Re: [CMake] Can/should we specify optional dependencies

From: Gavin Lambert (boost_at_[hidden])
Date: 2021-10-11 06:23:11

On 9/10/2021 03:40, Peter Dimov wrote:
> John Maddock wrote:
>> I have a regex bug report relating to linking to ICU when using CMake:
>> https://github.com/boostorg/regex/issues/143
>>
>> Is there a correct way to specify in the CMake file that if you use header X,
>> then you should also link to library Y, but otherwise not?
> 
> I don't see how. CMake doesn't know whether someone will include
> header X or not.

Provided that you can assume that the config (in the form of a #define) 
is visible to every compilation unit (or at least those that consume the 
library in some fashion), then you could make the header file itself 
define and do nothing (or perhaps raise a #error) if not enabled in the 
configuration.

This way, attempts to use the header will fail unless enabled in the 
config, which would allow you to add dependencies as required.

I'm not sure how straightforward it is to do this in CMake, although I 
thought "building everything with the same settings" is explicitly one 
of its goals.