://crascit.com
>
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