...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
As explained in the Concepts section,
Boost.Intrusive containers need a ValueTraits
class to perform transformations
between nodes and user values. ValueTraits
can be explicitly configured (using the value_traits<>
option) or implicitly configured (using
hooks and their base_hook<>
/member_hook<>
options). ValueTraits
contains
all the information to glue the value_type
of the containers and the node to be used in node algorithms, since these types
can be different. Apart from this, ValueTraits
also stores information about the link policy of the values to be inserted.
Instead of using Boost.Intrusive predefined
hooks a user might want to develop customized containers, for example, using
nodes that are optimized for a specific application or that are compatible
with a legacy ABI. A user might want to have only two additional pointers in
his class and insert the class in a doubly linked list sometimes and in a singly
linked list in other situations. You can't achieve this using Boost.Intrusive
predefined hooks. Now, instead of using base_hook<...>
or member_hook<...>
options the user will specify the
value_traits<...>
options. Let's see how we can do this:
ValueTraits
has the following
interface:
#include <boost/pointer_to_other.hpp> #include <boost/intrusive/link_mode.hpp> struct my_value_traits { typedef implementation_defined node_traits; typedef implementation_defined value_type; typedef node_traits::node_ptr node_ptr; typedef node_traits::const_node_ptr const_node_ptr; typedef boost::pointer_to_other<node_ptr, value_type>::type pointer; typedef boost::pointer_to_other<node_ptr, const value_type>::type const_pointer; static const link_mode_type link_mode = some_linking_policy; static node_ptr to_node_ptr (value_type &value); static const_node_ptr to_node_ptr (const value_type &value); static pointer to_value_ptr (node_ptr n); static const_pointer to_value_ptr (const_node_ptr n); };
Let's explain each type and function:
slist
,
node_traits
should
follow the interface needed by circular_slist_algorithms
.
list
,
node_traits
should
follow the interface needed by circular_list_algorithms
.
set
/multiset
, node_traits
should follow the interface
needed by rbtree_algorithms
.
unordered_set
/
unordered_multiset
,
node_traits
should
follow the interface needed by circular_slist_algorithms
.
node_traits::node_ptr
.
node_traits::const_node_ptr
.
node_traits::node
but it can be different (for example,
node_traits::node
can be a member type of value_type
). If value_type
and node_traits::node
are the same type, the to_node_ptr
and to_value_ptr
functions are trivial.
value_type
.
It must be the same pointer type as node_ptr
:
If node_ptr
is node*
,
pointer
must be value_type*
.
If node_ptr
is smart_ptr<node_traits::node>
,
pointer
must be smart_ptr<value_type>
.
This can be generically achieved using boost::pointer_to_other
utility from Boost SmartPointers defined
in <boost/pointer_to_other.hpp>
.
const value_type
. It must be the same pointer
type as node_ptr
: If node_ptr
is node*
, const_pointer
must be const value_type*
. If node_ptr
is smart_ptr<node_traits::node>
,
const_pointer
must be
smart_ptr<const value_type>
This can be generically achieved using
boost::pointer_to_other
utility from Boost SmartPointers defined in <boost/pointer_to_other.hpp>
.
value_traits
needs
some additional work or checks from the container. The types are enumerations
defined in the link_mode.hpp
header. These are the possible types:
normal_link
:
If this linking policy is specified in a ValueTraits
class as the link mode, containers configured with such ValueTraits
won't set the hooks of
the erased values to a default state. Containers also won't check that
the hooks of the new values are default initialized.
safe_link
:
If this linking policy is specified as the link mode in a ValueTraits
class, containers configured
with this ValueTraits
will set the hooks of the erased values to a default state. Containers
also will check that the hooks of the new values are default initialized.
auto_unlink
:
Same as "safe_link" but containers with constant-time size
features won't be compatible with ValueTraits
configured with this policy. Containers also know that a value can
be silently erased from the container without using any function provided
by the containers.
Let's define our own value_traits
class to be able to use Boost.Intrusive
containers with an old C structure whose definition can't be changed. That
legacy type has two pointers that can be used to build singly and doubly
linked lists: in singly linked lists we only need a pointer, whereas in doubly
linked lists, we need two pointers. Since we only have two pointers, we can't
insert the object in both a singly and a doubly linked list at the same time.
