---
breadcrumbs:
- - /chromium-os/developer-library/reference
  - ChromiumOS > Reference
page_name: cpp-pref
title: C++ Prefs
---

[TOC]

Prefs is a system for storing configuration data keyed to the user session (i.e.
the sorts of data that would be stored in config files in most other software
projects). Although originally designed for storing Chrome's user preferences,
it now sees use more generally to store all sorts of feature data. It is capable
of representing any JSON data structure (strings, numbers, booleans, lists,
mappings -- see
[`base::Value`](https://source.chromium.org/chromium/chromium/src/+/main:base/values.h)).

Prefs are not meant to store large collections of data, which could result in a
large memory overhead. See more
[here](https://chromium.googlesource.com/chromium/src/+/HEAD/chrome/browser/prefs/README.md).

All prefs need to be registered by a
[PrefRegistry](https://source.chromium.org/chromium/chromium/src/+/main:components/prefs/pref_registry.h;drc=8ba1bad80dc22235693a0dd41fe55c0fd2dbdabd),
which is often triggered by
[BrowserContextKeyedServiceFactory::RegisterProfilePrefs()](https://source.chromium.org/chromium/chromium/src/+/refs/heads/main:components/keyed_service/content/browser_context_keyed_service_factory.h;l=174;drc=47042255a0d8acfbcf58cb0eea4607ec574b8419).

For prefs to be recognized in the ChromeOS UI, the pref name must be added to
`GetAllowlistedKeys()` in
[`prefs_util.cc`](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/extensions/api/settings_private/prefs_util.cc;l=176)

Active prefs can be viewed on Chromebooks by entering `chrome://prefs-internals`
and `chrome://local-state` into the Chrome tab search bar to view active Profile
Prefs and Local State Prefs, respectively.
* Note that not all Local State prefs will be shown on Chromebooks as some contain
PII: to view these removed prefs on a test Chromebook, deploy code with the
[pref removing
logic](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/local_state/local_state_ui.cc;l=45-54;drc=a0f1d14499cc76db560360f1286b25584e2aee91) commented out.

To learn more about using Prefs to write code, see
go/chromium-cookbook-policy-prefs. If you'd like to understand prefs more
deeply, read ["User Sessions"](/chromium-os/developer-library/reference/user-sessions/user-sessions).