Question

增强文档（http://www.boost.org/doc/libs/1_55_0/doc/html/intrusive.html）指出侵入式容器是为list（单/双链接），set和multiset实现的。我找不到地图的实现。有没有更深层次的理由，还是只是等待实施？

Answer 1

这是因为map<Key, Value>实际上是set<std::pair<Key const, Value>>，而Boost.Intrusive和Boost.MultiIndex集允许您使用其中一个值成员作为键。换句话说，map如果find可以接受可比较的密钥，而不是搜索which has been a long-standing unresolved issue with std::map and std::set的整个值，则无需{{1}}：

关联容器查找函数（find，lower_bound，upper_bound， equal_range）只接受key_type的参数，要求用户构造（隐式地或显式地）key_type的对象来进行查找。这可能很昂贵，例如构造一个大对象来搜索集合当比较器函数只查看对象的一个字段时。那里用户之间的强烈愿望是能够使用其他类型进行搜索与key_type相当。

Answer 2

自version 1.59 Boost.Intrusive supports map-like associative containers以来。它比标准的类地图容器更灵活，因为程序员不必定义任何std::pair类似的结构来定义key_type和mapped_type。新的选项key_of_value被添加到侵入式集接口中，用于定义值的哪一部分应被视为key_type以及如何获取它。这样做是为了在使用Boost.Intrusive时简化类似地图的使用。从Boost文档中获取的示例代码：

#include <boost/static_assert.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/intrusive/set.hpp>
#include <boost/intrusive/unordered_set.hpp>
#include <vector>
#include <cassert>

using namespace boost::intrusive;

class MyClass : public set_base_hook<>
              , public unordered_set_base_hook<>
{
   public:
   int first;
   explicit MyClass(int i) : first(i){}
};

//key_of_value function object, must:
//- be default constructible (and lightweight)
//- define the key type using "type"
//- define an operator() taking "const value_type&" and
//    returning "const type &"
struct first_int_is_key
{
   typedef int type;

   const type & operator()(const MyClass& v) const
   {  return v.first;  }
};

//Define omap like ordered and unordered classes 
typedef set< MyClass, key_of_value<first_int_is_key> > OrderedMap;
typedef unordered_set< MyClass, key_of_value<first_int_is_key> > UnorderedMap;

int main()
{
   BOOST_STATIC_ASSERT((boost::is_same<  OrderedMap::key_type, int>::value));
   BOOST_STATIC_ASSERT((boost::is_same<UnorderedMap::key_type, int>::value));

   //Create several MyClass objects, each one with a different value
   //and insert them into the omap
   std::vector<MyClass> values;
   for(int i = 0; i < 100; ++i)  values.push_back(MyClass(i));

   //Create ordered/unordered maps and insert values
   OrderedMap   omap(values.begin(), values.end());
   UnorderedMap::bucket_type buckets[100];
   UnorderedMap umap(values.begin(), values.end(), UnorderedMap::bucket_traits(buckets, 100));

   //Test each element using the key_type (int)
   for(int i = 0; i != 100; ++i){
      assert(omap.find(i) != omap.end());
      assert(umap.find(i) != umap.end());
      assert(omap.lower_bound(i) != omap.end());
      assert(++omap.lower_bound(i) == omap.upper_bound(i));
      assert(omap.equal_range(i).first != omap.equal_range(i).second);
      assert(umap.equal_range(i).first != umap.equal_range(i).second);
   }

   //Count and erase by key
   for(int i = 0; i != 100; ++i){
      assert(1 == omap.count(i));
      assert(1 == umap.count(i));
      assert(1 == omap.erase(i));
      assert(1 == umap.erase(i));
   }
   assert(omap.empty());
   assert(umap.empty());

   return 0;
}

为什么没有boost :: intrusive :: map？

2 个答案: