In some applications, we want to execute one or more important action before accessing an object. An example is accessing sensitive information. Before allowing any user to access sensitive information, we want to make sure that the user has sufficient privileges. A similar situation exists in operating systems. A user is required to have administrative privileges to install new programs system-wide.

The important action is not necessarily related to security issues. Lazy initialization [j.mp/wikilazy] is another case; we want to delay the creation of a computationally expensive object until the first time the user actually needs to use it.

Such actions are typically performed using the Proxy design pattern. The pattern gets its name from the proxy (also known as surrogate) object used to perform an important action before accessing the actual object. There are four different well-known proxy types [GOF95, page 234], [j.mp/proxypat]. They are as follows: