<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><HTML><HEAD><TITLE>Introduction</TITLE><META NAME="Generator" content="Microsoft Visual Studio .NET 7.1"><base target = "content"></HEAD><BODY><H1>Introduction</H1><p>There are (at least) three orthogonal challenges to designing generic associative containers:</p><ol><li> The choice of underlying data-structure affects not only the performance of containers, but their semantics as well. <i>E.g.</i>, containers based on trees store elements by a given order, while containers based on hash tables store elements in a meaningless (and probably time-varying) order; containers based on node-based trees can guarantee exception-free element erasing, while containers based on vector-based trees cannot. This complicates generic manipulation of associative containers based on different underlying data-structures.</li><li>Underlying data-structures can act very differently given different policies. <i>E.g.</i>, the policy by which a hash table translates a hash value into a position within a table affects performance dramatically; certain policies can make containers based on trees support order statistics (<i>i.e.</i>, queries on the order of stored elements) or other useful queries. This complicates the policy design of an associative container based on a given data-structure.</li><li>Various mapping semantics are appropriate in different settings. <i>E.g.</i>, in some cases a unique mapping between each key and a datum is appropriate (such as the STL's <tt>std::map</tt> guarantees); in other cases, unique storage of keys is required (such as the STL's <tt>std::set</tt> guarantees); in other cases, more complex mapping semantics are required. This complicates generic manipulation of associative containers with different mapping semantics.</li></ol><p><tt>pb_assoc</tt> attempts to address these problems safely and efficiently.</p></BODY></HTML>