This is the definition of the old node:
#include <boost/intrusive/link_mode.hpp> #include <boost/intrusive/list.hpp> #include <boost/intrusive/slist.hpp> #include <vector> //This node is the legacy type we can't modify and we want to insert in //intrusive list and slist containers using only two pointers, since //we know the object will never be at the same time in both lists. struct legacy_value { legacy_value *prev_; legacy_value *next_; int id_; };
Now we have to define a NodeTraits class that will implement the functions/typedefs that will make the legacy node compatible with Boost.Intrusive algorithms. After that, we'll define a ValueTraits class that will configure Boost.Intrusive containers:
//Define our own NodeTraits that will configure singly and doubly linked //list algorithms. Note that this node traits is compatible with //circular_slist_algorithms and circular_list_algorithms. namespace bi = boost::intrusive; struct legacy_node_traits { typedef legacy_value node; typedef legacy_value * node_ptr; typedef const legacy_value * const_node_ptr; static node *get_next(const node *n) { return n->next_; } static void set_next(node *n, node *next) { n->next_ = next; } static node *get_previous(const node *n) { return n->prev_; } static void set_previous(node *n, node *prev) { n->prev_ = prev; } }; //This ValueTraits will configure list and slist. In this case, //legacy_node_traits::node is the same as the //legacy_value_traits::value_type so to_node_ptr/to_value_ptr //functions are trivial. struct legacy_value_traits { typedef legacy_node_traits node_traits; typedef node_traits::node_ptr node_ptr; typedef node_traits::const_node_ptr const_node_ptr; typedef legacy_value value_type; typedef legacy_value * pointer; typedef const legacy_value * const_pointer; static const bi::link_mode_type link_mode = bi::normal_link; static node_ptr to_node_ptr (value_type &value) { return node_ptr(&value); } static const_node_ptr to_node_ptr (const value_type &value) { return const_node_ptr(&value); } static pointer to_value_ptr(node_ptr n) { return pointer(n); } static const_pointer to_value_ptr(const_node_ptr n) { return const_pointer(n); } };
Defining a value traits class that simply defines value_type
as legacy_node_traits::node
is a common approach when defining
customized intrusive containers, so Boost.Intrusive
offers a templatized trivial_value_traits
class that does exactly what we want:
#include <boost/intrusive/trivial_value_traits.hpp> //Now we can define legacy_value_traits just with a single line using namespace boost::intrusive; typedef trivial_value_traits<legacy_node_traits, normal_link> legacy_value_traits;
Now we can just define the containers that will store the legacy abi objects and write a little test:
//Now define an intrusive list and slist that will store legacy_value objects typedef bi::value_traits<legacy_value_traits> ValueTraitsOption; typedef bi::list<legacy_value, ValueTraitsOption> LegacyAbiList; typedef bi::slist<legacy_value, ValueTraitsOption> LegacyAbiSlist; template<class List> bool test_list() { typedef std::vector<legacy_value> Vect; //Create legacy_value objects, with a different internal number Vect legacy_vector; for(int i = 0; i < 100; ++i){ legacy_value value; value.id_ = i; legacy_vector.push_back(value); } //Create the list with the objects List mylist(legacy_vector.begin(), legacy_vector.end()); //Now test both lists typename List::const_iterator bit(mylist.begin()), bitend(mylist.end()); typename Vect::const_iterator it(legacy_vector.begin()), itend(legacy_vector.end()); //Test the objects inserted in our list for(; it != itend; ++it, ++bit) if(&*bit != &*it) return false; return true; } int main() { return test_list<LegacyAbiList>() && test_list<LegacyAbiSlist>() ? 0 : 1; }
As seen, several key elements of Boost.Intrusive can be reused with custom user types, if the user does not want to use the provided Boost.Intrusive facilities.
In the previous example, legacy_node_traits::node
type and legacy_value_traits::value_type
are the same type, but this is not necessary. It's possible to have several
ValueTraits
defining the
same node_traits
type (and
thus, the same node_traits::node
).
This reduces the number of node algorithm instantiations, but now ValueTraits::to_node_ptr
and ValueTraits::to_value_ptr
functions need to offer conversions between both types. Let's see a small
example:
First, we'll define the node to be used in the algorithms. For a linked list, we just need a node that stores two pointers:
#include <boost/intrusive/link_mode.hpp> #include <boost/intrusive/list.hpp> #include <vector> //This is the node that will be used with algorithms. struct simple_node { simple_node *prev_; simple_node *next_; };
Now we'll define two different types that will be inserted in intrusive lists
and a templatized ValueTraits
that will work for both types:
class base_1{}; class base_2{}; struct value_1 : public base_1, public simple_node { int id_; }; struct value_2 : public base_1, public base_2, public simple_node { float id_; }; //Define the node traits. A single node_traits will be enough. struct simple_node_traits { typedef simple_node node; typedef node * node_ptr; typedef const node * const_node_ptr; static node *get_next(const node *n) { return n->next_; } static void set_next(node *n, node *next) { n->next_ = next; } static node *get_previous(const node *n) { return n->prev_; } static void set_previous(node *n, node *prev) { n->prev_ = prev; } }; //A templatized value traits for value_1 and value_2 template<class ValueType> struct simple_value_traits { typedef simple_node_traits node_traits; typedef node_traits::node_ptr node_ptr; typedef node_traits::const_node_ptr const_node_ptr; typedef ValueType value_type; typedef ValueType * pointer; typedef const ValueType * const_pointer; static const boost::intrusive::link_mode_type link_mode = boost::intrusive::normal_link; static node_ptr to_node_ptr (value_type &value) { return node_ptr(&value); } static const_node_ptr to_node_ptr (const value_type &value) { return const_node_ptr(&value); } static pointer to_value_ptr(node_ptr n) { return static_cast<value_type*>(n); } static const_pointer to_value_ptr(const_node_ptr n) { return static_cast<const value_type*>(n); } };
Now define two containers. Both containers will instantiate the same list
algorithms (circular_list_algorithms<simple_node_traits>
), due to the fact that the value traits
used to define the containers provide the same node_traits
type:
//Now define two intrusive lists. Both lists will use the same algorithms: // circular_list_algorithms<simple_node_traits> using namespace boost::intrusive; typedef list <value_1, value_traits<simple_value_traits<value_1> > > Value1List; typedef list <value_2, value_traits<simple_value_traits<value_2> > > Value2List;
All Boost.Intrusive containers using predefined
hooks use this technique to minimize code size: all possible list
containers created with predefined hooks that define the same VoidPointer
type share the same list algorithms.
