Smart pointers in aptitude IMPORTANT NOTE: All (or most of) the smart pointers documented here are deprecated in favor of boost::shared_ptr, which is simpler, more flexible, and probably more efficient (although I haven't checked that). New code should use shared_ptr and/or scoped_ptr. aptitude uses smart pointers in a number of ways internally. These techniques generally simplify the code and make it easy to program safely, but C++ being what it is, you should be aware of how they work and what the caveats are. * threading As of this writing, none of the smart pointers lock their reference counts (implementing locks in the imm::wtree class caused the problem resolver's running time to increase by 50%!). Some of the less-commonly-used ones may get locking in the future, but for the time being you should handle objects that are given to another thread carefully -- do deep copies of anything that's reference-counted. (this is only the imm::* stuff at the moment, so the problem is managable) For more information on threading, see README.THREADS. * reference-counted immutable values aptitude employs reference-counting of immutable objects in several places. For the most part this is invisible to clients; the typical implementation looks like this: class foo { class foo_impl { ... public: void m1(); void m2(int); ... }; public: void m1(); void m2(int); ... }; When you create a new "foo" object, a corresponding foo_impl is created with reference-count 1. Reference counts are managed in all the ways you expect, and the object is deleted when you're done with it. This use of reference-counting can be viewed as a performance hack: since the objects are immutable, pass-by-value is indistinguishable from pass-by-reference; if we can prove that no strong cycles will be created, it's always safe to reference-count objects like this. * auto_ptr for temporary return values When building up a temporary return value, several routines use auto_ptrs to store intermediates: auto_ptr v1 = parse_T_from_string(str1); auto_ptr v2 = parse_T_from_string(str2); The advantage here is that you can freely throw exceptions and/or break out of the function with an early "return" without worrying about which of {v1,v2} have to be deleted. To actually return them, you'd do something like this: return pair(v1.release(), v2.release()); The "release" calls tell v1 and v2 that they no longer "own" their respective pointers. * reference-counting of display widgets Display widgets are also reference-counted. This is a generalization of the idea that "the enclosing widget owns this one"; it provides more support for extending the lifetime of a widget if, for instance, you want to extract values from it after its parent is done with it. Unfortunately, this reference counting is NOT fully transparent, and you should be aware of some basic principles: - As with any reference counting scheme, it is up to you to avoid creating strong cycles. By default, the only non-transient strong references are references from a parent to a child; your best bet is to ensure that all additional strong references either have stack lifetime or do not close cycles. If you must create a cycle, make sure it gets explicitly broken before all external references to it are lost. - Reference-counting is done via the generic ref_ptr class (ref_ptr.h). This is a templated class, like auto_ptr, and generally works as you expect. To enforce the use of ref_ptr, all widget constructors are protected; static ::create methods are provided to actually allocate a new widget. Because the cwidget::widget class initializes its reference count to 1, the ::create method should explicitly decref() its return value; see the existing ::create routines for examples. To make code a bit more readable, adopt the convention of creating typedefs for ref_ptr wrappers around new classes. For instance, if you have just created the class vs_moo, add the following line to its header file: typedef ref_ptr vs_moo_ref; - Watch out for deletion of "this". If you aren't sure whether "this" will be deleted in a method, I recommend creating a method-scoped strong reference to it: ref_ptr thisref = this; Doing so will prevent "this" from being deleted until the current method terminates and is probably good practice in general. - Beware sigc::bind. sigc::bind is an easy way to create bad circularities; moreover, it's actually unsafe to bind a ref_ptr as a slot argument. The solution adopted in cwidget is to exploit sigc++ weak references. If w is a ref_ptr, then rather than closing w, you should close over w.weak_ref(). Unfortunately, w.weak_ref() will appear to the callee as a C++ reference, not a ref_ptr: if w has type T, you need a slot that accepts a T&, not a ref_ptr. To solve this problem, it is conventional for widgets defining public interfaces that accept a ref_ptr to define a corresponding _bare method that accepts a T&; the _bare method should simply instantiate a ref_ptr and call the main interface. For instance, void add_widget(const ref_ptr &w); void add_widget_bare(cwidget::widget &w) { add_widget(ref_ptr(&w)); } Obviously this is less than ideal, but it will work. Be aware, though, that the bound argument is a *weak* reference: if there are no strong references to a bound widget, the signal connection will simply disappear.