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From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2021-06-10 21:36:50

On 6/9/21 2:05 PM, Rainer Deyke via Boost wrote:
> On 08.06.21 09:33, Gavin Lambert via Boost wrote:
>> On 8/06/2021 4:53 pm, Edward Diener wrote:
>>> I do not know CMake, so maybe my comment is irrelevant, but it seems
>>> natural to me that a header-only library would always choose to use
>>> some other dependent library as header-only, even when that other
>>> library had static or shared variants. In other words I applaud the
>>> decision of Boost.Test to provide a header-only variant and can not
>>> even begin to process the fact that CMake can not deal with
>>> Boost.Test as a header-only library, if that is indeed the case.
>> A problem occurs when program P uses library A as static or shared and
>> then wants to use header-only library B which also uses library A.
>> For this to work, one of the following must be true:
>>    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.

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.

> A header-only library B can depend on symbols being provided by the
> calling program P, which can delegate the task to library A, or to
> another library C which provides the same interface as A.  There should
> be no direct connection from B to A.  Depending on the design of B, this
> may involve #including a header from A before #including a header from
> B, or it can involve making the headers from A available to B so that B
> can #include them directly.  Where these symbols and/or headers come
> from is the business of P, not of B.

Reverse dependencies are rather uncommon, and it's not what is being
discussed here. The question is how the header-only library selects the
targets to depend on.

I think, it should not select any specific target unless it absolutely
must (i.e. it won't work otherwise). In terms of the earlier example,
the library B should depend on the generic target A. Now, that target A
can be defined as a header-only, shared or static variant, depending on
the user's choice or some default.

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