[CMake] Setting a value in a sub-project

[Jakob van Bethlehem]

Ah, nice. Good to know. But then still that cache variable is not created
until *after* it was set by Foo, because it is only created when adding the
Bar subdirectory, which explains the output.

Sincerely,
Jakob


On Fri, May 20, 2016 at 1:18 PM, Petr Kmoch <petr.kmoch at gmail.com> wrote:

> The situation already involves a cache variable, though: `option(abc "cmt"
> ON)` is just syntactic sugar for `set(abc ON CACHE BOOL "cmt")`.
>
> Petr
>
>
> On 20 May 2016 at 13:03, Jakob van Bethlehem <jsvanbethlehem at gmail.com>
> wrote:
>
>> You don't have to create a cache variable for this. Put yourself in the
>> position of CMake;
>> * While scanning the CMakeLists.txt of Foo CMake encounters the set(OPT1
>> ON FORCE) command, which tells CMake to create a *variable* called OPT1
>> with value ON
>> * Then CMake is asked to include Bar
>> * While scanning Bar, CMake encounters the option() command, so it will
>> create an option called OPT1
>> * end then.... nothing, CMake finished scanning
>>
>> Only the second time around, when CMake encounters the set(OPT1..)
>> command, it will have gained knowledge of the presence of the OPT1 option,
>> and hence it will realize it needs to change that option instead of
>> creating a variable with that name.
>>
>> So to me, your output is exactly as expected. I suspect if you include
>> Bar before setting the option, you will get the behaviour you expected.
>> This, to me, makes perfect sense, as Bar is a dependency of Foo, and hence
>> needs to be setup before you start setting up Foo. You wouldn't compile Foo
>> before compiling Bar, so why would that be different for the configuration
>> step.
>>
>> Sincerely,
>> Jakob
>>
>> On Fri, May 20, 2016 at 11:24 AM, Doug Cuthbertson <
>> doug.cuthbertson at gmail.com> wrote:
>>
>>>
>>> CMake (version 3.5.2) surprised me with how it's passing values to
>>> sub-projects, so I was wondering if this is expected behavior. Here's an
>>> example of what I mean. Let's say I have a project Foo in a directory of
>>> the same name. It contains a third-party library called Bar which has a
>>> CMakeLists.txt file that looks like:
>>>
>>>     cmake_minimum_required(VERSION 2.8.12)
>>>
>>>     option(OPT1
>>>       "Set to OFF|ON (default is OFF) to control build of Bar library"
>>> OFF)
>>>
>>>     if(OPT1)
>>>       message("Bar: OPT1 is on")
>>>     else(OPT1)
>>>       message("Bar: OPT1 is off")
>>>     endif(OPT1)
>>>
>>> I then create CMakeLists.txt for Foo that sets OPT1 to ON and includes
>>> Bar:
>>>
>>>     cmake_minimum_required(VERSION 2.8.12)
>>>
>>>     set(OPT1 ON FORCE)
>>>     if(OPT1)
>>>       message("Foo: OPT1 is on")
>>>     else(OPT1)
>>>       message("Foo: OPT1 is off")
>>>     endif(OPT1)
>>>     add_subdirectory(Bar)
>>>
>>> The first time I run cmake the message output is:
>>>
>>>     Foo: OPT1 is on
>>>     Bar: OPT1 is off
>>>
>>> If I run cmake again, I get:
>>>
>>>     Foo: OPT1 is on
>>>     Bar: OPT1 is on
>>>
>>> If this is expected behavior, is there any way I can ensure Bar receives
>>> the value of OPT1 the first time? It makes a huge difference when the
>>> option controls, for example, whether a static or dynamic library will be
>>> built.
>>>
>>> Thanks,
>>> Doug
>>>
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>>
>>
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>> Powered by www.kitware.com
>>
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