Boost C++ Libraries

STL and most other containers value initialize new elements in common operations like vector::resize(size_type n) or explicit vector::vector(size_type n).

In some performance-sensitive environments, where vectors are used as a replacement for variable-size buffers for file or network operations, value initialization is a cost that is not negligible as elements are going to be overwritten by an external source shortly after new elements are added to the container.

Boost.Container offers two new members for vector, static_vector and stable_vector: explicit container::container(size_type n, default_init_t) and explicit container::resize(size_type n, default_init_t), where new elements are constructed using default initialization.

When filling associative containers big performance gains can be achieved if the input range to be inserted is guaranteed by the user to be ordered according to the predicate. This can happen when inserting values from a set to a multiset or between different associative container families ([multi]set/map vs. flat_[multi]set/map).

Boost.Container has some overloads for constructors and insertions taking an ordered_unique_range_t or an ordered_range_t tag parameters as the first argument. When an ordered_unique_range_t overload is used, the user notifies the container that the input range is ordered according to the container predicate and has no duplicates. When an ordered_range_t overload is used, the user notifies the container that the input range is ordered according to the container predicate but it might have duplicates. With this information, the container can avoid multiple predicate calls and improve insertion times.

set, multiset, map and multimap associative containers are implemented as binary search trees which offer the needed complexity and stability guarantees required by the C++ standard for associative containers.

Boost.Container offers the possibility to configure at compile time some parameters of the binary search tree implementation. This configuration is passed as the last template parameter and defined using the utility class tree_assoc_options.

The following parameters can be configured:

See the following example to see how tree_assoc_options can be used to customize these containers:

#include <boost/container/set.hpp>
#include <cassert>

int main ()
{
   using namespace boost::container;

   //First define several options
   //

   //This option specifies an AVL tree based associative container
   typedef tree_assoc_options< tree_type<avl_tree> >::type AVLTree;

   //This option specifies an AVL tree based associative container
   //disabling node size optimization.
   typedef tree_assoc_options< tree_type<avl_tree>
                             , optimize_size<false> >::type AVLTreeNoSizeOpt;

   //This option specifies an Splay tree based associative container
   typedef tree_assoc_options< tree_type<splay_tree> >::type SplayTree;

   //Now define new tree-based associative containers
   //

   //AVLTree based set container
   typedef set<int, std::less<int>, std::allocator<int>, AVLTree> AvlSet;

   //AVLTree based set container without size optimization
   typedef set<int, std::less<int>, std::allocator<int>, AVLTreeNoSizeOpt> AvlSetNoSizeOpt;

   //Splay tree based multiset container
   typedef multiset<int, std::less<int>, std::allocator<int>, SplayTree> SplayMultiset;

   //Use them
   //
   AvlSet avl_set;
   avl_set.insert(0);
   assert(avl_set.find(0) != avl_set.end());

   AvlSetNoSizeOpt avl_set_no_szopt;
   avl_set_no_szopt.insert(1);
   avl_set_no_szopt.insert(1);
   assert(avl_set_no_szopt.count(1) == 1);

   SplayMultiset splay_mset;
   splay_mset.insert(2);
   splay_mset.insert(2);
   assert(splay_mset.count(2) == 2);
   return 0;
}

In the first C++ standard list::size() was not required to be constant-time, and that caused some controversy in the C++ community. Quoting Howard Hinnant's On List Size paper:

C++11 definitely required size() to be O(1), so range splice became O(N). However, Howard Hinnant's paper proposed a new splice overload so that even O(1) list:size() implementations could achieve O(1) range splice when the range size was known to the caller:

This new splice signature allows the client to pass the distance of the input range in. This information is often available at the call site. If it is passed in, then the operation is constant time, even with an O(1) size.

Boost.Container implements this overload for list and a modified version of it for slist (as slist::size() is also O(1)).

Many C++ programmers have ever wondered where does good old realloc fit in C++. And that's a good question. Could we improve vector performance using memory expansion mechanisms to avoid too many copies? But vector is not the only container that could benefit from an improved allocator interface: we could take advantage of the insertion of multiple elements in list using a burst allocation mechanism that could amortize costs (mutex locks, free memory searches...) that can't be amortized when using single node allocation strategies.

These improvements require extending the STL allocator interface and use make use of a new general purpose allocator since new and delete don't offer expansion and burst capabilities.

The following extended allocators are provided:

Use them simply specifying the new allocator in the corresponding template argument of your favourite container:

#include <boost/container/vector.hpp>
#include <boost/container/flat_set.hpp>
#include <boost/container/list.hpp>
#include <boost/container/set.hpp>

//"allocator" is a general purpose allocator that can reallocate
//memory, something useful for vector and flat associative containers
#include <boost/container/allocator.hpp>

//"adaptive_pool" is a node allocator, specially suited for
//node-based containers
#include <boost/container/adaptive_pool.hpp>

int main ()
{
   using namespace boost::container;

   //A vector that can reallocate memory to implement faster insertions
   vector<int, allocator<int> > extended_alloc_vector;

   //A flat set that can reallocate memory to implement faster insertions
   flat_set<int, std::less<int>, allocator<int> > extended_alloc_flat_set;

   //A list that can manages nodes to implement faster
   //range insertions and deletions
   list<int, adaptive_pool<int> > extended_alloc_list;

   //A set that can recycle nodes to implement faster
   //range insertions and deletions
   set<int, std::less<int>, adaptive_pool<int> > extended_alloc_set;

   //Now user them as always
   extended_alloc_vector.push_back(0);
   extended_alloc_flat_set.insert(0);
   extended_alloc_list.push_back(0);
   extended_alloc_set.insert(0);

   //...
   return 0;
}