Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

The details follow later, but basically policies can be set by either:

Defining some macros that change the default behaviour: this is the recommended method for setting installation-wide policies.
By instantiating a statistical distribution object with an explicit policy: this is mainly reserved for ad hoc policy changes.
By passing a policy to a special function as an optional final argument: this is mainly reserved for ad hoc policy changes.
By using some helper macros to define a set of functions or distributions in the current namespace that use a specific policy: this is the recommended method for setting policies on a project- or translation-unit-wide basis.

The following sections introduce these methods in more detail.