- Aka: Surrogate
- Intent: Provide a surrogate or placeholder for another object to control access to it
Structure §
Image from: Gamma, Helm, Johnson, and Vissides
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Image from: Gamma, Helm, Johnson, and Vissides
Applicability §
- Applicable when there is a need for a more versatile or sophisticated reference to an object than a pointer
Types §
Remote §
- Provides a local representative for an object in a different address space.
Virtual §
- Creates expensive objects on demand.
Protection §
- Controls access to the original object (i.e. if objects should have different access rights
Smart reference §
- Performs additional actions when an object is referenced such as
- Loading a persistent object into memory when first referenced
- Counting the number of references to the real object so it can be freed when there are no more references.
- Checking the real object is locked so no object can access it
Consequences §
- Introduces a level of indirection when accessing an object:
- Remote proxies hide that an object resides in a different address space
- Virtual proxies perform optimizations
- Protection and Smart proxies allow housekeeping tasks on access.
- Postponing the copy process to reduce its cost.
Implementation §
- Exploit overloading operators such as -> in C++.
- Proxy doesn’t always have to know the type of the subject (unless the proxy is a virtual proxy)
- Consider how clients will refer to subjects before they are instantiated.