Re: Using Boost.Test header-only from CMake

From: Rainer Deyke (rdeyke_at_[hidden])
Date: 2021-06-11 09:17:29

On 10.06.21 23:36, Andrey Semashev via Boost wrote:
> On 6/9/21 2:05 PM, Rainer Deyke via Boost wrote:
>> On 08.06.21 09:33, Gavin Lambert via Boost wrote:
>>> For this to work, one of the following must be true:
>>> Â Â  1. library B can figure out how program P chose to link to it and 
>>> links to it the exact same way.
>>
>> A header-only library cannot *link* to anything, or it is not 
>> header-only.
> 
> CMake defines targets for header-only libraries, which can have 
> dependencies on other targets, including on static or shared libraries. 
> So the program that "links" to the header-only library picks up its 
> dependencies recursively. As a result, the linker is invoked with all 
> (binary) libraries in the dependency tree.

If this functionality works the way I think does, then I think it should 
not be used.

> Whether you can consider such library header-only in the first place is 
> a philosophical question. I do, because otherwise any library that calls 
> the standard library cannot be called header-only.

Actually, the standard library is a great example of what I am talking 
about.  A header-only library (B) should not care about the specific 
implementation of the standard library (A) being used by the program 
(P).  There is no link dependency from B to A.  There is a requirement 
from B to P that P must provide /a/ implementation of the C++ standard 
library, but B doesn't care if it is libstdc++ or libc++ or even a 
custom standard library implementation that is part of P and not a 
separate library at all.

Again, to make my point absolutely clear: header-only library B "using" 
library A does not imply a dependency from B to A.  Instead, it implies 
a requirement on a program P that uses B to also link to A.  Header-only 
libraries use interfaces.  Programs link to implementations.

-- 
Rainer Deyke (rainerd_at_[hidden])