What are proxy classes in c plus plus?

Answer 1

A proxy is defined as any entity that acts on behalf of another entity. For instance, a proxy server is a server that you use to make network calls on your behalf. The proxy server effectively hides your identity from the network because the network only sees the proxy server.

A proxy class is a similar concept -- it is simply a class that acts on behalf of another class. Proxy classes are typically used to simplify the interface to a larger, more complex object.

Note that this is not the same as deriving one object from another. Although you can achieve the same sort of thing with derivation, a proxy class contains a member pointer to the class it acts upon, it does not derive from it. Thus it is free to override the class behaviour, but does not inherit any of its underlying complexity.

"Wrapper" classes are a form of proxy. They contain a class member pointer but they expose a limited or simplified interface to that class member, making more complex calls to that class on your behalf.

Proxy classes can also be used as a reference counting mechanism. Rather than having multiple copies of the same complex object, you can have several lightweight proxy classes all pointing to a single instance of an object, each of which acts on its behalf. Copying lightweight objects does not copy the original object, thus reducing the memory footprint of that object, and when all the lightweight classes finally fall from scope, the original object also falls from scope.

Answer 2

A proxy class is a class that acts on behalf of another class. There are many uses for proxy classes, but generally they are used to simplify a complex object's interface, by hiding details that are of no relevance within the object's current context. Although it may be better to declare a stripped-down version of the object, this is only practical when you have access to the original class implementation (in which case the full-blown class can be derived from the stripped-down class). Rather than re-invent wheels, accessing an existing class by proxy offers a more convenient workaround.

Reference counting mechanisms are another example of proxy classes. When working with a large and complex object, it is often undesirable to copy the object, but there may be a need to have several references to it. A proxy class ensures that there is only ever one instance of the object, and multiple references are handled by multiple instances of the lightweight proxy, thus reducing the memory footprint. When all instances of the proxy fall from scope, the object itself falls from scope.

Note that proxy classes do not derive from the classes they represent. They simply maintain a pointer to the object and expose a limited (or simplified) interface to the object.