establishing functional requirements - level of detail [closed]

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W开发者_JAVA技巧here do you draw the line between functional requirements and aspects of design?

If i can illustrate, one function of my system is that it must implement a user hierarchy.

In this instance would the definition of what those hierarchies are come under requirements (assuming you already know what the user hierarchies are) or would they come under the design aspect of a project?

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Where do you draw the line between functional requirements and aspects of design?

Requirements is what the code needs to do. Design gets into how it will do it, with class hierarchies, design patterns used, etc.

Requirements:

There must be A, B, and C types of user. Type A is only able to edit their own account, type B can edit their own account and post to hidden forums, type C can edit all accounts and post to all forums. The application must have a small memory footprint because blah blah blah....

Design:

Each instance of the user class will contain a member variable of a permissions object. The permissions object is a wrapper for a bit field because memory is tight, but has several convenience methods, called foo, bar, and baz. foo does XXX and is public, (eventually design gets very low level and is turned into code).

I'm not saying that would be the actual design, but that's how the design should analyze the requirements and eventually become a working implementation.