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HydraExpress Servlet Development Guide
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6.3 Using rwsf::Attribute

Objects within the HydraExpress Agent exchange information using attributes. Each request object, session object, and context object contains a collection of attributes. Servlets and filters use these attribute collections to transfer and store information between requests.

Each attribute is an instance of rwsf::Attribute. Class rwsf::Attribute is a container that holds a single object. The contained object must provide a copy constructor and assignment operator that do not change the semantics or validity of the original object. The contained object must also have a default constructor. To copy an object into an attribute, use operator<<(). Conversely, to copy an object out of an attribute, use operator>>().

The following code sample copies a string into a servlet attribute, then copies the string from the attribute to a different string:

//1Copies the string "Servlets in C++" from source to stringAttr.
//2Changes source to contain the string "HydraExpress". Since stringAttr contains a copy of the original string, stringAttr still contains "Servlets in C++".
//3Copies the string "Servlets in C++" from stringAttr to destination.

At the end of the sample, destination and stringAttr each contain a separate copy of the string "Servlets in C++", while source contains the string "HydraExpress".

There are two important limitations to rwsf::Attribute. First, the compiler cannot always detect attempts to extract an attribute into an object of an incompatible type. In these cases, the extraction operator throws rwsf::Exception when the type of the destination does not match the type of the object stored in the attribute.

Second, an attribute can only store a single object. An attribute cannot store an array or a string literal. However, an attribute can store an object that encapsulates the same data. For example, the code below stores a string instead of a literal character string:

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