Build/host dependency of header-only C++ packages

[sdebionne]

When a C++ header-only package foo depends on another header only-package bar, is there a way to ensure that the downstream project get both installed as host requirements?

foobar <--depends-- foo <--depends-- bar

with

package:
  name: foobar

requirements:
  host:
    - foo

package:
  name: foo

requirements:
  host:
    - bar

With the current implementation, only foo gets installed.

Is there any way to export the build/host requirements of a package to the downstream projects transitively, maybe something like build_export?

[marcelotrevisani]

If you add the run requirements on the bar project
everything which is there will be at the host section as well.

[marcelotrevisani]

You can also do
https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html#export-runtime-requirements

or

https://docs.conda.io/projects/conda-build/en/latest/resources/define-metadata.html#run-constrained

[marcelotrevisani]

Did it solve your problem?

[sdebionne]

Well I thought of using runtime requirements briefly but then the headers would be installed with the dowstream project foobar, no?

What I think might be needed is the twin of run_exports:

# meta.yaml of foo 
build:
  host_exports:
    - bar

so anyone using foo as a host dependency would automatically get bar installed in the host environment.

My workaround is to list both foo and bar as host requirements:

# meta.yaml of foobar 
requirements:
  host:
    - foo
    - bar # this should not be needed

[sdebionne]

I had a quick look on how this situation is handled in Vcpkg:

Build-Depends

Vcpkg does not distinguish between build-only dependencies and runtime dependencies.

I guess it's the same for Conda. Maybe I should rename this issue to make it clear it is about build-only dependencies (and actually not specific to header-only libraries).

[msarahan]

It's counterintuitive for sure, but here if you add a run requirement on bar for foo, them it will be present in the host section for foobar, but will not be a run dependency for foobar. You can think of the host section as using the run requirements for all of the specs on the host section. If foo and bar both had run_exports, foo's run exports would be in foobar, but bar's would not necessarily, because run exports are not transitive.

[github-actions]

Hi there, thank you for your contribution!

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Thanks!

[jakirkham]

Just a note, to discuss adding host_exports, filed issue: conda/ceps#77