[CMake] Generator expressions containing spaces

Tue Apr 24 13:03:31 EDT 2018

2018-04-24 18:58 GMT+02:00 Elvis Stansvik <elvis.stansvik at orexplore.com>:
> 2018-04-23 14:11 GMT+02:00 CHEVRIER, Marc <marc.chevrier at sap.com>:
>> The space is used to separate arguments passed to COMMAND. So your generator
>> expression is splitted before evaluation and elements are no longer valid
>> generator expression.
>>
>>
>>
>> So, to solve your problem, encapsulate the generator expression inside
>> quotes. And apply the following advices for correct result:
>>
>> Separate command from args
>> Use variable to list your arguments and add option COMMAND_EXPAND_LISTS
>> (available with version 3.8) to avoid “spurious” semi-colons in the result
>>
>>
>>
>> Your example reworked:
>>
>>
>>
>> Set (args foo bar)
>>
>> add_custom_target(foo)
>>
>> add_custom_command(TARGET foo POST_BUILD
>>
>>   COMMAND $<1:echo> "$<1 :${args}>"
>>
>>   COMMAND_EXPAND_LISTS
>>
>>   )
>>
>
> Interesting thread. I was sort of in the same boat, having a
>
> add_compile_options(
>     "$<$<CXX_COMPILER_ID:GNU>:-Wall>"
>     "$<$<CXX_COMPILER_ID:GNU>:-Wextra>"
>     "$<$<CXX_COMPILER_ID:GNU>:-Werror>"
>
>     "$<$<CXX_COMPILER_ID:Clang>:-Wall>"
>     "$<$<CXX_COMPILER_ID:Clang>:-Wextra>"
>     "$<$<CXX_COMPILER_ID:Clang>:-Werror>"
>
>     "$<$<CXX_COMPILER_ID:AppleClang>:-Wall>"
>     "$<$<CXX_COMPILER_ID:AppleClang>:-Wextra>"
>     "$<$<CXX_COMPILER_ID:AppleClang>:-Werror>"
> )

Sidenote: I know that in this case, since I want the same flags for
all three of these compilers, I could "simplify" it using an
$<OR:?[,?]...>, but that's a separate thing (and I decided against it
since it gives such long unreadable lines).

Elvis

>
> and of course wishing I could just write something like
>
> add_compile_options(
>     $<$<CXX_COMPILER_ID:GNU>:-Wall -Wextra -Werror>
>     $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -Werror>
>     $<$<CXX_COMPILER_ID:AppleClang>:-Wall -Wextra -Werror>
> )
>
> instead.
>
> I can't depend on CMake 3.8+ just yet, but I'll remember your solution
> for when I can.
>
> I don't quite like having to introduce variables though. Is there no
> way to solve this without having to first stuff the elements in a list
> and use that?
>
> Cheers,
> Elvis
>
>>
>>
>>
>>
>> From: CMake <cmake-bounces at cmake.org> on behalf of Yves Frederix
>> <yves.frederix+cmake at gmail.com>
>> Date: Monday 23 April 2018 at 13:08
>> To: "cmake at cmake.org" <cmake at cmake.org>
>> Subject: [CMake] Generator expressions containing spaces
>>
>>
>>
>> Hi,
>>
>>
>>
>> I recently learned that the COMMAND line in a custom command supports
>> generator expressions. In particular, what makes this powerful is that if
>> the expression evaluates to the empty string, no corresponding code will be
>> added to the target (as documented in the docs).
>>
>>
>>
>> While this works very nicely for single-string command, I fail to make it
>> work for commands consisting of multiple space-separated strings:
>>
>>
>>
>> ```
>>
>> ~/tmp/genex_with_spaces$ cat CMakeLists.txt
>> cmake_minimum_required(VERSION 3.6)
>>
>>
>>
>> add_custom_target(foo)
>>
>> add_custom_command(TARGET foo POST_BUILD
>>
>>   COMMAND $<1:echo bar>
>>
>>   )
>>
>>
>>
>> ~/tmp/genex_with_spaces$ grep bar build/CMakeFiles/foo.dir/build.make
>> "\$$<1:echo" bar>
>>
>> ```
>>
>>
>>
>> As can be seen, the generator expression is not expanded.
>>
>>
>>
>> My question is now whether I am doing something wrong (is there a correct
>> way of dealing with spaces in the context of generator expressions?) or
>> might this be an inherent limitation of generator expression in general?
>>
>>
>>
>> As a workaround, the only thing that seems to work is to put each of the
>> space-separated components of the command in (typically identical) genexes:
>>
>>
>>
>> ```
>>
>> ~/tmp/genex_with_spaces$ cat CMakeLists.txt
>> cmake_minimum_required(VERSION 3.6)
>>
>>
>>
>> add_custom_target(foo)
>>
>> add_custom_command(TARGET foo POST_BUILD
>>
>>   COMMAND $<1:echo> $<1:bar>
>>
>>   )
>>
>>
>>
>> ~/tmp/genex_with_spaces$ grep bar build/CMakeFiles/foo.dir/build.make
>> echo bar
>>
>> ```
>>
>>
>> Of course, while this works, things becomes very unreadable if the genex is
>> more complex.
>>
>>
>>
>> Other things that I have tried but failed:
>>
>> escape the space with a backslash -> this quotes the entire expression
>> inside the genex.
>> define the command directly as a list inside the genex -> this removes
>> spaces between the different components of the command and quotes that
>> result.
>> treat the command as a list, perform string operations to wrap each element
>> in the desired regex and convert semicolon to spaces -> again results in the
>> command being quoted as a whole.
>>
>>
>>
>> Any advice is highly appreciated.
>>
>>
>>
>> Thanks,
>>
>> Yves
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> --
>>
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