[CMake] Setting a value in a sub-project

[Petr Kmoch]

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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>
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>
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