Tuesday, November 28, 2006

scopes and RAII

Classes in Hyper cannot have a destructor. I wanted it to be this way because the language uses a garbage collector, and then the execution of a destructor cannot be always guaranteed. So destructors would not be reliable anyway if you want it to release acquired resources. And without destructors there is currently no way to do RAII. This is not acceptable in my opinion. So we need at least one way to do it.

This is my proposition. I would like to introduce a new block statement, "scope". This would take care of resource acquisition and disposal. You would give it an object that support 2 methods: "enterScope()" and "leaveScope()". The scope block would call the first method upon entering the scope, and would make sure that the second one is called whenever execution leaves the scope (i.e. normal exit at the end, plus exceptions and jump statements like "return", "break", etc.). The first form of "scope" would support the declaration of a variable, with or without an initializer. But I think it would also be useful to have an anonymous variable (i.e. to give it no name) and/or to have no variable at all and simply use a temporary object on the stack.


procedure test(x : * Xyz)
# indented output
scope i : Indenter = x.getIndenter() # increase indentation
i.printLn("Hello world.")
# ...
end # get indentation back to the previous level

scope this.getMutex() # lock mutex
# ...
end # unlock mutex
end test

The first example uses an explicit variable and the second uses no variable at all. In the second example the mutex returned by "getMutex()" is used as the scope object. In this case the result of the scope expression is probably a pointer (or a reference) to a field, but it does not necessarily has to be a pointer.

This reminds me about how to treat temporaries. I suggest to let them exist until the function they appear in returns. This would be ideal for use with restricted pointers. Any temporary could then be pointed to by a restricted pointer. And those could be stored in variables, so it would be necessary to keep the temporaries alive until the function returns.

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