Install a package on specific platforms only

[avivanoff]

Let's say I have the following vcpkg manifest:

{
  "dependencies": [
    {
      "name": "nlohmann-json",
      "version>=": "3.11.3"
    }
  ]
}

Is there a way to specify that I only want this dependency on Cent 7?

[autoantwort]

There are platform expressions, but CentOS is not a known platform: https://learn.microsoft.com/en-us/vcpkg/reference/vcpkg-json#platform-expression

[avivanoff]

I know about platform expressions. But as you pointed out there is no way to specify a specific distribution and/or version. Any other options?

[dg0yt]

I assume that

• your software depends on nlohmann-json for more platforms than just "Cent 7",
• you actually want to use a system-provided nlohmann-json on platforms other than "Cent 7".

In this case you need to actively provide an empty (or minimal) overlay port for the general nlohmann-json dependency.

[avivanoff]

Could you elaborate on "empty (or minimal) overlay port"? My knowledge of vcpkg is not that advanced.

[dg0yt]

"Overlay ports" are separate from the main registry and added to the build via --overlay-ports=<dir>. Cf.
https://learn.microsoft.com/en-us/vcpkg/consume/install-locally-modified-package
The key is to provide the overlay ports options on the platforms where you need it (i.e. not on "Cent 7").

"Empty" means that the port doesn't install anything but just satisfies the vcpkg dependency resolution. It needs a vcpkg.json to declare the name, version etc., and a portfile.cmake with just set(VCPKG_POLICY_EMPTY_PACKAGE enabled).
In some cases, ports also carry wrappers or similar modifications of downstream behavior. Then an empty port might not be enough - you might need to add a "minimal" subset of the adapters.

[avivanoff]

Since we are building from the same source on multiple platforms, can the overlay port be configured to provide empty/minimal based on the target platform name/version?

[dg0yt]

It is up to your scripting, including CMake scripting before the project command - that's the point where the manifest mode installation takes place.