Concept: Default features

Article
03/26/2024

This article describes the concept of default features, their implications, and strategies for effective management.

Default features are a set of features automatically included when a port is installed, unless explicitly disabled. These features ensure that a library maintains a baseline level of functionality, even as the complexity of its dependency graph increases.

Note

Not all ports contain default features.

Role of default features

Default features provide a user-friendly experience by ensuring essential functionalities are available in libraries with modular features.

For instance, a library capable of extracting various archive formats might have each format as an optional feature. Without default features, the library might not include any extraction algorithm out of the box, complicating initial use.

Packages installed by vcpkg will always include their default features, unless explicitly requested otherwise. This behavior applies to packages required directly by the user through a manifest file or vcpkg install invocation, and to packages required as dependencies.

Disabling default features in manifest mode

Add the "default-features": false attribute to a dependency declaration to disable installation of its default features.

Disabling the automatic installation of default features is only possible for user requested dependencies.

If you want to disable default features for a transitive dependency, you need to promote it to a direct dependency on your project's manifest.

Disabling default features in classic mode

Add the special core feature as part of the requested features of the packages you want to install.

Note

Similar to how manifest mode works, if you want to disable default features for a transitive dependency; you need to promote it to a direct installation request in your command line.

Example 1: Including a library with default features

{
  "name": "my-application",
  "version": "0.15.2",
  "dependencies": [
    "extract-any"
  ]
}

In this scenario, extract-any is installed with its default features, ensuring immediate functionality.

Example 2: Disabling default features (manifest mode)

{
  "name": "my-application",
  "version": "0.15.2",
  "dependencies": [
    {
      "name": "extract-any",
      "default-features": false
    }
  ]
}

Here, extract-any is installed without its default features, allowing for a tailored setup.

Example 3: Disabling default features (classic mode)

vcpkg install extract-any[core]

Here, extract-any is installed without its default features by using the special core feature.

Default features interactions

Installing a port with default features (default behavior): vcpkg install extract-any installs extract-any with its default features (extract-any[format-a]), due to the absence of specific feature requests.
Specifying a feature without disabling defaults: vcpkg install extract-any[format-b] leads to the installation of extract-any with both the specified (format-b) and default (format-a) features (extract-any[format-a,format-b]).
Explicitly disabling default features: vcpkg install extract-any[core] installs extract-any without any default features, as [core] explicitly excludes them.
Transitive dependencies and default features: vcpkg install extract-any[format-b] portB might result in extract-any[format-a,format-b] portB being installed if portB depends on extract-any[format-a], but does not explicitly disable extract-any's default features.
Combining ports with varied feature specifications: vcpkg install extract-any[format-b] portB installs extract-any[format-a, format-b] portB, including extract-any's default features unless explicitly excluded.
Comprehensive disabling of default features: vcpkg install extract-any[core] portB ensures that extract-any is installed with only the explicitly requested features, excluding the defaults.

Managing default features

To effectively manage default features within your projects:

Be Explicit: Specify only the features you need. Use [core] to disable default features when necessary.
Inspect Dependencies: Use vcpkg depend-info <port> to understand dependency resolutions and adjust your configurations accordingly.
Utilize Overrides and Baselines: Leverage versioning and baseline features for more control over dependency resolutions.

For more information, see the following:

Adopting a clear strategy for managing default features will help streamline your project's dependency management, ensuring you include only what's necessary for your application's functionality.

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