ceph/src/boost/libs/xpressive/doc/tracking_ptr.qbk

   1 [/
   2  / Copyright (c) 2008 Eric Niebler
   3  /
   4  / Distributed under the Boost Software License, Version 1.0. (See accompanying
   5  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   6  /]
   7
   8 [section Cycle collection with [^tracking_ptr<>]]
   9
  10 In xpressive, regex objects can refer to each other and themselves by value or by reference.
  11 In addition, they ref-count their referenced regexes to keep them alive. This creates the possibility
  12 for cyclic reference counts, and raises the possibility of memory leaks. xpressive avoids leaks
  13 by using a type called `tracking_ptr<>`. This doc describes at a high level how `tracking_ptr<>`
  14 works.
  15
  16 [h2 Constraints]
  17
  18 Our solution must meet the following design constraints:
  19
  20 * No dangling references: All objects referred to directly or indirectly must be kept alive
  21   as long as the references are needed.
  22 * No leaks: all objects must be freed eventually.
  23 * No user intervention: The solution must not require users to explicitly invoke some cycle
  24   collection routine.
  25 * Clean-up is no-throw: The collection phase will likely be called from a destructor, so it
  26   must never throw an exception under any circumstance.
  27
  28 [h2 Handle-Body Idiom]
  29
  30 To use `tracking_ptr<>`, you must separate your type into a handle and a body. In the case of
  31 xpressive, the handle type is called `basic_regex<>` and the body is called `regex_impl<>`. The
  32 handle will store a `tracking_ptr<>` to the body.
  33
  34 The body type must inherit from `enable_reference_tracking<>`. This gives the body the bookkeeping
  35 data structures that `tracking_ptr<>` will use. In particular, it gives the body:
  36
  37 # `std::set<shared_ptr<body> > refs_` : collection of bodies to which this body refers, and
  38 # `std::set<weak_ptr<body> > deps_` : collection of bodies which refer to this body.
  39
  40 [h2 References and Dependencies]
  41
  42 We refer to (1) above as the "references" and (2) as the "dependencies". It is crucial to the
  43 understanding of `tracking_ptr<>` to recognize that the set of references includes both those
  44 objects that are referred to directly as well as those that are referred to indirectly (that
  45 is, through another reference). The same is true for the set of dependencies. In other words,
  46 each body holds a ref-count directly to every other body that it needs.
  47
  48 Why is this important?  Because it means that when a body no longer has a handle referring
  49 to it, all its references can be released immediately without fear of creating dangling references.
  50
  51 References and dependencies cross-pollinate. Here's how it works:
  52
  53 # When one object acquires another as a reference, the second object acquires the first as
  54   a dependency.
  55 # In addition, the first object acquires all of the second object's references, and the second
  56   object acquires all of the first object's dependencies.
  57 # When an object picks up a new reference, the reference is also added to all dependent objects.
  58 # When an object picks up a new dependency, the dependency is also added to all referenced
  59   objects.
  60 # An object is never allowed to have itself as a dependency. Objects may have themselves as
  61   references, and often do.
  62
  63 Consider the following code:
  64
  65     sregex expr;
  66     {
  67         sregex group  = '(' >> by_ref(expr) >> ')';                 // (1)
  68         sregex fact   = +_d | group;                                // (2)
  69         sregex term   = fact >> *(('*' >> fact) | ('/' >> fact));   // (3)
  70         expr          = term >> *(('+' >> term) | ('-' >> term));   // (4)
  71     }                                                               // (5)
  72
  73 Here is how the references and dependencies propagate, line by line:
  74
  75 [table
  76 [[Expression][Effects]]
  77 [[1) `sregex group  = '(' >> by_ref(expr) >> ')';`]
  78 [[^group: cnt=1 refs={expr} deps={}\n
  79 expr:  cnt=2 refs={} deps={group}]]]
  80
  81 [[2) `sregex fact   = +_d | group;`]
  82 [[^group: cnt=2 refs={expr} deps={fact}\n
  83 expr:  cnt=3 refs={} deps={group,fact}\n
  84 fact:  cnt=1 refs={expr,group} deps={}]]]
  85
  86 [[3) `sregex term   = fact >> *(('*' >> fact) | ('/' >> fact));`]
  87 [[^group: cnt=3 refs={expr} deps={fact,term}\n
  88 expr:  cnt=4 refs={} deps={group,fact,term}\n
  89 fact:  cnt=2 refs={expr,group} deps={term}\n
  90 term:  cnt=1 refs={expr,group,fact} deps={}]]]
  91
  92 [[4) `expr          = term >> *(('+' >> term) | ('-' >> term));`]
  93 [[^group: cnt=5 refs={expr,group,fact,term} deps={expr,fact,term}\n
  94 expr:  cnt=5 refs={expr,group,fact,term} deps={group,fact,term}\n
  95 fact:  cnt=5 refs={expr,group,fact,term} deps={expr,group,term}\n
  96 term:  cnt=5 refs={expr,group,fact,term} deps={expr,group,fact}]]]
  97
  98 [[5) `}`]
  99 [[^expr:  cnt=2 refs={expr,group,fact,term} deps={group,fact,term}]]]
 100 ]
 101
 102 This shows how references and dependencies propagate when creating cycles of objects. After
 103 line (4), which closes the cycle, every object has a ref-count on every other object, even
 104 to itself. So how does this not leak? Read on.