The previous example can be further simplified using the derivation_value_traits
class to define a value traits class with a value that stores the simple_node
as a base class:
#include <boost/intrusive/derivation_value_traits.hpp> //value_1, value_2, simple_node and simple_node_traits are defined //as in the previous example... //... using namespace boost::intrusive; //Now define the needed value traits using typedef derivation_value_traits<value_1, simple_node_traits, normal_link> ValueTraits1; typedef derivation_value_traits<value_2, simple_node_traits, normal_link> ValueTraits2; //Now define the containers typedef list <value1, value_traits<ValueTraits1> > Value1List; typedef list <value2, value_traits<ValueTraits2> > Value2List;
We can even choose to store simple_node
as a member of value_1
and
value_2
classes and use
member_value_traits
to define the needed value traits classes:
class base_1{}; class base_2{}; struct value_1 : public base_1, public simple_node { int id_; simple_node node_; }; struct value_2 : public base_1, public base_2, public simple_node { simple_node node_; float id_; }; using namespace boost::intrusive; typedef member_value_traits <value_1, simple_node_traits, &value_1::node_, normal_link> ValueTraits1; typedef member_value_traits <value_2, simple_node_traits, &value_2::node_, normal_link> ValueTraits2; //Now define two intrusive lists. Both lists will use the same algorithms: // circular_list_algorithms<simple_node_traits> typedef list <value_1, value_traits<ValueTraits1> > Value1List; typedef list <value_2, value_traits<ValueTraits2> > Value2List;
Until now all shown custom value traits are stateless, that is, the transformation between nodes and values is implemented in terms of static functions. It's possible to use stateful value traits so that we can separate nodes and values and avoid modifying types to insert nodes. Boost.Intrusive differentiates between stateful and stateless value traits by checking if the ValueTraits class is empty:
Using stateful value traits it's possible to create containers of non-copyable/moveble objects without modifying the definition of the class to be inserted. This interesting property is achieved without using global variables (stateless value traits could use global variables to achieve the same goal), so:
Stateful value traits have many advantages but also some downsides:
operator*()
and operator->()
).
An easy and useful example of stateful value traits is when an array of values can be indirectly introduced in a list guaranteeing no additional allocation apart from the initial resource reservation:
#include <boost/intrusive/list.hpp> using namespace boost::intrusive; //This type is not modifiable so we can't store hooks or custom nodes typedef int identifier_t; //This value traits will associate elements from an array of identifiers with //elements of an array of nodes. The element i of the value array will use the //node i of the node array: struct stateful_value_traits { typedef list_node_traits<void*> node_traits; typedef node_traits::node node; typedef node * node_ptr; typedef const node * const_node_ptr; typedef identifier_t value_type; typedef identifier_t * pointer; typedef const identifier_t * const_pointer; static const link_mode_type link_mode = normal_link; stateful_value_traits(pointer ids, node_ptr node_array) : ids_(ids), nodes_(node_array) {} ///Note: non static functions! node_ptr to_node_ptr (value_type &value) { return this->nodes_ + (&value - this->ids_); } const_node_ptr to_node_ptr (const value_type &value) const { return this->nodes_ + (&value - this->ids_); } pointer to_value_ptr(node_ptr n) { return this->ids_ + (n - this->nodes_); } const_pointer to_value_ptr(const_node_ptr n) const { return this->ids_ + (n - this->nodes_); } private: pointer ids_; node_ptr nodes_; }; int main() { const int NumElements = 100; //This is an array of ids that we want to "store" identifier_t ids [NumElements]; //This is an array of nodes that is necessary to form the linked list list_node_traits<void*>::node nodes [NumElements]; //Initialize id objects, each one with a different number for(int i = 0; i != NumElements; ++i) ids[i] = i; //Define a list that will "link" identifiers using external nodes typedef list<identifier_t, value_traits<stateful_value_traits> > List; //This list will store ids without modifying identifier_t instances //Stateful value traits must be explicitly passed in the constructor. List my_list (stateful_value_traits (ids, nodes)); //Insert ids in reverse order in the list for(identifier_t * it(&ids[0]), *itend(&ids[NumElements]); it != itend; ++it) my_list.push_front(*it); //Now test lists List::const_iterator list_it (my_list.cbegin()); identifier_t *it_val(&ids[NumElements-1]), *it_rbeg_val(&ids[0]-1); //Test the objects inserted in the base hook list for(; it_val != it_rbeg_val; --it_val, ++list_it) if(&*list_it != &*it_val) return 1; return 0; }