 105
 106 [h2 Cycle Breaking]
 107
 108 Now that the bodies have their sets of references and dependencies, the hard part is done.
 109 All that remains is to decide when and where to break the cycle. That is the job of `tracking_ptr<>`,
 110 which is part of the handle. The `tracking_ptr<>` holds 2 `shared_ptr`s. The first, obviously,
 111 is the `shared_ptr<body>` -- the reference to the body to which this handle refers. The other
 112 `shared_ptr` is used to break the cycle. It ensures that when all the handles to a body go out
 113 of scope, the body's set of references is cleared.
 114
 115 This suggests that more than one handle can refer to a body. In fact, `tracking_ptr<>` gives
 116 you copy-on-write semantics -- when you copy a handle, the body is shared. That makes copies
 117 very efficient. Eventually, all the handles to a particular body go out of scope. When that
 118 happens, the ref count to the body might still be greater than 0 because some other body (or
 119 this body itself!) might be holding a reference to it. However, we are certain that the cycle-breaker's
 120 ref-count goes to 0 because the cycle-breaker only lives in handles. No more handles, no more
 121 cycle-breakers.
 122
 123 What does the cycle-breaker do? Recall that the body has a set of references of type
 124 `std::set<shared_ptr<body> >`. Let's call this type "references_type". The cycle-breaker is a
 125 `shared_ptr<references_type>`. It uses a custom deleter, which is defined as follows:
 126
 127   template<typename DerivedT>
 128   struct reference_deleter
 129   {
 130       void operator ()(std::set<shared_ptr<DerivedT> > *refs) const
 131       {
 132           refs->clear();
 133       }
 134   };
 135
 136 The job of to the cycle breaker is to ensure that when the last handle to a body goes away,
 137 the body's set of references is cleared. That's it.
 138
 139 We can clearly see how this guarantees that all bodies are cleaned up eventually. Once every
 140 handle has gone out of scope, all the bodies' sets of references will be cleared, leaving none
 141 with a non-zero ref-count. No leaks, guaranteed.
 142
 143 It's a bit harder to see how this guarantees no dangling references. Imagine that there are 3
 144 bodies: A, B and C. A refers to B which refers to C. Now all the handles to B go out of scope,
 145 so B's set of references is cleared. Doesn't this mean that C gets deleted, even though it
 146 is being used (indirectly) by A? It doesn't. This situation can never occur because we propagated
 147 the references and dependencies above such that A will be holding a reference directly to C
 148 in addition to B. When B's set of references is cleared, no bodies get deleted, because they
 149 are all still in use by A.
 150
 151 [h2 Future Work]
 152
 153 All these `std::set`s and `shared_ptr`s and `weak_ptr`s! Very inefficient. I used them because
 154 they were handy. I could probably do better.
 155
 156 Also, some objects stick around longer than they need to. Consider:
 157
 158     sregex b;
 159     {
 160         sregex a = _;
 161         b = by_ref(a);
 162         b = _;
 163     }
 164     // a is still alive here!
 165
 166 Due to the way references and dependencies are propagated, the `std::set` of references can only
 167 grow. It never shrinks, even when some references are no longer needed. For xpressive this
 168 isn't an issue. The graphs of referential objects generally stay small and isolated. If someone
 169 were to try to use `tracking_ptr<>` as a general ref-count-cycle-collection mechanism, this problem
 170 would have to be addressed.
 171
 172 [endsect]